Programming Rust

Fast, Safe Systems Development

Jim Blandy and Jason Orendorff

Programming Rust

by Jim Blandy and Jason Orendorff

Copyright © 2018 Jim Blandy, Jason Orendorff. All rights reserved.

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Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472.

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December 2017: First Edition

Revision History for the First Edition

2017-11-20: First Release

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978-1-491-92728-1

[M]

Preface

Rust is a language for systems programming.

This bears some explanation these days, as systems programming is unfamiliar to most working programmers. Yet it underlies everything we do.

You close your laptop. The operating system detects this, suspends all the running programs, turns off the screen, and puts the computer to sleep. Later, you open the laptop: the screen and other components are powered up again, and each program is able to pick up where it left off. We take this for granted. But systems programmers wrote a lot of code to make that happen.

Systems programming is for:

Operating systems

Device drivers of all kinds

Filesystems

Databases

Code that runs in very cheap devices, or devices that must be extremely reliable

Cryptography

Media codecs (software for reading and writing audio, video, and image files)

Media processing (for example, speech recognition or photo editing software)

Memory management (for example, implementing a garbage collector)

Text rendering (the conversion of text and fonts into pixels)

Implementing higher-level programming languages (like JavaScript and Python)

Networking

Virtualization and software containers

Scientific simulations

Games

In short, systems programming is resource-constrained programming. It is programming when every byte and every CPU cycle counts.

The amount of systems code involved in supporting a basic app is staggering.

This book will not teach you systems programming. In fact, this book covers many details of memory management that might seem unnecessarily abstruse at first, if you haven’t already done some systems programming on your own. But if you are a seasoned systems programmer, you’ll find that Rust is something exceptional: a new tool that eliminates major, well-understood problems that have plagued a whole industry for decades.

Who Should Read This Book

If you’re already a systems programmer, and you’re ready for an alternative to C++, this book is for you. If you’re an experienced developer in any programming language, whether that’s C#, Java, Python, JavaScript, or something else, this book is for you too.

However, you don’t just need to learn Rust. To get the most out of the language, you also need to gain some experience with systems programming. We recommend reading this book while also implementing some systems programming side projects in Rust. Build something you’ve never built before, something that takes advantage of Rust’s speed, concurrency, and safety. The list of topics at the beginning of this preface should give you some ideas.

Why We Wrote This Book

We set out to write the book we wished we had when we started learning Rust. Our goal was to tackle the big, new concepts in Rust up front and head-on, presenting them clearly and in depth so as to minimize learning by trial and error.

Navigating This Book

The first two chapters of this book introduce Rust and provide a brief tour before we move on to the fundamental data types in Chapter 3. Chapters 4 and 5 address the core concepts of ownership and references. We recommend reading these first five chapters through in order.

Chapters 6 through 10 cover the basics of the language: expressions (Chapter 6), error handling (Chapter 7), crates and modules (Chapter 8), structs (Chapter 9), and enums and patterns (Chapter 10). It’s all right to skim a little here, but don’t skip the chapter on error handling. Trust us.

Chapter 11 covers traits and generics, the last two big concepts you need to know. Traits are like interfaces in Java or C#. They’re also the main way Rust supports integrating your types into the language itself. Chapter 12 shows how traits support operator overloading, and Chapter 13 covers many more utility traits.

Understanding traits and generics unlocks the rest of the book. Closures and iterators, two key power tools that you won’t want to miss, are covered in Chapters 14 and 15, respectively. You can read the remaining chapters in any order, or just dip into them as needed. They cover the rest of the language: collections (Chapter 16), strings and text (Chapter 17), input and output (Chapter 18), concurrency (Chapter 19), macros (Chapter 20), and unsafe code (Chapter 21).

Conventions Used in This Book

The following typographical conventions are used in this book:

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Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

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Acknowledgments

The book you are holding has benefited greatly from the attention of our official technical reviewers: Brian Anderson, Matt Brubeck, J. David Eisenberg, and Jack Moffitt.

Many other unofficial reviewers read early drafts and provided invaluable feedback. We would like to thank Eddy Bruel, Nick Fitzgerald, Michael Kelly, Jeffrey Lim, Jakob Olesen, Gian-Carlo Pascutto, Larry Rabinowitz, Jaroslav Šnajdr, and Joe Walker for their thoughtful comments. Jeff Walden and Nicolas Pierron were especially generous with their time, reviewing almost the entire book. Like any programming venture, a programming book thrives on quality bug reports. Thank you.

Mozilla was extremely accommodating of our work on this project, even though it fell outside our official responsibilities and competed with them for our attention. We are grateful to our managers, Dave Camp, Naveed Ihsanullah, Tom Tromey, and Joe Walker, for their support. They take a long view of what Mozilla is about; we hope these results justify the faith they placed in us.

We would also like to express our appreciation for everyone at O’Reilly who helped bring this project to fruition, especially our editors Brian MacDonald and Jeff Bleiel.

Most of all, our heartfelt thanks to our wives and children for their unwavering love, enthusiasm, and patience.

Chapter 1. Why Rust?

In certain contexts—for example the context Rust is targeting—being 10x or even 2x faster than the competition is a make-or-break thing. It decides the fate of a system in the market, as much as it would in the hardware market.

Graydon Hoare

All computers are now parallel...
Parallel programming is programming.

Michael McCool et al., Structured Parallel Programming

TrueType parser flaw
used by nation-state attacker for surveillance;
all software is security-sensitive.

Andy Wingo

Systems programming languages have come a long way in the 50 years since we started using high-level languages to write operating systems, but two problems in particular have proven difficult to crack:

It’s difficult to write secure code. It’s especially difficult to manage memory correctly in C and C++. Users have been suffering with the consequences for decades, in the form of security holes dating back at least as far as the 1988 Morris worm.

It’s very difficult to write multithreaded code, which is the only way to exploit the abilities of modern machines. Even experienced programmers approach threaded code with caution: concurrency can introduce broad new classes of bugs and make ordinary bugs much harder to reproduce.

Enter Rust: a safe, concurrent language with the performance of C and C++.

Rust is a new systems programming language developed by Mozilla and a community of contributors. Like C and C++, Rust gives developers fine control over the use of memory, and maintains a close relationship between the primitive operations of the language and those of the machines it runs on, helping developers anticipate their code’s costs. Rust shares the ambitions Bjarne Stroustrup articulates for C‍++ in his paper “Abstraction and the C++ Machine Model:”

In general, C++ implementations obey the zero-overhead principle: What you don’t use, you don’t pay for. And further: What you do use, you couldn’t hand code any better.

To these Rust adds its own goals of memory safety and trustworthy concurrency.

The key to meeting all these promises is Rust’s novel system of ownership, moves, and borrows, checked at compile time and carefully designed to complement Rust’s flexible static type system. The ownership system establishes a clear lifetime for each value, making garbage collection unnecessary in the core language, and enabling sound but flexible interfaces for managing other sorts of resources like sockets and file handles. Moves transfer values from one owner to another, and borrowing lets code use a value temporarily without affecting its ownership. Since many programmers will have never encountered these features in this form before, we explain them in detail in Chapters 4 and 5.

These same ownership rules also form the foundation of Rust’s trustworthy concurrency model. Most languages leave the relationship between a mutex and the data it’s meant to protect to the comments; Rust can actually check at compile time that your code locks the mutex while it accesses the data. Most languages admonish you to be sure not to use a data structure yourself after you’ve given it to another thread; Rust checks that you don’t. Rust is able to prevent data races at compile time.

Rust is not really an object-oriented language, although it has some object-oriented characteristics. Rust is not a functional language, although it does tend to make the influences on a computation’s result more explicit, as functional languages do. Rust resembles C and C++ to an extent, but many idioms from those languages don’t apply, so typical Rust code does not deeply resemble C or C++ code. It’s probably best to reserve judgement about what sort of language Rust is, and see what you think once you’ve become comfortable with the language.

To get feedback on the design in a real-world setting, Mozilla has developed Servo, a new web browser engine, in Rust. Servo’s needs and Rust’s goals are well matched: a browser must perform well and handle untrusted data securely. Servo uses Rust’s safe concurrency to put the full machine to work on tasks that would be impractical to parallelize in C or C++. In fact, Servo and Rust have grown up together, with Servo using the latest new language features, and Rust evolving based on feedback from Servo’s developers.

Type Safety

Rust is a type-safe language. But what do we mean by “type safety”? Safety sounds good, but what exactly are we being kept safe from?

Here’s the definition of undefined behavior from the 1999 standard for the C programming language, known as C99:

undefined behavior

behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

Consider the following C program:

int main(int argc, char **argv) {
  unsigned long a[1];
  a[3] = 0x7ffff7b36cebUL;
  return 0;
}

According to C99, because this program accesses an element off the end of the array a, its behavior is undefined, meaning that it can do anything whatsoever. When we ran this program on Jim’s laptop, it produced the following output:

undef: Error: .netrc file is readable by others.
undef: Remove password or make file unreadable by others.

Then it crashed. Jim’s laptop doesn’t even have a .netrc file. If you try it yourself, it will probably do something entirely different.

The machine code the C compiler generated for this main function happens to place the array a on the stack three words before the return address, so storing 0x7ffff7b36cebUL in a[3] changes poor main’s return address to point into the midst of code in the C standard library that consults one’s .netrc file for a password. When main returns, execution resumes not in main’s caller, but at the machine code for these lines from the library:

warnx(_("Error: .netrc file is readable by others."));
warnx(_("Remove password or make file unreadable by others."));
            goto bad;

In allowing an array reference to affect the behavior of a subsequent return statement, the C compiler is fully standards-compliant. An undefined operation doesn’t just produce an unspecified result: it is allowed to cause the program to do anything at all.

The C99 standard grants the compiler this carte blanche to allow it to generate faster code. Rather than making the compiler responsible for detecting and handling odd behavior like running off the end of an array, the standard makes the programmer responsible for ensuring those conditions never arise in the first place.

Empirically speaking, we’re not very good at that. While a student at the University of Utah, researcher Peng Li modified C and C++ compilers to make the programs they translated report when they executed certain forms of undefined behavior. He found that nearly all programs do, including those from well-respected projects that hold their code to high standards. And undefined behavior often leads to exploitable security holes in practice. The Morris worm propagated itself from one machine to another using an elaboration of the technique shown before, and this kind of exploit remains in widespread use today.

In light of that example, let’s define some terms. If a program has been written so that no possible execution can exhibit undefined behavior, we say that program is well defined. If a language’s safety checks ensure that every program is well defined, we say that language is type safe.

A carefully written C or C++ program might be well defined, but C and C++ are not type safe: the program shown earlier has no type errors, yet exhibits undefined behavior. By contrast, Python is type safe. Python is willing to spend processor time to detect and handle out-of-range array indices in a friendlier fashion than C:

>>> a = [0]
>>> a[3] = 0x7ffff7b36ceb
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range
>>>

Python raised an exception, which is not undefined behavior: the Python documentation specifies that the assignment to a[3] should raise an IndexError exception, as we saw. Certainly, a module like ctypes that provides unconstrained access to the machine can introduce undefined behavior into Python, but the core language itself is type safe. Java, JavaScript, Ruby, and Haskell are similar in this way.

Note that being type safe is independent of whether a language checks types at compile time or at runtime: C checks at compile time, and is not type safe; Python checks at runtime, and is type safe.

It is ironic that the dominant systems programming languages, C and C++, are not type safe, while most other popular languages are. Given that C and C++ are meant to be used to implement the foundations of a system, entrusted with implementing security boundaries and placed in contact with untrusted data, type safety would seem like an especially valuable quality for them to have.

This is the decades-old tension Rust aims to resolve: it is both type safe and a systems programming language. Rust is designed for implementing those fundamental system layers that require performance and fine-grained control over resources, yet still guarantees the basic level of predictability that type safety provides. We’ll look at how Rust manages this unification in more detail in later parts of this book.

Rust’s particular form of type safety has surprising consequences for multithreaded programming. Concurrency is notoriously difficult to use correctly in C and C++; developers usually turn to concurrency only when single-threaded code has proven unable to achieve the performance they need. But Rust guarantees that concurrent code is free of data races, catching any misuse of mutexes or other synchronization primitives at compile time. In Rust, you can use concurrency without worrying that you’ve made your code impossible for any but the most accomplished programmers to work on.

Rust has an escape valve from the safety rules, for when you absolutely have to use a raw pointer. This is called unsafe code, and while most Rust programs don’t need it, we’ll show how to use it and how it fits into Rust’s overall safety scheme in Chapter 21.

Like those of other statically typed languages, Rust’s types can do much more than simply prevent undefined behavior. An accomplished Rust programmer uses types to ensure values are used not just safely but meaningfully, in a way that’s consistent with the application’s intent. In particular, Rust’s traits and generics, described in Chapter 11, provide a succinct, flexible, and performant way to describe characteristics that a group of types has in common, and then take advantage of those commonalities.

Our aim in this book is to give you the insights you need not just to write programs in Rust, but to put the language to work ensuring that those programs are both safe and correct, and to anticipate how they will perform. In our experience, Rust is a major step forward in systems programming, and we want to help you take advantage of it.

Chapter 2. A Tour of Rust

Toute l’expérience d’un individu est construit sur le plan de son langage.
(An individual’s experience is built entirely in terms of his language.)

Henri Delacroix

In this chapter we’ll look at several short programs to see how Rust’s syntax, types, and semantics fit together to support safe, concurrent, and efficient code. We’ll walk through the process of downloading and installing Rust, show some simple mathematical code, try out a web server based on a third-party library, and use multiple threads to speed up the process of plotting the Mandelbrot set.

Downloading and Installing Rust

The best way to install Rust is to use rustup, the Rust installer. Go to https://rustup.rs and follow the instructions there.

You can, alternatively, go to https://www.rust-lang.org, click Downloads, and get pre-built packages for Linux, macOS, and Windows. Rust is also included in some operating system distributions. We prefer rustup because it’s a tool for managing Rust installations, like RVM for Ruby or NVM for Node. For example, when a new version of Rust is released, you’ll be able to upgrade with zero clicks by typing rustup update.

In any case, once you’ve completed the installation, you should have three new commands available at your command line:

$ cargo --version
cargo 0.18.0 (fe7b0cdcf 2017-04-24)
$ rustc --version
rustc 1.17.0 (56124baa9 2017-04-24)
$ rustdoc --version
rustdoc 1.17.0 (56124baa9 2017-04-24)
$

Here, the $ is the command prompt; on Windows, this would be C:\> or something similar. In this transcript we run the three commands we installed, asking each to report which version it is. Taking each command in turn:

cargo is Rust’s compilation manager, package manager, and general-purpose tool. You can use Cargo to start a new project, build and run your program, and manage any external libraries your code depends on.

rustc is the Rust compiler. Usually we let Cargo invoke the compiler for us, but sometimes it’s useful to run it directly.

rustdoc is the Rust documentation tool. If you write documentation in comments of the appropriate form in your program’s source code, rustdoc can build nicely formatted HTML from them. Like rustc, we usually let Cargo run rustdoc for us.

As a convenience, Cargo can create a new Rust package for us, with some standard metadata arranged appropriately:

$ cargo new --bin hello
     Created binary (application) `hello` project

This command creates a new package directory named hello, and the --bin flag directs Cargo to prepare this as an executable, not a library. Looking inside the package’s top-level directory:

$ cd hello
$ ls -la
total 24
drwxrwxr-x.  4 jimb jimb 4096 Sep 22 21:09 .
drwx------. 62 jimb jimb 4096 Sep 22 21:09 ..
drwxrwxr-x.  6 jimb jimb 4096 Sep 22 21:09 .git
-rw-rw-r--.  1 jimb jimb    7 Sep 22 21:09 .gitignore
-rw-rw-r--.  1 jimb jimb   88 Sep 22 21:09 Cargo.toml
drwxrwxr-x.  2 jimb jimb 4096 Sep 22 21:09 src
$

We can see that Cargo has created a file Cargo.toml to hold metadata for the package. At the moment this file doesn’t contain much:

[package]
name = "hello"
version = "0.1.0"
authors = ["You <you@example.com>"]

[dependencies]

If our program ever acquires dependencies on other libraries, we can record them in this file, and Cargo will take care of downloading, building, and updating those libraries for us. We’ll cover the Cargo.toml file in detail in Chapter 8.

Cargo has set up our package for use with the git version control system, creating a .git metadata subdirectory, and a .gitignore file. You can tell Cargo to skip this step by specifying --vcs none on the command line.

The src subdirectory contains the actual Rust code:

$ cd src
$ ls -l
total 4
-rw-rw-r--. 1 jimb jimb 45 Sep 22 21:09 main.rs

It seems that Cargo has begun writing the program on our behalf. The main.rs file contains the text:

fn main() {
    println!("Hello, world!");
}

In Rust, you don’t even need to write your own “Hello, World!” program. And this is the extent of the boilerplate for a new Rust program: two files, totaling nine lines.

We can invoke the cargo run command from any directory in the package to build and run our program:

$ cargo run
   Compiling hello v0.1.0 (file:///home/jimb/rust/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 0.27 secs
     Running `/home/jimb/rust/hello/target/debug/hello`
Hello, world!
$

Here, Cargo has invoked the Rust compiler, rustc, and then run the executable it produced. Cargo places the executable in the target subdirectory at the top of the package:

$ ls -l ../target/debug
total 580
drwxrwxr-x. 2 jimb jimb   4096 Sep 22 21:37 build
drwxrwxr-x. 2 jimb jimb   4096 Sep 22 21:37 deps
drwxrwxr-x. 2 jimb jimb   4096 Sep 22 21:37 examples
-rwxrwxr-x. 1 jimb jimb 576632 Sep 22 21:37 hello
-rw-rw-r--. 1 jimb jimb    198 Sep 22 21:37 hello.d
drwxrwxr-x. 2 jimb jimb     68 Sep 22 21:37 incremental
drwxrwxr-x. 2 jimb jimb   4096 Sep 22 21:37 native
$ ../target/debug/hello
Hello, world!
$

When we’re through, Cargo can clean up the generated files for us:

$ cargo clean
$ ../target/debug/hello
bash: ../target/debug/hello: No such file or directory
$

A Simple Function

Rust’s syntax is deliberately unoriginal. If you are familiar with C, C++, Java, or JavaScript, you can probably find your way through the general structure of a Rust program. Here is a function that computes the greatest common divisor of two integers, using Euclid’s algorithm:

fn gcd(mut n: u64, mut m: u64) -> u64 {
    assert!(n != 0 && m != 0);
    while m != 0 {
        if m < n {
            let t = m;
            m = n;
            n = t;
        }
        m = m % n;
    }
    n
}

The fn keyword (pronounced “fun”) introduces a function. Here, we’re defining a function named gcd, which takes two parameters n and m, each of which is of type u64, an unsigned 64-bit integer. The -> token precedes the return type: our function returns a u64 value. Four-space indentation is standard Rust style.

Rust’s machine integer type names reflect their size and signedness: i32 is a signed 32-bit integer; u8 is an unsigned eight-bit integer (used for “byte” values), and so on. The isize and usize types hold pointer-sized signed and unsigned integers, 32 bits long on 32-bit platforms, and 64 bits long on 64-bit platforms. Rust also has two floating-point types, f32 and f64, which are the IEEE single- and double-precision floating-point types, like float and double in C and C++.

By default, once a variable is initialized, its value can’t be changed, but placing the mut keyword (pronounced “mute”, short for mutable) before the parameters n and m allows our function body to assign to them. In practice, most variables don’t get assigned to; the mut keyword on those that do can be a helpful hint when reading code.

The function’s body starts with a call to the assert! macro, verifying that neither argument is zero. The ! character marks this as a macro invocation, not a function call. Like the assert macro in C and C++, Rust’s assert! checks that its argument is true, and if it is not, terminates the program with a helpful message including the source location of the failing check; this kind of abrupt termination is called a panic. Unlike C and C++, in which assertions can be skipped, Rust always checks assertions regardless of how the program was compiled. There is also a debug_assert! macro, whose assertions are skipped when the program is compiled for speed.

The heart of our function is a while loop containing an if statement and an assignment. Unlike C and C++, Rust does not require parentheses around the conditional expressions, but it does require curly braces around the statements they control.

A let statement declares a local variable, like t in our function. We don’t need to write out t’s type, as long as Rust can infer it from how the variable is used. In our function, the only type that works for t is u64, matching m and n. Rust only infers types within function bodies: you must write out the types of function parameters and return values, as we did before. If we wanted to spell out t’s type, we could write:

let t: u64 = m;

Rust has a return statement, but the gcd function doesn’t need one. If a function body ends with an expression that is not followed by a semicolon, that’s the function’s return value. In fact, any block surrounded by curly braces can function as an expression. For example, this is an expression that prints a message and then yields x.cos() as its value:

{
    println!("evaluating cos x");
    x.cos()
}

It’s typical in Rust to use this form to establish the function’s value when control “falls off the end” of the function, and use return statements only for explicit early returns from the midst of a function.

Writing and Running Unit Tests

Rust has simple support for testing built into the language. To test our gcd function, we can write:

#[test]
fn test_gcd() {
    assert_eq!(gcd(14, 15), 1);

    assert_eq!(gcd(2 * 3 * 5 * 11 * 17,
                   3 * 7 * 11 * 13 * 19),
               3 * 11);
}

Here we define a function named test_gcd, which calls gcd and checks that it returns correct values. The #[test] atop the definition marks test_gcd as a test function, to be skipped in normal compilations, but included and called automatically if we run our program with the cargo test command. Let’s assume we’ve edited our gcd and test_gcd definitions into the hello package we created at the beginning of the chapter. If our current directory is somewhere within the package’s subtree, we can run the tests as follows:

$ cargo test
   Compiling hello v0.1.0 (file:///home/jimb/rust/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 0.35 secs
     Running /home/jimb/rust/hello/target/debug/deps/hello-2375a82d9e9673d7

running 1 test
test test_gcd ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

$

We can have test functions scattered throughout our source tree, placed next to the code they exercise, and cargo test will automatically gather them up and run them all.

The #[test] marker is an example of an attribute. Attributes are an open-ended system for marking functions and other declarations with extra information, like attributes in C++ and C#, or annotations in Java. They’re used to control compiler warnings and code style checks, include code conditionally (like #ifdef in C and C++), tell Rust how to interact with code written in other languages, and so on. We’ll see more examples of attributes as we go.

Handling Command-Line Arguments

If we want our program to take a series of numbers as command-line arguments and print their greatest common divisor, we can replace the main function with the following:

use std::io::Write;
use std::str::FromStr;

fn main() {
    let mut numbers = Vec::new();

    for arg in std::env::args().skip(1) {
        numbers.push(u64::from_str(&arg)
                     .expect("error parsing argument"));
    }

    if numbers.len() == 0 {
        writeln!(std::io::stderr(), "Usage: gcd NUMBER ...").unwrap();
        std::process::exit(1);
    }

    let mut d = numbers[0];
    for m in &numbers[1..] {
        d = gcd(d, *m);
    }

    println!("The greatest common divisor of {:?} is {}",
             numbers, d);
}

This is a large block of code, so let’s take it piece by piece:

use std::io::Write;
use std::str::FromStr;

The use declarations bring the two traits Write and FromStr into scope. We’ll cover traits in detail in Chapter 11, but for now we’ll simply say that a trait is a collection of methods that types can implement. Although we never use the names Write or FromStr elsewhere in the program, a trait must be in scope in order to use its methods. In the present case:

Any type that implements the Write trait has a write_fmt method that writes formatted text to a stream. The std::io::Stderr type implements Write, and we’ll use the writeln! macro to print error messages; that macro expands to code that uses the write_fmt method.

Any type that implements the FromStr trait has a from_str method that tries to parse a value of that type from a string. The u64 type implements FromStr, and we’ll call u64::from_str to parse our command-line arguments.

Moving on to the program’s main function:

fn main() {

Our main function doesn’t return a value, so we can simply omit the -> and type that would normally follow the parameter list.

let mut numbers = Vec::new();

We declare a mutable local variable numbers, and initialize it to an empty vector. Vec is Rust’s growable vector type, analogous to C++’s std::vector, a Python list, or a JavaScript array. Even though vectors are designed to be grown and shrunk dynamically, we must still mark the variable mut for Rust to let us push numbers onto the end of it.

The type of numbers is Vec<u64>, a vector of u64 values, but as before, we don’t need to write that out. Rust will infer it for us, in part because what we push onto the vector are u64 values, but also because we pass the vector’s elements to gcd, which accepts only u64 values.

for arg in std::env::args().skip(1) {

Here we use a for loop to process our command-line arguments, setting the variable arg to each argument in turn, and evaluating the loop body.

The std::env::args function returns an iterator, a value that produces each argument on demand, and indicates when we’re done. Iterators are ubiquitous in Rust; the standard library includes other iterators that produce the elements of a vector, the lines of a file, messages received on a communications channel, and almost anything else that makes sense to loop over. Rust’s iterators are very efficient: the compiler is usually able to translate them into the same code as a handwritten loop. We’ll show how this works and give examples in Chapter 15.

Beyond their use with for loops, iterators include a broad selection of methods you can use directly. For example, the first value produced by the iterator returned by std::env::args is always the name of the program being run. We want to skip that, so we call the iterator’s skip method to produce a new iterator that omits that first value.

numbers.push(u64::from_str(&arg)
             .expect("error parsing argument"));

Here we call u64::from_str to attempt to parse our command-line argument arg as an unsigned 64-bit integer. Rather than a method we’re invoking on some u64 value we have at hand, u64::from_str is a function associated with the u64 type, akin to a static method in C++ or Java. The from_str function doesn’t return a u64 directly, but rather a Result value that indicates whether the parse succeeded or failed. A Result value is one of two variants:

A value written Ok(v), indicating that the parse succeeded and v is the value produced

A value written Err(e), indicating that the parse failed and e is an error value explaining why

Functions that perform input or output or otherwise interact with the operating system all return Result types whose Ok variants carry successful results—the count of bytes transferred, the file opened, and so on—and whose Err variants carry an error code from the system. Unlike most modern languages, Rust does not have exceptions: all errors are handled using either Result or panic, as outlined in Chapter 7.

We check the success of our parse by using Result’s expect method. If the result is some Err(e), expect prints a message that includes a description of e, and exits the program immediately. However, if the result is Ok(v), expect simply returns v itself, which we are finally able to push onto the end of our vector of numbers.

if numbers.len() == 0 {
    writeln!(std::io::stderr(), "Usage: gcd NUMBER ...").unwrap();
    std::process::exit(1);
}

There’s no greatest common divisor of an empty set of numbers, so we check that our vector has at least one element, and exit the program with an error if it doesn’t. We use the writeln! macro to write our error message to the standard error output stream, provided by std::io::stderr(). The .unwrap() call is a terse way to check that the attempt to print the error message did not itself fail; an expect call would work too, but that’s probably not worth it.

let mut d = numbers[0];
for m in &numbers[1..] {
    d = gcd(d, *m);
}

This loop uses d as its running value, updating it to stay the greatest common divisor of all the numbers we’ve processed so far. As before, we must mark d as mutable, so that we can assign to it in the loop.

The for loop has two surprising bits to it. First, we wrote for m in &numbers[1..]; what is the & operator for? Second, we wrote gcd(d, *m); what is the * in *m for? These two details are complementary to each other.

Up to this point, our code has operated only on simple values like integers that fit in fixed-size blocks of memory. But now we’re about to iterate over a vector, which could be of any size whatsoever—possibly very large. Rust is cautious when handling such values: it wants to leave the programmer in control over memory consumption, making it clear how long each value lives, while still ensuring memory is freed promptly when no longer needed.

So when we iterate, we want to tell Rust that ownership of the vector should remain with numbers; we are merely borrowing its elements for the loop. The & operator in &numbers[1..] borrows a reference to the vector’s elements from the second onward. The for loop iterates over the referenced elements, letting m borrow each element in succession. The * operator in *m dereferences m, yielding the value it refers to; this is the next u64 we want to pass to gcd. Finally, since numbers owns the vector, Rust automatically frees it when numbers goes out of scope at the end of main.

Rust’s rules for ownership and references are key to Rust’s memory management and safe concurrency; we discuss them in detail in Chapter 4 and its companion, Chapter 5. You’ll need to be comfortable with those rules to be comfortable in Rust, but for this introductory tour, all you need to know is that &x borrows a reference to x, and that *r is the value that the reference r refers to.

Continuing our walk through the program:

println!("The greatest common divisor of {:?} is {}",
         numbers, d);

Having iterated over the elements of numbers, the program prints the results to the standard output stream. The println! macro takes a template string, substitutes formatted versions of the remaining arguments for the {...} forms as they appear in the template string, and writes the result to the standard output stream.

Unlike C and C++, which require main to return zero if the program finished successfully, or a nonzero exit status if something went wrong, Rust assumes that if main returns at all, the program finished successfully. Only by explicitly calling functions like expect or std::process::exit can we cause the program to terminate with an error status code.

The cargo run command allows us to pass arguments to our program, so we can try out our command-line handling:

$ cargo run 42 56
   Compiling hello v0.1.0 (file:///home/jimb/rust/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38 secs
     Running `/home/jimb/rust/hello/target/debug/hello 42 56`
The greatest common divisor of [42, 56] is 14
$ cargo run 799459 28823 27347
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `/home/jimb/rust/hello/target/debug/hello 799459 28823 27347`
The greatest common divisor of [799459, 28823, 27347] is 41
$ cargo run 83
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `/home/jimb/rust/hello/target/debug/hello 83`
The greatest common divisor of [83] is 83
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `/home/jimb/rust/hello/target/debug/hello`
Usage: gcd NUMBER ...
$

We’ve used a few features from Rust’s standard library in this section. If you’re curious about what else is available, we strongly encourage you to try out Rust’s online documentation. It has a live search feature that makes exploration easy, and even includes links to the source code. The rustup command automatically installs a copy on your computer when you install Rust itself. You can view the standard library documentation in your browser with the command:

$ rustup doc --std

You can also view it on the web at https://doc.rust-lang.org/.

A Simple Web Server

One of Rust’s strengths is the freely available collection of library packages published on the website crates.io. The cargo command makes it easy for our own code to use a crates.io package: it will download the right version of the package, build it, and update it as requested. A Rust package, whether a library or an executable, is called a crate; Cargo and crates.io both derive their names from this term.

To show how this works, we’ll put together a simple web server using the iron web framework, the hyper HTTP server, and various other crates on which they depend. As shown in Figure 2-1, our website will prompt the user for two numbers, and compute their greatest common divisor.

Figure 2-1. Web page offering to compute GCD

First, we’ll have Cargo create a new package for us, named iron-gcd:

$ cargo new --bin iron-gcd
     Created binary (application) `iron-gcd` project
$ cd iron-gcd
$

Then, we’ll edit our new project’s Cargo.toml file to list the packages we want to use; its contents should be as follows:

[package]
name = "iron-gcd"
version = "0.1.0"
authors = ["You <you@example.com>"]

[dependencies]
iron = "0.5.1"
mime = "0.2.3"
router = "0.5.1"
urlencoded = "0.5.0"

Each line in the [dependencies] section of Cargo.toml gives the name of a crate on crates.io, and the version of that crate we would like to use. There may well be versions of these crates on crates.io newer than those shown here, but by naming the specific versions we tested this code against, we can ensure the code will continue to compile even as new versions of the packages are published. We’ll discuss version management in more detail in Chapter 8.

Note that we need only name those packages we’ll use directly; cargo takes care of bringing in whatever other packages those need in turn.

For our first iteration, we’ll keep the web server simple: it will serve only the page that prompts the user for numbers to compute with. In iron-gcd/src/main.rs, we’ll place the following text:

extern crate iron;
#[macro_use] extern crate mime;

use iron::prelude::*;
use iron::status;

fn main() {
    println!("Serving on http://localhost:3000...");
    Iron::new(get_form).http("localhost:3000").unwrap();
}

fn get_form(_request: &mut Request) -> IronResult<Response> {
    let mut response = Response::new();

    response.set_mut(status::Ok);
    response.set_mut(mime!(Text/Html; Charset=Utf8));
    response.set_mut(r#"
        <title>GCD Calculator</title>
        <form action="/gcd" method="post">
          <input type="text" name="n"/>
          <input type="text" name="n"/>
          <button type="submit">Compute GCD</button>
        </form>
    "#);

    Ok(response)
}

We start with two extern crate directives, which make the iron and mime crates that we cited in our Cargo.toml file available to our program. The #[macro_use] attribute before the extern crate mime item alerts Rust that we plan to use macros exported by this crate.

Next, we have use declarations to bring in some of those crates’ public features. The declaration use iron::prelude::* makes all the public names of the iron::prelude module directly visible in our own code. Generally, it’s preferable to spell out the name you wish to use, as we did for iron::status; but by convention, when a module is named prelude, that means that its exports are intended to provide the sort of general facilities that any user of the crate will probably need. So in this case, a wildcard use directive makes a bit more sense.

Our main function is simple: it prints a message reminding us how to connect to our server, calls Iron::new to create a server, and then sets it listening on TCP port 3000 on the local machine. We pass the get_form function to Iron::new, indicating that the server should use that function to handle all requests; we’ll refine this shortly.

The get_form function itself takes a mutable reference, written &mut, to a Request value representing the HTTP request we’ve been called to handle. While this particular handler function never uses its _request parameter, we’ll see one later that does. For the time being, giving the parameter a name beginning with _ tells Rust that we expect the variable to be unused, so it shouldn’t warn us about it.

In the body of the function, we build a Response value. The set_mut method uses its argument’s type to decide which part of the response to set, so each call to set_mut is actually setting a different part of response: passing status::Ok sets the HTTP status; passing the media type of the content (using the handy mime! macro that we imported from the mime crate) sets the Content-Type header; and passing a string sets the response body.

Since the response text contains a lot of double quotes, we write it using the Rust “raw string” syntax: the letter r, zero or more hash marks (that is, the # character), a double quote, and then the contents of the string, terminated by another double quote followed by the same number of hash marks. Any character may occur within a raw string without being escaped, including double quotes; in fact, no escape sequences like \" are recognized. We can always ensure the string ends where we intend by using more hash marks around the quotes than ever appear in the text.

Our function’s return type, IronResult<Response>, is another variant of the Result type we encountered earlier: this is either Ok(r) for some successful Response value r, or Err(e) for some error value e. We construct our return value Ok(response) at the bottom of the function body, using the “last expression” syntax to implicitly specify the function’s return value.

Having written main.rs, we can use the cargo run command to do everything needed to set it running: fetching the needed crates, compiling them, building our own program, linking everything together, and starting it up:

$ cargo run
    Updating registry `https://github.com/rust-lang/crates.io-index`
 Downloading iron v0.5.1
 Downloading urlencoded v0.5.0
 Downloading router v0.5.1
 Downloading hyper v0.10.8
 Downloading lazy_static v0.2.8
 Downloading bodyparser v0.5.0
...
   Compiling conduit-mime-types v0.7.3
   Compiling iron v0.5.1
   Compiling router v0.5.1
   Compiling persistent v0.3.0
   Compiling bodyparser v0.5.0
   Compiling urlencoded v0.5.0
   Compiling iron-gcd v0.1.0 (file:///home/jimb/rust/iron-gcd)
     Running `target/debug/iron-gcd`
Serving on http://localhost:3000...

At this point, we can visit the given URL in our browser and see the page shown earlier in Figure 2-1.

Unfortunately, clicking Compute GCD doesn’t do anything, other than navigate our browser to the URL http://localhost:3000/gcd, which then shows the same page; in fact, every URL on our server does this. Let’s fix that next, using the Router type to associate different handlers with different paths.

First, let’s arrange to be able to use Router without qualification, by adding the following declarations to iron-gcd/src/main.rs:

extern crate router;
use router::Router;

Rust programmers typically gather all their extern crate and use declarations together toward the top of the file, but this isn’t strictly necessary: Rust allows declarations to occur in any order, as long as they appear at the appropriate level of nesting. (Macro definitions and extern crate items with #[macro_use] attributes are exceptions to this rule: they must appear before they are used.)

We can then modify our main function to read as follows:

fn main() {
    let mut router = Router::new();

    router.get("/", get_form, "root");
    router.post("/gcd", post_gcd, "gcd");

    println!("Serving on http://localhost:3000...");
    Iron::new(router).http("localhost:3000").unwrap();
}

We create a Router, establish handler functions for two specific paths, and then pass this Router as the request handler to Iron::new, yielding a web server that consults the URL path to decide which handler function to call.

Now we are ready to write our post_gcd function:

extern crate urlencoded;

use std::str::FromStr;
use urlencoded::UrlEncodedBody;

fn post_gcd(request: &mut Request) -> IronResult<Response> {
    let mut response = Response::new();

    let form_data = match request.get_ref::<UrlEncodedBody>() {
        Err(e) => {
            response.set_mut(status::BadRequest);
            response.set_mut(format!("Error parsing form data: {:?}\n", e));
            return Ok(response);
        }
        Ok(map) => map
    };

    let unparsed_numbers = match form_data.get("n") {
        None => {
            response.set_mut(status::BadRequest);
            response.set_mut(format!("form data has no 'n' parameter\n"));
            return Ok(response);
        }
        Some(nums) => nums
    };

    let mut numbers = Vec::new();
    for unparsed in unparsed_numbers {
        match u64::from_str(&unparsed) {
            Err(_) => {
                response.set_mut(status::BadRequest);
                response.set_mut(
                    format!("Value for 'n' parameter not a number: {:?}\n",
                            unparsed));
                return Ok(response);
            }
            Ok(n) => { numbers.push(n); }
        }
    }

    let mut d = numbers[0];
    for m in &numbers[1..] {
        d = gcd(d, *m);
    }

    response.set_mut(status::Ok);
    response.set_mut(mime!(Text/Html; Charset=Utf8));
    response.set_mut(
        format!("The greatest common divisor of the numbers {:?} is <b>{}</b>\n",
                numbers, d));
    Ok(response)
}

The bulk of this function is a series of match expressions, which will be unfamiliar to C, C++, Java, and JavaScript programmers, but a welcome sight to those who work with Haskell and OCaml. We’ve mentioned that a Result is either a value Ok(s) for some success value s, or Err(e) for some error value e. Given some Result res, we can check which variant it is and access whichever value it holds with a match expression of the form:

match res {
    Ok(success) => { ... },
    Err(error)  => { ... }
}

This is a conditional, like an if statement or a switch statement in C: if res is Ok(v), then it runs the first branch, with the variable success set to v. Similarly, if res is Err(e), it runs the second branch with error set to e. The success and error variables are each local to their branch. The value of the entire match expression is the value of the branch that runs.

The beauty of a match expression is that the program can only access the value of a Result by first checking which variant it is; one can never misinterpret a failure value as a successful completion. Whereas in C and C++ it’s a common error to forget to check for an error code or a null pointer, in Rust, these mistakes are caught at compile time. This simple measure is a significant advance in usability.

Rust allows you to define your own types like Result with value-carrying variants, and use match expressions to analyze them. Rust calls these types enums; you may know them from other languages as algebraic data types. We describe enumerations in detail in Chapter 10.

Now that you can read match expressions, the structure of post_gcd should be clear:

It calls request.get_ref::<UrlEncodedBody>() to parse the request’s body as a table mapping query parameter names to arrays of values; if this parse fails, it reports the error back to the client. The ::<UrlEncodedBody> part of the method call is a type parameter indicating which part of the Request get_ref should retrieve. In this case, the UrlEncodedBody type refers to the body, parsed as a URL-encoded query string. We’ll talk more about type parameters in the next section.

Within that table, it finds the value of the parameter named "n", which is where the HTML form places the numbers entered into the web page. This value will be not a single string but a vector of strings, as query parameter names can be repeated.

It walks the vector of strings, parsing each one as an unsigned 64-bit number, and returning an appropriate failure page if any of the strings fail to parse.

Finally, it computes the numbers’ greatest common divisor as before, and constructs a response describing the results. The format! macro uses the same kind of string template as the writeln! and println! macros, but returns a string value, rather than writing the text to a stream.

The last remaining piece is the gcd function we wrote earlier. With that in place, you can interrupt any servers you might have left running, and rebuild and restart the program:

$ cargo run
   Compiling iron-gcd v0.1.0 (file:///home/jimb/rust/iron-gcd)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/iron-gcd`
Serving on http://localhost:3000...

This time, by visiting http://localhost:3000, entering some numbers, and clicking the Compute GCD button, you should actually see some results (Figure 2-2).

Figure 2-2. Web page showing results of computing GCD

Concurrency

One of Rust’s great strengths is its support for concurrent programming. The same rules that ensure Rust programs are free of memory errors also ensure threads can share memory only in ways that avoid data races. For example:

If you use a mutex to coordinate threads making changes to a shared data structure, Rust ensures that you can’t access the data except when you’re holding the lock, and releases the lock automatically when you’re done. In C and C++, the relationship between a mutex and the data it protects is left to the comments.

If you want to share read-only data among several threads, Rust ensures that you cannot modify the data accidentally. In C and C++, the type system can help with this, but it’s easy to get it wrong.

If you transfer ownership of a data structure from one thread to another, Rust makes sure you have indeed relinquished all access to it. In C and C++, it’s up to you to check that nothing on the sending thread will ever touch the data again. If you don’t get it right, the effects can depend on what happens to be in the processor’s cache and how many writes to memory you’ve done recently. Not that we’re bitter.

In this section, we’ll walk you through the process of writing your second multi-threaded program.

Although you probably weren’t aware of it, you’ve already written your first: the Iron web framework you used to implement the Greatest Common Divisor server uses a pool of threads to run request handler functions. If the server receives simultaneous requests, it may run the get_form and post_gcd functions in several threads at once. That may come as a bit of a shock, since we certainly didn’t have concurrency in mind when we wrote those functions. But Rust guarantees this is safe to do, no matter how elaborate your server gets: if your program compiles, it is free of data races. All Rust functions are thread-safe.

This section’s program plots the Mandelbrot set, a fractal produced by iterating a simple function on complex numbers. Plotting the Mandelbrot set is often called an embarrassingly parallel algorithm, because the pattern of communication between the threads is so simple; we’ll cover more complex patterns in Chapter 19, but this task demonstrates some of the essentials.

To get started, we’ll create a fresh Rust project:

$ cargo new --bin mandelbrot
     Created binary (application) `mandelbrot` project

All the code will go in mandelbrot/src/main.rs, and we’ll add some dependencies to mandelbrot/Cargo.toml.

Before we get into the concurrent Mandelbrot implementation, we need to describe the computation we’re going to perform.

What the Mandelbrot Set Actually Is

When reading code, it’s helpful to have a concrete idea of what it’s trying to do, so let’s take a short excursion into some pure mathematics. We’ll start with a simple case, and then add complicating details until we arrive at the calculation at the heart of the Mandelbrot set.

Here’s an infinite loop, written using Rust’s dedicated syntax for that, a loop statement:

fn square_loop(mut x: f64) {
    loop {
        x = x * x;
    }
}

In real life, Rust can see that x is never used for anything, and so might not bother computing its value. But for the time being, assume the code runs as written. What happens to the value of x? Squaring any number smaller than 1 makes it smaller, so it approaches zero; squaring 1 yields 1; squaring a number larger than 1 makes it larger, so it approaches infinity; and squaring a negative number makes it positive, after which it behaves as one of the prior cases (Figure 2-3).

Figure 2-3. Effects of repeatedly squaring a number

So depending on the value you pass to square_loop, x either approaches zero, stays at 1, or approaches infinity.

Now consider a slightly different loop:

fn square_add_loop(c: f64) {
    let mut x = 0.;
    loop {
        x = x * x + c;
    }
}

This time, x starts at zero, and we tweak its progress in each iteration by adding in c after squaring it. This makes it harder to see how x fares, but some experimentation shows that if c is greater than 0.25, or less than –2.0, then x eventually becomes infinitely large; otherwise, it stays somewhere in the neighborhood of zero.

The next wrinkle: instead of using f64 values, consider the same loop using complex numbers. The num crate on crates.io provides a complex number type we can use, so we must add a line for num to the [dependencies] section in our program’s Cargo.toml file. Here’s the entire file, up to this point (we’ll be adding more later):

[package]
name = "mandelbrot"
version = "0.1.0"
authors = ["You <you@example.com>"]

[dependencies]
num = "0.1.27"

Now we can write the penultimate version of our loop:

extern crate num;
use num::Complex;

#[allow(dead_code)]
fn complex_square_add_loop(c: Complex<f64>) {
    let mut z = Complex { re: 0.0, im: 0.0 };
    loop {
        z = z * z + c;
    }
}

It’s traditional to use z for complex numbers, so we’ve renamed our looping variable. The expression Complex { re: 0.0, im: 0.0 } is the way we write complex zero using the num crate’s Complex type. Complex is a Rust structure type (or struct), defined like this:

struct Complex<T> {
    /// Real portion of the complex number
    re: T,

    /// Imaginary portion of the complex number
    im: T
}

The preceding code defines a struct named Complex, with two fields, re and im. Complex is a generic structure: you can read the <T> after the type name as “for any type T”. For example, Complex<f64> is a complex number whose re and im fields are f64 values, Complex<f32> would use 32-bit floats, and so on. Given this definition, an expression like Complex { re: R, im: I } produces a Complex value with its re field initialized to R, and its im field initialized to I.

The num crate arranges for *, +, and other arithmetic operators to work on Complex values, so the rest of the function works just like the prior version, except that it operates on points on the complex plane, not just points along the real number line. We’ll explain how you can make Rust’s operators work with your own types in Chapter 12.

Finally, we’ve reached the destination of our pure math excursion. The Mandelbrot set is defined as the set of complex numbers c for which z does not fly out to infinity. Our original simple squaring loop was predictable enough: any number greater than 1 or less than –1 flies away. Throwing a + c into each iteration makes the behavior a little harder to anticipate: as we said earlier, values of c greater than 0.25 or less than –2 cause z to fly away. But expanding the game to complex numbers produces truly bizarre and beautiful patterns, which are what we want to plot.

Since a complex number c has both real and imaginary components c.re and c.im, we’ll treat these as the x and y coordinates of a point on the Cartesian plane, and color the point black if c is in the Mandelbrot set, or a lighter color otherwise. So for each pixel in our image, we must run the preceding loop on the corresponding point on the complex plane, see whether it escapes to infinity or orbits around the origin forever, and color it accordingly.

The infinite loop takes a while to run, but there are two tricks for the impatient. First, if we give up on running the loop forever and just try some limited number of iterations, it turns out that we still get a decent approximation of the set. How many iterations we need depends on how precisely we want to plot the boundary. Second, it’s been shown that, if z ever once leaves the circle of radius two centered at the origin, it will definitely fly infinitely far away from the origin eventually.

So here’s the final version of our loop, and the heart of our program:

extern crate num;
use num::Complex;

/// Try to determine if `c` is in the Mandelbrot set, using at most `limit`
/// iterations to decide.
///
/// If `c` is not a member, return `Some(i)`, where `i` is the number of
/// iterations it took for `c` to leave the circle of radius two centered on the
/// origin. If `c` seems to be a member (more precisely, if we reached the
/// iteration limit without being able to prove that `c` is not a member),
/// return `None`.
fn escape_time(c: Complex<f64>, limit: u32) -> Option<u32> {
    let mut z = Complex { re: 0.0, im: 0.0 };
    for i in 0..limit {
        z = z*z + c;
        if z.norm_sqr() > 4.0 {
            return Some(i);
        }
    }

    None
}

This function takes the complex number c that we want to test for membership in the Mandelbrot set, and a limit on the number of iterations to try before giving up and declaring c to probably be a member.

The function’s return value is an Option<u32>. Rust’s standard library defines the Option type as follows:

enum Option<T> {
    None,
    Some(T),
}

Option is an enumerated type, often called an enum, because its definition enumerates several variants that a value of this type could be: for any type T, a value of type Option<T> is either Some(v), where v is a value of type T; or None, indicating no T value is available. Like the Complex type we discussed earlier, Option is a generic type: you can use Option<T> to represent an optional value of any type T you like.

In our case, escape_time returns an Option<u32> to indicate whether c is in the Mandelbrot set—and if it’s not, how long we had to iterate to find that out. If c is not in the set, escape_time returns Some(i), where i is the number of the iteration at which z left the circle of radius two. Otherwise, c is apparently in the set, and escape_time returns None.

for i in 0..limit {

The earlier examples showed for loops iterating over command-line arguments and vector elements; this for loop simply iterates over the range of integers starting with 0 and up to (but not including) limit.

The z.norm_sqr() method call returns the square of z’s distance from the origin. To decide whether z has left the circle of radius two, instead of computing a square root, we just compare the squared distance with 4.0, which is faster.

You may have noticed that we use /// to mark the comment lines above the function definition; the comments above the members of the Complex structure start with /// as well. These are documentation comments; the rustdoc utility knows how to parse them, together with the code they describe, and produce online documentation. The documentation for Rust’s standard library is written in this form. We describe documentation comments in detail in Chapter 8.

The rest of the program is concerned with deciding which portion of the set to plot at what resolution, and distributing the work across several threads to speed up the calculation.

Parsing Pair Command-Line Arguments

The program needs to take several command-line arguments controlling the resolution of the image we’ll write, and the portion of the Mandelbrot set the image shows. Since these command-line arguments all follow a common form, here’s a function to parse them:

use std::str::FromStr;

/// Parse the string `s` as a coordinate pair, like `"400x600"` or `"1.0,0.5"`.
///
/// Specifically, `s` should have the form <left><sep><right>, where <sep> is
/// the character given by the `separator` argument, and <left> and <right> are both
/// strings that can be parsed by `T::from_str`.
///
/// If `s` has the proper form, return `Some<(x, y)>`. If it doesn't parse
/// correctly, return `None`.
fn parse_pair<T: FromStr>(s: &str, separator: char) -> Option<(T, T)> {
    match s.find(separator) {
        None => None,
        Some(index) => {
            match (T::from_str(&s[..index]), T::from_str(&s[index + 1..])) {
                (Ok(l), Ok(r)) => Some((l, r)),
                _ => None
            }
        }
    }
}

#[test]
fn test_parse_pair() {
    assert_eq!(parse_pair::<i32>("",        ','), None);
    assert_eq!(parse_pair::<i32>("10,",     ','), None);
    assert_eq!(parse_pair::<i32>(",10",     ','), None);
    assert_eq!(parse_pair::<i32>("10,20",   ','), Some((10, 20)));
    assert_eq!(parse_pair::<i32>("10,20xy", ','), None);
    assert_eq!(parse_pair::<f64>("0.5x",    'x'), None);
    assert_eq!(parse_pair::<f64>("0.5x1.5", 'x'), Some((0.5, 1.5)));
}

The definition of parse_pair is a generic function:

fn parse_pair<T: FromStr>(s: &str, separator: char) -> Option<(T, T)> {

You can read the clause <T: FromStr> aloud as, “For any type T that implements the FromStr trait...”. This effectively lets us define an entire family of functions at once: parse_pair::<i32> is a function that parses pairs of i32 values; parse_pair::<f64> parses pairs of floating-point values; and so on. This is very much like a function template in C++. A Rust programmer would call T a type parameter of parse_pair. When you use a generic function, Rust will often be able to infer type parameters for you, and you won’t need to write them out as we did in the test code.

Our return type is Option<(T, T)>: either None, or a value Some((v1, v2)), where (v1, v2) is a tuple of two values, both of type T. The parse_pair function doesn’t use an explicit return statement, so its return value is the value of the last (and the only) expression in its body:

match s.find(separator) {
    None => None,
    Some(index) => {
        ...
    }
}

The String type’s find method searches the string for a character that matches separator. If find returns None, meaning that the separator character doesn’t occur in the string, the entire match expression evaluates to None, indicating that the parse failed. Otherwise, we take index to be the separator’s position in the string.

match (T::from_str(&s[..index]), T::from_str(&s[index + 1..])) {
    (Ok(l), Ok(r)) => Some((l, r)),
    _ => None
}

This begins to show off the power of the match expression. The argument to the match is this tuple expression:

(T::from_str(&s[..index]), T::from_str(&s[index + 1..]))

The expressions &s[..index] and &s[index + 1..] are slices of the string, preceding and following the separator. The type parameter T’s associated from_str function takes each of these and tries to parse them as a value of type T, producing a tuple of results. This is what we match against:

(Ok(l), Ok(r)) => Some((l, r)),

This pattern matches only if both elements of the tuple are Ok variants of the Result type, indicating that both parses succeeded. If so, Some((l, r)) is the value of the match expression, and hence the return value of the function.

_ => None

The wildcard pattern _ matches anything, and ignores its value. If we reach this point, then parse_pair has failed, so we evaluate to None, again providing the return value of the function.

Now that we have parse_pair, it’s easy to write a function to parse a pair of floating-point coordinates and return them as a Complex<f64> value:

/// Parse a pair of floating-point numbers separated by a comma as a complex
/// number.
fn parse_complex(s: &str) -> Option<Complex<f64>> {
    match parse_pair(s, ',') {
        Some((re, im)) => Some(Complex { re, im }),
        None => None
    }
}

#[test]
fn test_parse_complex() {
    assert_eq!(parse_complex("1.25,-0.0625"),
               Some(Complex { re: 1.25, im: -0.0625 }));
    assert_eq!(parse_complex(",-0.0625"), None);
}

The parse_complex function calls parse_pair, builds a Complex value if the coordinates were parsed successfully, and passes failures along to its caller.

If you were reading closely, you may have noticed that we used a shorthand notation to build the Complex value. It’s common to initialize a struct’s fields with variables of the same name, so rather than forcing you to write Complex { re: re, im: im }, Rust lets you simply write Complex { re, im }. This is modeled on similar notations in JavaScript and Haskell.

Mapping from Pixels to Complex Numbers

The program needs to work in two related coordinate spaces: each pixel in the output image corresponds to a point on the complex plane. The relationship between these two spaces depends on which portion of the Mandelbrot set we’re going to plot, and the resolution of the image requested, as determined by command-line arguments. The following function converts from image space to complex number space:

/// Given the row and column of a pixel in the output image, return the
/// corresponding point on the complex plane.
///
/// `bounds` is a pair giving the width and height of the image in pixels.
/// `pixel` is a (column, row) pair indicating a particular pixel in that image.
/// The `upper_left` and `lower_right` parameters are points on the complex
/// plane designating the area our image covers.
fn pixel_to_point(bounds: (usize, usize),
                  pixel: (usize, usize),
                  upper_left: Complex<f64>,
                  lower_right: Complex<f64>)
    -> Complex<f64>
{
    let (width, height) = (lower_right.re - upper_left.re,
                           upper_left.im - lower_right.im);
    Complex {
        re: upper_left.re + pixel.0 as f64 * width  / bounds.0 as f64,
        im: upper_left.im - pixel.1 as f64 * height / bounds.1 as f64
        // Why subtraction here? pixel.1 increases as we go down,
        // but the imaginary component increases as we go up.
    }
}

#[test]
fn test_pixel_to_point() {
    assert_eq!(pixel_to_point((100, 100), (25, 75),
                              Complex { re: -1.0, im:  1.0 },
                              Complex { re:  1.0, im: -1.0 }),
               Complex { re: -0.5, im: -0.5 });
}

Figure 2-4 illustrates the calculation pixel_to_point performs.

Figure 2-4. The relationship between the complex plane and the image’s pixels

The code of pixel_to_point is simply calculation, so we won’t explain it in detail. However, there are a few things to point out. Expressions with this form refer to tuple elements:

pixel.0

This refers to the first element of the tuple pixel.

pixel.0 as f64

This is Rust’s syntax for a type conversion: this converts pixel.0 to an f64 value. Unlike C and C++, Rust generally refuses to convert between numeric types implicitly; you must write out the conversions you need. This can be tedious, but being explicit about which conversions occur and when is surprisingly helpful. Implicit integer conversions seem innocent enough, but historically they have been a frequent source of bugs and security holes in real-world C and C++ code.

Plotting the Set

To plot the Mandelbrot set, for every pixel in the image, we simply apply escape_time to the corresponding point on the complex plane, and color the pixel depending on the result:

/// Render a rectangle of the Mandelbrot set into a buffer of pixels.
///
/// The `bounds` argument gives the width and height of the buffer `pixels`,
/// which holds one grayscale pixel per byte. The `upper_left` and `lower_right`
/// arguments specify points on the complex plane corresponding to the upper-
/// left and lower-right corners of the pixel buffer.
fn render(pixels: &mut [u8],
          bounds: (usize, usize),
          upper_left: Complex<f64>,
          lower_right: Complex<f64>)
{
    assert!(pixels.len() == bounds.0 * bounds.1);

    for row in 0 .. bounds.1 {
        for column in 0 .. bounds.0 {
            let point = pixel_to_point(bounds, (column, row),
                                       upper_left, lower_right);
            pixels[row * bounds.0 + column] =
                match escape_time(point, 255) {
                    None => 0,
                    Some(count) => 255 - count as u8
                };
        }
    }
}

This should all look pretty familiar at this point.

pixels[row * bounds.0 + column] =
    match escape_time(point, 255) {
        None => 0,
        Some(count) => 255 - count as u8
    };

If escape_time says that point belongs to the set, render colors the corresponding pixel black (0). Otherwise, render assigns darker colors to the numbers that took longer to escape the circle.

Writing Image Files

The image crate provides functions for reading and writing a wide variety of image formats, along with some basic image manipulation functions. In particular, it includes an encoder for the PNG image file format, which this program uses to save the final results of the calculation. In order to use image, add the following line to the [dependencies] section of Cargo.toml:

image = "0.13.0"

With that in place, we can write:

extern crate image;

use image::ColorType;
use image::png::PNGEncoder;
use std::fs::File;

/// Write the buffer `pixels`, whose dimensions are given by `bounds`, to the
/// file named `filename`.
fn write_image(filename: &str, pixels: &[u8], bounds: (usize, usize))
    -> Result<(), std::io::Error>
{
    let output = File::create(filename)?;

    let encoder = PNGEncoder::new(output);
    encoder.encode(&pixels,
                   bounds.0 as u32, bounds.1 as u32,
                   ColorType::Gray(8))?;

    Ok(())
}

The operation of this function is pretty straightforward: it opens a file and tries to write the image to it. We pass the encoder the actual pixel data from pixels, and its width and height from bounds, and then a final argument that says how to interpret the bytes in pixels: the value ColorType::Gray(8) indicates that each byte is an eight-bit grayscale value.

That’s all straightforward. What’s interesting about this function is how it copes when something goes wrong. If we encounter an error, we need to report that back to our caller. As we’ve mentioned before, fallible functions in Rust should return a Result value, which is either Ok(s) on success, where s is the successful value, or Err(e) on failure, where e is an error code. So what are write_image’s success and error types?

When all goes well, our write_image function has no useful value to return; it wrote everything interesting to the file. So its success type is the unit type (), so called because it has only one value, also written (). The unit type is akin to void in C and C++.

When an error occurs, it’s because either File::create wasn’t able to create the file, or encoder.encode wasn’t able to write the image to it; the I/O operation returned an error code. The return type of File::create is Result<std::fs::File, std::io::Error>, while that of encoder.encode is Result<(), std::io::Error>, so both share the same error type, std::io::Error. It makes sense for our write_image function to do the same.

Consider the call to File::create. If that returns Ok(f) for a successfully opened File value f, then write_image can proceed to write the image data to f. But if File::create returns Err(e) for an error code e, write_image should immediately return Err(e) as its own return value. The call to encoder.encode must be handled similarly: failure should result in an immediate return, passing along the error code.

The ? operator exists to make these checks convenient. Instead of spelling everything out, and writing:

let output = match File::create(filename) {
    Ok(f) => { f }
    Err(e) => { return Err(e); }
};

you can use the equivalent and much more legible:

let output = File::create(filename)?;

Note

It’s a common beginner’s mistake to attempt to use ? in the main function. However, since main has no return value, this won’t work; you should use Result’s expect method instead. The ? operator is only useful within functions that themselves return Result.

There’s another shorthand we could use here. Because return types of the form Result<T, std::io::Error> for some type T are so common—this is often the right type for a function that does I/O—the Rust standard library defines a shorthand for it. In the std::io module, we have the definitions:

// The std::io::Error type.
struct Error { ... };

// The std::io::Result type, equivalent to the usual `Result`, but
// specialized to use std::io::Error as the error type.
type Result<T> = std::result::Result<T, Error>

If we bring this definition into scope with a use std::io::Result declaration, we can write write_image’s return type more tersely as Result<()>. This is the form you will often see when reading the documentation for functions in std::io, std::fs, and elsewhere.

A Concurrent Mandelbrot Program

Finally, all the pieces are in place, and we can show you the main function, where we can put concurrency to work for us. First, a nonconcurrent version for simplicity:

use std::io::Write;

fn main() {
    let args: Vec<String> = std::env::args().collect();

    if args.len() != 5 {
        writeln!(std::io::stderr(),
                 "Usage: mandelbrot FILE PIXELS UPPERLEFT LOWERRIGHT")
            .unwrap();
        writeln!(std::io::stderr(),
                 "Example: {} mandel.png 1000x750 -1.20,0.35 -1,0.20",
                 args[0])
            .unwrap();
        std::process::exit(1);
    }

    let bounds = parse_pair(&args[2], 'x')
        .expect("error parsing image dimensions");
    let upper_left = parse_complex(&args[3])
        .expect("error parsing upper left corner point");
    let lower_right = parse_complex(&args[4])
        .expect("error parsing lower right corner point");

    let mut pixels = vec![0; bounds.0 * bounds.1];

    render(&mut pixels, bounds, upper_left, lower_right);

    write_image(&args[1], &pixels, bounds)
        .expect("error writing PNG file");
}

After collecting the command-line arguments into a vector of Strings, we parse each one and then begin calculations.

let mut pixels = vec![0; bounds.0 * bounds.1];

A macro call vec![v; n] creates a vector n elements long whose elements are initialized to v, so the preceding code creates a vector of zeros whose length is bounds.0 * bounds.1, where bounds is the image resolution parsed from the command line. We’ll use this vector as a rectangular array of one-byte grayscale pixel values, as shown in Figure 2-5.

Figure 2-5. Using a vector as a rectangular array of pixels

The next line of interest is this:

render(&mut pixels, bounds, upper_left, lower_right);

This calls the render function to actually compute the image. The expression &mut pixels borrows a mutable reference to our pixel buffer, allowing render to fill it with computed grayscale values, even while pixels remains the vector’s owner. The remaining arguments pass the image’s dimensions, and the rectangle of the complex plane we’ve chosen to plot.

write_image(&args[1], &pixels, bounds)
    .expect("error writing PNG file");

Finally, we write the pixel buffer out to disk as a PNG file. In this case, we pass a shared (nonmutable) reference to the buffer, since write_image should have no need to modify the buffer’s contents.

The natural way to distribute this calculation across multiple processors is to divide the image into sections, one per processor, and let each processor color the pixels assigned to it. For simplicity, we’ll break it into horizontal bands, as shown in Figure 2-6. When all processors have finished, we can write out the pixels to disk.

Figure 2-6. Dividing the pixel buffer into bands for parallel rendering

The crossbeam crate provides a number of valuable concurrency facilities, including a scoped thread facility that does exactly what we need here. To use it, we must add the following line to our Cargo.toml file:

crossbeam = "0.2.8"

Then, we must add the following line to the top of our main.rs file:

extern crate crossbeam;

Then we need to take out the single line calling render, and replace it with the following:

let threads = 8;
let rows_per_band = bounds.1 / threads + 1;

{
    let bands: Vec<&mut [u8]> =
        pixels.chunks_mut(rows_per_band * bounds.0).collect();
    crossbeam::scope(|spawner| {
        for (i, band) in bands.into_iter().enumerate() {
            let top = rows_per_band * i;
            let height = band.len() / bounds.0;
            let band_bounds = (bounds.0, height);
            let band_upper_left =
                pixel_to_point(bounds, (0, top), upper_left, lower_right);
            let band_lower_right =
                pixel_to_point(bounds, (bounds.0, top + height),
                               upper_left, lower_right);

            spawner.spawn(move || {
                render(band, band_bounds, band_upper_left, band_lower_right);
            });
        }
    });
}

Breaking this down in the usual way:

let threads = 8;
let rows_per_band = bounds.1 / threads + 1;

Here we decide to use eight threads.¹ Then we compute how many rows of pixels each band should have. Since the height of a band is rows_per_band and the overall width of the image is bounds.0, the area of a band, in pixels, is rows_per_band * bounds.0. We round the row count upward, to make sure the bands cover the entire image even if the height isn’t a multiple of threads.

let bands: Vec<&mut [u8]> =
    pixels.chunks_mut(rows_per_band * bounds.0).collect();

Here we divide the pixel buffer into bands. The buffer’s chunks_mut method returns an iterator producing mutable, nonoverlapping slices of the buffer, each of which encloses rows_per_band * bounds.0 pixels—in other words, rows_per_band complete rows of pixels. The last slice that chunks_mut produces may contain fewer rows, but each row will contain the same number of pixels. Finally, the iterator’s collect method builds a vector holding these mutable, nonoverlapping slices.

Now we can put the crossbeam library to work:

crossbeam::scope(|spawner| { ... });

The argument |spawner| { ... } is a Rust closure expression. A closure is a value that can be called as if it were a function. Here, |spawner| is the argument list, and { ... } is the body of the function. Note that, unlike functions declared with fn, we don’t need to declare the types of a closure’s arguments; Rust will infer them, along with its return type.

In this case, crossbeam::scope calls the closure, passing as the spawner argument a value the closure can use to create new threads. The crossbeam::scope function waits for all such threads to finish execution before returning itself. This behavior allows Rust to be sure that such threads will not access their portions of pixels after it has gone out of scope, and allows us to be sure that when crossbeam::scope returns, the computation of the image is complete.

for (i, band) in bands.into_iter().enumerate() {

Here we iterate over the pixel buffer’s bands. The into_iter() iterator gives each iteration of the loop body exclusive ownership of one band, ensuring that only one thread can write to it at a time. We explain how this works in detail in Chapter 5. Then, the enumerate adapter produces tuples pairing each vector element with its index.

let top = rows_per_band * i;
let height = band.len() / bounds.0;
let band_bounds = (bounds.0, height);
let band_upper_left =
    pixel_to_point(bounds, (0, top), upper_left, lower_right);
let band_lower_right =
    pixel_to_point(bounds, (bounds.0, top + height),
                   upper_left, lower_right);

Given the index and the actual size of the band (recall that the last one might be shorter than the others), we can produce a bounding box of the sort render requires, but one that refers only to this band of the buffer, not the entire image. Similarly, we repurpose the renderer’s pixel_to_point function to find where the band’s upper-left and lower-right corners fall on the complex plane.

spawner.spawn(move || {
    render(band, band_bounds, band_upper_left, band_lower_right);
});

Finally, we create a thread, running the closure move || { ... }. This syntax is a bit strange to read: it denotes a closure of no arguments whose body is the { ... } form. The move keyword at the front indicates that this closure takes ownership of the variables it uses; in particular, only the closure may use the mutable slice band.

As we mentioned earlier, the crossbeam::scope call ensures that all threads have completed before it returns, meaning that it is safe to save the image to a file, which is our next action.

Running the Mandelbrot Plotter

We’ve used several external crates in this program: num for complex number arithmetic; image for writing PNG files; and crossbeam for the scoped thread creation primitives. Here’s the final Cargo.toml file including all those dependencies:

[package]
name = "mandelbrot"
version = "0.1.0"
authors = ["You <you@example.com>"]

[dependencies]
crossbeam = "0.2.8"
image = "0.13.0"
num = "0.1.27"

With that in place, we can build and run the program:

$ cargo build --release
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling bitflags v0.3.3
   ...
   Compiling png v0.4.3
   Compiling image v0.13.0
   Compiling mandelbrot v0.1.0 (file:///home/jimb/rust/mandelbrot)
    Finished release [optimized] target(s) in 42.64 secs
$ time target/release/mandelbrot mandel.png 4000x3000 -1.20,0.35 -1,0.20
real    0m1.750s
user    0m6.205s
sys     0m0.026s
$

Here, we’ve used the Unix time program to see how long the program took to run; note that even though we spent more than six seconds of processor time computing the image, the elapsed real time was less than two seconds. You can verify that a substantial portion of that real time is spent writing the image file by commenting out the code that does so; on the laptop where this code was tested, the concurrent version reduces the Mandelbrot calculation time proper by a factor of almost four. We’ll show how to substantially improve on this in Chapter 19.

This command should create a file called mandel.png, which you can view with your system’s image viewing program or in a web browser. If all has gone well, it should look like Figure 2-7.

Figure 2-7. Results from parallel Mandelbrot program

Safety Is Invisible

In the end, the parallel program we ended up with is not substantially different from what we might write in any other language: we apportion pieces of the pixel buffer out among the processors; let each one work on its piece separately; and when they’ve all finished, present the result. So what is so special about Rust’s concurrency support?

What we haven’t shown here is all the Rust programs we cannot write. The code we looked at in this chapter partitions the buffer among the threads correctly, but there are many small variations on that code that do not (and thus introduce data races); not one of those variations will pass the Rust compiler’s static checks. A C or C++ compiler will cheerfully help you explore the vast space of programs with subtle data races; Rust tells you, up front, when something could go wrong.

In Chapters 4 and 5, we’ll describe Rust’s rules for memory safety. Chapter 19 explains how these rules also ensure proper concurrency hygiene. But for those to make sense, it’s essential to get a grounding in Rust’s fundamental types, which we’ll cover in the next chapter.

¹ The num_cpus crate provides a function that returns the number of CPUs available on the current system.

Chapter 3. Basic Types

There are many, many types of books in the world, which makes good sense, because there are many, many types of people, and everybody wants to read something different.

Lemony Snicket

Rust’s types serve several goals:

Safety

By checking a program’s types, the Rust compiler rules out whole classes of common mistakes. By replacing null pointers and unchecked unions with type-safe alternatives, Rust is even able to eliminate errors that are common sources of crashes in other languages.

Efficiency

Programmers have fine-grained control over how Rust programs represent values in memory, and can choose types they know the processor will handle efficiently. Programs needn’t pay for generality or flexibility they don’t use.

Concision

Rust manages all of this without requiring too much guidance from the programmer in the form of types written out in the code. Rust programs are usually less cluttered with types than the analogous C++ program would be.

Rather than using an interpreter or a just-in-time compiler, Rust is designed to use ahead-of-time compilation: the translation of your entire program to machine code is completed before it ever begins execution. Rust’s types help an ahead-of-time compiler choose good machine-level representations for the values your program operates on: representations whose performance you can predict, and which give you full access to the machine’s capabilities.

Rust is a statically typed language: without actually running the program, the compiler checks that every possible path of execution will use values only in ways consistent with their types. This allows Rust to catch many programming mistakes early, and is crucial to Rust’s safety guarantees.

Compared to a dynamically typed language like JavaScript or Python, Rust requires more planning from you up front: you must spell out the types of functions’ parameters and return values, members of struct types, and a few other constructs. However, two features of Rust make this less trouble than you might expect:

Given the types that you did spell out, Rust will infer most of the rest for you. In practice, there’s often only one type that will work for a given variable or expression; when this is the case, Rust lets you leave out the type. For example, you could spell out every type in a function, like this:

fn build_vector() -> Vec<i16> {
    let mut v: Vec<i16> = Vec::<i16>::new();
    v.push(10i16);
    v.push(20i16);
    v
}

But this is cluttered and repetitive. Given the function’s return type, it’s obvious that v must be a Vec<i16>, a vector of 16-bit signed integers; no other type would work. And from that it follows that each element of the vector must be an i16. This is exactly the sort of reasoning Rust’s type inference applies, allowing you to instead write:

fn build_vector() -> Vec<i16> {
    let mut v = Vec::new();
    v.push(10);
    v.push(20);
    v
}

These two definitions are exactly equivalent; Rust will generate the same machine code either way. Type inference gives back much of the legibility of dynamically typed languages, while still catching type errors at compile time.

Functions can be generic: when a function’s purpose and implementation are general enough, you can define it to work on any set of types that meet the necessary criteria. A single definition can cover an open-ended set of use cases.

In Python and JavaScript, all functions work this way naturally: a function can operate on any value that has the properties and methods the function will need. (This is the characteristic often called duck typing: if it quacks like a duck, it’s a duck.) But it’s exactly this flexibility that makes it so difficult for those languages to detect type errors early; testing is often the only way to catch such mistakes. Rust’s generic functions give the language a degree of the same flexibility, while still catching all type errors at compile time.

Despite their flexibility, generic functions are just as efficient as their nongeneric counterparts. We’ll discuss generic functions in detail in Chapter 11.

The rest of this chapter covers Rust’s types from the bottom up, starting with simple machine types like integers and floating-point values, and then showing how to compose them into more complex structures. Where appropriate, we’ll describe how Rust represents values of these types in memory, and their performance characteristics.

Here’s a summary of the sorts of types you’ll see in Rust. This table shows Rust’s primitive types, some very common types from the standard library, and some examples of user-defined types:

Type
Description
Values

i8, i16, i32, i64,
u8, u16, u32, u64
Signed and unsigned integers,
of given bit width
42,
-5i8, 0x400u16, 0o100i16,
20_922_789_888_000u64,
b'*' (u8 byte literal)

isize, usize
Signed and unsigned integers,
the same size as an address on the machine (32 or 64 bits)
137,
-0b0101_0010isize,
0xffff_fc00usize

f32, f64
IEEE floating-point numbers,
single and double precision
1.61803, 3.14f32, 6.0221e23f64

bool
Boolean
true, false

char
Unicode character, 32 bits wide
'*', '\n', '字', '\x7f', '\u{CA0}'

(char, u8, i32)
Tuple: mixed types allowed
('%', 0x7f, -1)

()
“unit” (empty) tuple
()

struct S { x: f32, y: f32 }
Named-field struct
S { x: 120.0, y: 209.0 }

struct T(i32, char);
Tuple-like struct
T(120, 'X')

struct E;
Unit-like struct; has no fields
E

enum Attend { OnTime, Late(u32) }
Enumeration, algebraic data type
Attend::Late(5), Attend::OnTime

Box<Attend>
Box: owning pointer to value in heap
Box::new(Late(15))

&i32, &mut i32
Shared and mutable references: nonowning pointers that must not outlive their referent
&s.y, &mut v

String
UTF-8 string, dynamically sized
"ラーメン: ramen".to_string()

&str
Reference to str: nonowning pointer to UTF-8 text
"そば: soba", &s[0..12]

[f64; 4], [u8; 256]
Array, fixed length; elements all of same type
[1.0, 0.0, 0.0, 1.0],
[b' '; 256]

Vec<f64>
Vector, varying length; elements all of same type
vec![0.367, 2.718, 7.389]

&[u8], &mut [u8]
Reference to slice: reference to a portion of an array or vector, comprising pointer and length
&v[10..20], &mut a[..]

&Any, &mut Read
Trait object: reference to any value that implements a given set of methods
value as &Any,
&mut file as &mut Read

fn(&str, usize) -> isize
Pointer to function
i32::saturating_add

(Closure types have no written form)
Closure
|a, b| a*a + b*b

Most of these types are covered in this chapter, except for the following:

We give struct types their own chapter, Chapter 9.

We give enumerated types their own chapter, Chapter 10.

We describe trait objects in Chapter 11.

We describe the essentials of String and &str here, but provide more detail in Chapter 17.

We cover function and closure types in Chapter 14.

Machine Types

The footing of Rust’s type system is a collection of fixed-width numeric types, chosen to match the types that almost all modern processors implement directly in hardware, and the Boolean and character types.

The names of Rust’s numeric types follow a regular pattern, spelling out their width in bits, and the representation they use:

Size (bits) | Unsigned integer | Signed integer | Floating-point
8 | u8 | i8 |
16 | u16 | i16 |
32 | u32 | i32 | f32
64 | u64 | i64 | f64
Machine word | usize | isize |

Here, a machine word is a value the size of an address on the machine the code runs on, usually 32 or 64 bits.

Integer Types

Rust’s unsigned integer types use their full range to represent positive values and zero:

Type | Range
u8 | 0 to 2⁸–1 (0 to 255)
u16 | 0 to 2¹⁶−1 (0 to 65,535)
u32 | 0 to 2³²−1 (0 to 4,294,967,295)
u64 | 0 to 2⁶⁴−1 (0 to 18,446,744,073,709,551,615, or 18 quintillion)
usize | 0 to either 2³²−1 or 2⁶⁴−1

Rust’s signed integer types use the two’s complement representation, using the same bit patterns as the corresponding unsigned type to cover a range of positive and negative values:

Type | Range
i8 | −2⁷ to 2⁷−1 (−128 to 127)
i16 | −2¹⁵ to 2¹⁵−1 (−32,768 to 32,767)
i32 | −2³¹ to 2³¹−1 (−2,147,483,648 to 2,147,483,647)
i64 | −2⁶³ to 2⁶³−1 (−9,223,372,036,854,775,808 to 9,223,372,036,854,775,807)
isize | Either −2³¹ to 2³¹−1, or −2⁶³ to 2⁶³−1

Rust generally uses the u8 type for byte values. For example, reading data from a file or socket yields a stream of u8 values.

Unlike C and C++, Rust treats characters as distinct from the numeric types; a char is neither a u8 nor an i8. We describe Rust’s char type in “Characters”.

The usize and isize types are analogous to size_t and ptrdiff_t in C and C++. The usize type is unsigned and isize is signed. Their precision depends on the size of the address space on the target machine: they are 32 bits long on 32-bit architectures, and 64 bits long on 64-bit architectures. Rust requires array indices to be usize values. Values representing the sizes of arrays or vectors or counts of the number of elements in some data structure also generally have the usize type.

In debug builds, Rust checks for integer overflow in arithmetic:

let big_val = std::i32::MAX;
let x = big_val + 1;  // panic: arithmetic operation overflowed

In a release build, this addition would wrap to a negative number (unlike C++, where signed integer overflow is undefined behavior). But unless you want to give up debug builds forever, it’s a bad idea to count on it. When you want wrapping arithmetic, use the methods:

let x = big_val.wrapping_add(1);  // ok

Integer literals in Rust can take a suffix indicating their type: 42u8 is a u8 value, and 1729isize is an isize. You can omit the suffix on an integer literal, in which case Rust will try to infer a type for it from the context. That inference usually identifies a unique type, but sometimes any one of several types would work. In this case, Rust defaults to i32, if that is among the possibilities. Otherwise, Rust reports the ambiguity as an error.

The prefixes 0x, 0o, and 0b designate hexadecimal, octal, and binary literals.

To make long numbers more legible, you can insert underscores among the digits. For example, you can write the largest u32 value as 4_294_967_295. The exact placement of the underscores is not significant, so you can break hexadecimal or binary numbers into groups of four digits rather than three, as in 0xffff_ffff, or set off the type suffix from the digits, as in 127_u8.

Some examples of integer literals:

Literal | Type | Decimal value
116i8 | i8 | 116
0xcafeu32 | u32 | 51966
0b0010_1010 | Inferred | 42
0o106 | Inferred | 70

Although numeric types and the char type are distinct, Rust does provide byte literals, character-like literals for u8 values: b'X' represents the ASCII code for the character X, as a u8 value. For example, since the ASCII code for A is 65, the literals b'A' and 65u8 are exactly equivalent. Only ASCII characters may appear in byte literals.

There are a few characters that you cannot simply place after the single quote, because that would be either syntactically ambiguous or hard to read. The following characters require a backslash placed in front of them:

Character | Byte literal | Numeric equivalent
Single quote, ' | b'\'' | 39u8
Backslash, \ | b'\\' | 92u8
Newline | b'\n' | 10u8
Carriage return | b'\r' | 13u8
Tab | b'\t' | 9u8

For characters that are hard to write or read, you can write their code in hexadecimal instead. A byte literal of the form b'\xHH', where HH is any two-digit hexadecimal number, represents the byte whose value is HH. For example, you can write a byte literal for the ASCII “escape” control character as b'\x1b', since the ASCII code for “escape” is 27, or 1B in hexadecimal. Since byte literals are just another notation for u8 values, consider whether a simple numeric literal might be more legible: it probably makes sense to use b'\x1b' instead of simply 27 only when you want to emphasize that the value represents an ASCII code.

You can convert from one integer type to another using the as operator. We explain how conversions work in “Type Casts”, but here are some examples:

assert_eq!(   10_i8  as u16,    10_u16); // in range
assert_eq!( 2525_u16 as i16,  2525_i16); // in range

assert_eq!(   -1_i16 as i32,    -1_i32); // sign-extended
assert_eq!(65535_u16 as i32, 65535_i32); // zero-extended

// Conversions that are out of range for the destination
// produce values that are equivalent to the original modulo 2^N,
// where N is the width of the destination in bits. This
// is sometimes called "truncation".
assert_eq!( 1000_i16 as  u8,   232_u8);
assert_eq!(65535_u32 as i16,    -1_i16);

assert_eq!(   -1_i8  as u8,    255_u8);
assert_eq!(  255_u8  as i8,     -1_i8);

Like any other sort of value, integers can have methods. The standard library provides some basic operations, which you can look up in the online documentation. Note that the documentation contains separate pages for the type itself (search for “i32 (primitive type)”, say), and for the module dedicated to that type (search for “std::i32”). For example:

assert_eq!(2u16.pow(4), 16);            // exponentiation
assert_eq!((-4i32).abs(), 4);           // absolute value
assert_eq!(0b101101u8.count_ones(), 4); // population count

The type suffixes on the literals are required here: Rust can’t look up a value’s methods until it knows its type. In real code, however, there’s usually additional context to disambiguate the type, so the suffixes aren’t needed.

Floating-Point Types

Rust provides IEEE single- and double-precision floating-point types. Following the IEEE 754-2008 specification, these types include positive and negative infinities, distinct positive and negative zero values, and a not-a-number value:

Type | Precision | Range
f32 | IEEE single precision (at least 6 decimal digits) | Roughly –3.4 × 10³⁸ to +3.4 × 10³⁸
f64 | IEEE double precision (at least 15 decimal digits) | Roughly –1.8 × 10³⁰⁸ to +1.8 × 10³⁰⁸

Rust’s f32 and f64 correspond to the float and double types in C and C++ implementations that support IEEE floating point, and in Java, which always uses IEEE floating point.

Floating-point literals have the general form diagrammed in Figure 3-1.

Figure 3-1. A floating-point literal

Every part of a floating-point number after the integer part is optional, but at least one of the fractional part, exponent, or type suffix must be present, to distinguish it from an integer literal. The fractional part may consist of a lone decimal point, so 5. is a valid floating-point constant.

If a floating-point literal lacks a type suffix, Rust infers whether it is an f32 or f64 from the context, defaulting to f64 if both would be possible. (Similarly, C, C++, and Java all treat unsuffixed floating-point literals as double values.) For the purposes of type inference, Rust treats integer literals and floating-point literals as distinct classes: it will never infer a floating-point type for an integer literal, or vice versa.

Some examples of floating-point literals:

Literal | Type | Mathematical value
–1.5625 | Inferred | −(1 ⁹⁄₁₆)
2. | Inferred | 2
0.25 | Inferred | ¼
1e4 | Inferred | 10,000
40f32 | f32 | 40
9.109_383_56e-31f64 | f64 | Roughly 9.10938356 × 10–31

The standard library’s std::f32 and std::f64 modules define constants for the IEEE-required special values like INFINITY, NEG_INFINITY (negative infinity), NAN (the not-a-number value), and MIN and MAX (the largest and smallest finite values). The std::f32::consts and std::f64::consts modules provide various commonly used mathematical constants like E, PI, and the square root of two.

The f32 and f64 types provide a full complement of methods for mathematical calculations; for example, 2f64.sqrt() is the double-precision square root of two. The standard library documentation describes these under the names “f32 (primitive type)” and “f64 (primitive type)”. Some examples:

assert_eq!(5f32.sqrt() * 5f32.sqrt(), 5.); // exactly 5.0, per IEEE
assert_eq!(-1.01f64.floor(), -1.0);
assert!((-1. / std::f32::INFINITY).is_sign_negative());

As before, you usually won’t need to write out the suffixes in real code, because the context will determine the type. When it doesn’t, however, the error messages can be surprising. For example, the following doesn’t compile:

println!("{}", (2.0).sqrt());

Rust complains:

error: no method named `sqrt` found for type `{float}` in the current scope

This can be a little bewildering; where else but on a floating-point type would one expect to find a sqrt method? The solution is to spell out which type you intend, in one way or another:

println!("{}", (2.0_f64).sqrt());
println!("{}", f64::sqrt(2.0));

Unlike C and C++, Rust performs almost no numeric conversions implicitly. If a function expects an f64 argument, it’s an error to pass an i32 value as the argument. In fact, Rust won’t even implicitly convert an i16 value to an i32 value, even though every i16 value is also an i32 value. But the key word here is implicitly: you can always write out explicit conversions using the as operator: i as f64, or x as i32. The lack of implicit conversions sometimes makes a Rust expression more verbose than the analogous C or C++ code would be. However, implicit integer conversions have a well-established record of causing bugs and security holes; in our experience, the act of writing out numeric conversions in Rust has alerted us to problems we would otherwise have missed. We explain exactly how conversions behave in “Type Casts”.

The bool Type

Rust’s Boolean type, bool, has the usual two values for such types, true and false. Comparison operators like == and < produce bool results: the value of 2 < 5 is true.

Many languages are lenient about using values of other types in contexts that require a Boolean value: C and C++ implicitly convert characters, integers, floating-point numbers, and pointers to Boolean values, so they can be used directly as the condition in an if or while statement. Python permits strings, lists, dictionaries, and even sets in Boolean contexts, treating such values as true if they’re nonempty. Rust, however, is very strict: control structures like if and while require their conditions to be bool expressions, as do the short-circuiting logical operators && and ||. You must write if x != 0 { ... }, not simply if x { ... }.

Rust’s as operator can convert bool values to integer types:

assert_eq!(false as i32, 0);
assert_eq!(true  as i32, 1);

However, as won’t convert in the other direction, from numeric types to bool. Instead, you must write out an explicit comparison like x != 0.

Although a bool only needs a single bit to represent it, Rust uses an entire byte for a bool value in memory, so you can create a pointer to it.

Characters

Rust’s character type char represents a single Unicode character, as a 32-bit value.

Rust uses the char type for single characters in isolation, but uses the UTF-8 encoding for strings and streams of text. So, a String represents its text as a sequence of UTF-8 bytes, not as an array of characters.

Character literals are characters enclosed in single quotes, like '8' or '!'. You can use any Unicode character you like: '錆' is a char literal representing the Japanese kanji for sabi (rust).

As with byte literals, backslash escapes are required for a few characters:

Character | Rust character literal
Single quote, ' | '\''
Backslash, \ | '\\'
Newline | '\n'
Carriage return | '\r'
Tab | '\t'

If you prefer, you can write out a character’s Unicode code point in hexadecimal:

If the character’s code point is in the range U+0000 to U+007F (that is, if it is drawn from the ASCII character set), then you can write the character as '\xHH', where HH is a two-digit hexadecimal number. For example, the character literals '*' and '\x2A' are equivalent, because the code point of the character * is 42, or 2A in hexadecimal.

You can write any Unicode character as '\u{HHHHHH}', where HHHHHH is a hexadecimal number between one and six digits long. For example, the character literal '\u{CA0}' represents the character “ಠ”, a Kannada character used in the Unicode Look of Disapproval, “ಠ_ಠ”. The same literal could also be simply written as 'ಠ'.

A char always holds a Unicode code point in the range 0x0000 to 0xD7FF, or 0xE000 to 0x10FFFF. A char is never a surrogate pair half (that is, a code point in the range 0xD800 to 0xDFFF), or a value outside the Unicode codespace (that is, greater than 0x10FFFF). Rust uses the type system and dynamic checks to ensure char values are always in the permitted range.

Rust never implicitly converts between char and any other type. You can use the as conversion operator to convert a char to an integer type; for types smaller than 32 bits, the upper bits of the character’s value are truncated:

assert_eq!('*' as i32, 42);
assert_eq!('ಠ' as u16, 0xca0);
assert_eq!('ಠ' as i8, -0x60); // U+0CA0 truncated to eight bits, signed

Going in the other direction, u8 is the only type the as operator will convert to char: Rust intends the as operator to perform only cheap, infallible conversions, but every integer type other than u8 includes values that are not permitted Unicode code points, so those conversions would require runtime checks. Instead, the standard library function std::char::from_u32 takes any u32 value and returns an Option<char>: if the u32 is not a permitted Unicode code point, then from_u32 returns None; otherwise, it returns Some(c), where c is the char result.

The standard library provides some useful methods on characters, which you can look up in the online documentation by searching for “char (primitive type)”, and for the module “std::char”. For example:

assert_eq!('*'.is_alphabetic(), false);
assert_eq!('β'.is_alphabetic(), true);
assert_eq!('8'.to_digit(10), Some(8));
assert_eq!('ಠ'.len_utf8(), 3);
assert_eq!(std::char::from_digit(2, 10), Some('2'));

Naturally, single characters in isolation are not as interesting as strings and streams of text. We’ll describe Rust’s standard String type and text handling in general in “String Types”.

Tuples

A tuple is a pair, or triple, or quadruple, ... of values of assorted types. You can write a tuple as a sequence of elements, separated by commas and surrounded by parentheses. For example, ("Brazil", 1985) is a tuple whose first element is a statically allocated string, and whose second is an integer; its type is (&str, i32) (or whatever integer type Rust infers for 1985). Given a tuple value t, you can access its elements as t.0, t.1, and so on.

Tuples aren’t much like arrays: for one thing, each element of a tuple can have a different type, whereas an array’s elements must be all the same type. Further, tuples allow only constants as indices, like t.4. You can’t write t.i or t[i] to get the i’th element.

Rust code often uses tuple types to return multiple values from a function. For example, the split_at method on string slices, which divides a string into two halves and returns them both, is declared like this:

fn split_at(&self, mid: usize) -> (&str, &str);

The return type (&str, &str) is a tuple of two string slices. You can use pattern-matching syntax to assign each element of the return value to a different variable:

let text = "I see the eigenvalue in thine eye";
let (head, tail) = text.split_at(21);
assert_eq!(head, "I see the eigenvalue ");
assert_eq!(tail, "in thine eye");

This is more legible than the equivalent:

let text = "I see the eigenvalue in thine eye";
let temp = text.split_at(21);
let head = temp.0;
let tail = temp.1;
assert_eq!(head, "I see the eigenvalue ");
assert_eq!(tail, "in thine eye");

You’ll also see tuples used as a sort of minimal-drama struct type. For example, in the Mandelbrot program in Chapter 2, we need to pass the width and height of the image to the functions that plot it and write it to disk. We could declare a struct with width and height members, but that’s pretty heavy notation for something so obvious, so we just used a tuple:

/// Write the buffer `pixels`, whose dimensions are given by `bounds`, to the
/// file named `filename`.
fn write_image(filename: &str, pixels: &[u8], bounds: (usize, usize))
    -> Result<(), std::io::Error>
{ ... }

The type of the bounds parameter is (usize, usize), a tuple of two usize values. Admittedly, we could just as well write out separate width and height parameters, and the machine code would be about the same either way. It’s a matter of clarity. We think of the size as one value, not two, and using a tuple lets us write what we mean.

The other commonly used tuple type, perhaps surprisingly, is the zero-tuple (). This is traditionally called the unit type because it has only one value, also written (). Rust uses the unit type where there’s no meaningful value to carry, but context requires some sort of type nonetheless.

For example, a function that returns no value has a return type of (). The standard library’s std::mem::swap function has no meaningful return value; it just exchanges the values of its two arguments. The declaration for std::mem::swap reads:

fn swap<T>(x: &mut T, y: &mut T);

The <T> means that swap is generic: you can use it on references to values of any type T. But the signature omits the swap’s return type altogether, which is shorthand for returning the unit type:

fn swap<T>(x: &mut T, y: &mut T) -> ();

Similarly, the write_bitmap example we mentioned before has a return type of Result<(), std::io::Error>, meaning that the function returns a std::io::Error value if something goes wrong, but returns no value on success.

If you like, you may include a comma after a tuple’s last element: the types (&str, i32,) and (&str, i32) are equivalent, as are the expressions ("Brazil", 1985,) and ("Brazil", 1985). Rust consistently permits an extra trailing comma everywhere commas are used: function arguments, arrays, struct and enum definitions, and so on. This may look odd to human readers, but it can make diffs easier to read when entries are added and removed at the end of a list.

For consistency’s sake, there are even tuples that contain a single value. The literal ("lonely hearts",) is a tuple containing a single string; its type is (&str,). Here, the comma after the value is necessary to distinguish the singleton tuple from a simple parenthetic expression.

Pointer Types

Rust has several types that represent memory addresses.

This is a big difference between Rust and most languages with garbage collection. In Java, if class Tree contains a field Tree left;, then left is a reference to another separately created Tree object. Objects never physically contain other objects in Java.

Rust is different. The language is designed to help keep allocations to a minimum. Values nest by default. The value ((0, 0), (1440, 900)) is stored as four adjacent integers. If you store it in a local variable, you’ve got a local variable four integers wide. Nothing is allocated in the heap.

This is great for memory efficiency, but as a consequence, when a Rust program needs values to point to other values, it must use pointer types explicitly. The good news is that the pointer types used in safe Rust are constrained to eliminate undefined behavior, so pointers are much easier to use correctly in Rust than in C++.

We’ll discuss three pointer types here: references, boxes, and unsafe pointers.

References

A value of type &String (pronounced “ref String”) is a reference to a String value, a &i32 is a reference to an i32, and so on.

It’s easiest to get started by thinking of references as Rust’s basic pointer type. A reference can point to any value anywhere, stack or heap. The expression &x produces a reference to x; in Rust terminology, we say that it borrows a reference to x. Given a reference r, the expression *r refers to the value r points to. These are very much like the & and * operators in C and C++. And like a C pointer, a reference does not automatically free any resources when it goes out of scope.

Unlike C pointers, however, Rust references are never null: there is simply no way to produce a null reference in safe Rust. And Rust references are immutable by default:

&T

Immutable reference, like const T* in C.

&mut T

Mutable reference, like T* in C.

Another major difference is that Rust tracks the ownership and lifetimes of values, so mistakes like dangling pointers, double frees, and pointer invalidation are ruled out at compile time. Chapter 5 explains Rust’s rules for safe reference use.

Boxes

The simplest way to allocate a value in the heap is to use Box::new:

let t = (12, "eggs");
let b = Box::new(t);  // allocate a tuple in the heap

The type of t is (i32, &str), so the type of b is Box<(i32, &str)>. Box::new() allocates enough memory to contain the tuple on the heap. When b goes out of scope, the memory is freed immediately, unless b has been moved—by returning it, for example. Moves are essential to the way Rust handles heap-allocated values; we explain all this in detail in Chapter 4.

Raw Pointers

Rust also has the raw pointer types *mut T and *const T. Raw pointers really are just like pointers in C++. Using a raw pointer is unsafe, because Rust makes no effort to track what it points to. For example, raw pointers may be null, or they may point to memory that has been freed or that now contains a value of a different type. All the classic pointer mistakes of C++ are offered for your enjoyment.

However, you may only dereference raw pointers within an unsafe block. An unsafe block is Rust’s opt-in mechanism for advanced language features whose safety is up to you. If your code has no unsafe blocks (or if those it does have are written correctly), then the safety guarantees we emphasize throughout this book still hold. For details, see Chapter 21.

Arrays, Vectors, and Slices

Rust has three types for representing a sequence of values in memory:

The type [T; N] represents an array of N values, each of type T. An array’s size is a constant determined at compile time, and is part of the type; you can’t append new elements, or shrink an array.

The type Vec<T>, called a vector of Ts, is a dynamically allocated, growable sequence of values of type T. A vector’s elements live on the heap, so you can resize vectors at will: push new elements onto them, append other vectors to them, delete elements, and so on.

The types &[T] and &mut [T], called a shared slice of Ts and mutable slice of Ts, are references to a series of elements that are a part of some other value, like an array or vector. You can think of a slice as a pointer to its first element, together with a count of the number of elements you can access starting at that point. A mutable slice &mut [T] lets you read and modify elements, but can’t be shared; a shared slice &[T] lets you share access among several readers, but doesn’t let you modify elements.

Given a value v of any of these three types, the expression v.len() gives the number of elements in v, and v[i] refers to the i’th element of v. The first element is v[0], and the last element is v[v.len() - 1]. Rust checks that i always falls within this range; if it doesn’t, the expression panics. The length of v may be zero, in which case any attempt to index it will panic. i must be a usize value; you can’t use any other integer type as an index.

Arrays

There are several ways to write array values. The simplest is to write a series of values within square brackets:

let lazy_caterer: [u32; 6] = [1, 2, 4, 7, 11, 16];
let taxonomy = ["Animalia", "Arthropoda", "Insecta"];

assert_eq!(lazy_caterer[3], 7);
assert_eq!(taxonomy.len(), 3);

For the common case of a long array filled with some value, you can write [V; N], where V is the value each element should have, and N is the length. For example, [true; 10000] is an array of 10,000 bool elements, all set to true:

let mut sieve = [true; 10000];
for i in 2..100 {
    if sieve[i] {
        let mut j = i * i;
        while j < 10000 {
            sieve[j] = false;
            j += i;
        }
    }
}

assert!(sieve[211]);
assert!(!sieve[9876]);

You’ll see this syntax used for fixed-size buffers: [0u8; 1024] can be a one-kilobyte buffer, filled with zero bytes. Rust has no notation for an uninitialized array. (In general, Rust ensures that code can never access any sort of uninitialized value.)

An array’s length is part of its type and fixed at compile time. If n is a variable, you can’t write [true; n] to get an array of n elements. When you need an array whose length varies at runtime (and you usually do), use a vector instead.

The useful methods you’d like to see on arrays—iterating over elements, searching, sorting, filling, filtering, and so on—all appear as methods of slices, not arrays. But Rust implicitly converts a reference to an array to a slice when searching for methods, so you can call any slice method on an array directly:

let mut chaos = [3, 5, 4, 1, 2];
chaos.sort();
assert_eq!(chaos, [1, 2, 3, 4, 5]);

Here, the sort method is actually defined on slices, but since sort takes its operand by reference, we can use it directly on chaos: the call implicitly produces a &mut [i32] slice referring to the entire array. In fact, the len method we mentioned earlier is a slice method as well. We cover slices in more detail in “Slices”.

Vectors

A vector Vec<T> is a resizable array of elements of type T, allocated on the heap.

There are several ways to create vectors. The simplest is to use the vec! macro, which gives us a syntax for vectors that looks very much like an array literal:

let mut v = vec![2, 3, 5, 7];
assert_eq!(v.iter().fold(1, |a, b| a * b), 210);

But of course, this is a vector, not an array, so we can add elements to it dynamically:

v.push(11);
v.push(13);
assert_eq!(v.iter().fold(1, |a, b| a * b), 30030);

You can also build a vector by repeating a given value a certain number of times, again using a syntax that imitates array literals:

fn new_pixel_buffer(rows: usize, cols: usize) -> Vec<u8> {
    vec![0; rows * cols]
}

The vec! macro is equivalent to calling Vec::new to create a new, empty vector, and then pushing the elements onto it, which is another idiom:

let mut v = Vec::new();
v.push("step");
v.push("on");
v.push("no");
v.push("pets");
assert_eq!(v, vec!["step", "on", "no", "pets"]);

Another possibility is to build a vector from the values produced by an iterator:

let v: Vec<i32> = (0..5).collect();
assert_eq!(v, [0, 1, 2, 3, 4]);

You’ll often need to supply the type when using collect (as we’ve done here), because it can build many different sorts of collections, not just vectors. By making the type for v explicit, we’ve made it unambiguous which sort of collection we want.

As with arrays, you can use slice methods on vectors:

// A palindrome!
let mut v = vec!["a man", "a plan", "a canal", "panama"];
v.reverse();
// Reasonable yet disappointing:
assert_eq!(v, vec!["panama", "a canal", "a plan", "a man"]);

Here, the reverse method is actually defined on slices, but the call implicitly borrows a &mut [&str] slice from the vector, and invokes reverse on that.

Vec is an essential type to Rust—it’s used almost anywhere one needs a list of dynamic size—so there are many other methods that construct new vectors or extend existing ones. We’ll cover them in Chapter 16.

A Vec<T> consists of three values: a pointer to the heap-allocated buffer allocated to hold the elements; the number of elements that buffer has the capacity to store; and the number it actually contains now (in other words, its length). When the buffer has reached its capacity, adding another element to the vector entails allocating a larger buffer, copying the present contents into it, updating the vector’s pointer and capacity to describe the new buffer, and finally freeing the old one.

If you know the number of elements a vector will need in advance, instead of Vec::new you can call Vec::with_capacity to create a vector with a buffer large enough to hold them all, right from the start; then, you can add the elements to the vector one at a time without causing any reallocation. The vec! macro uses a trick like this, since it knows how many elements the final vector will have. Note that this only establishes the vector’s initial size; if you exceed your estimate, the vector simply enlarges its storage as usual.

Many library functions look for the opportunity to use Vec::with_capacity instead of Vec::new. For example, in the collect example, the iterator 0..5 knows in advance that it will yield five values, and the collect function takes advantage of this to pre-allocate the vector it returns with the correct capacity. We’ll see how this works in Chapter 15.

Just as a vector’s len method returns the number of elements it contains now, its capacity method returns the number of elements it could hold without reallocation:

let mut v = Vec::with_capacity(2);
assert_eq!(v.len(), 0);
assert_eq!(v.capacity(), 2);

v.push(1);
v.push(2);
assert_eq!(v.len(), 2);
assert_eq!(v.capacity(), 2);

v.push(3);
assert_eq!(v.len(), 3);
assert_eq!(v.capacity(), 4);

The capacities you’ll see in your code may differ from those shown here. Vec and the system’s heap allocator may round up requests, even in the with_capacity case.

You can insert and remove elements wherever you like in a vector, although these operations shift all the elements after the insertion point forward or backward, so they may be slow if the vector is long:

let mut v = vec![10, 20, 30, 40, 50];

// Make the element at index 3 be 35.
v.insert(3, 35);
assert_eq!(v, [10, 20, 30, 35, 40, 50]);

// Remove the element at index 1.
v.remove(1);
assert_eq!(v, [10, 30, 35, 40, 50]);

You can use the pop method to remove the last element and return it. More precisely, popping a value from a Vec<T> returns an Option<T>: None if the vector was already empty, or Some(v) if its last element had been v:

let mut v = vec!["carmen", "miranda"];
assert_eq!(v.pop(), Some("miranda"));
assert_eq!(v.pop(), Some("carmen"));
assert_eq!(v.pop(), None);

You can use a for loop to iterate over a vector:

// Get our command-line arguments as a vector of Strings.
let languages: Vec<String> = std::env::args().skip(1).collect();
for l in languages {
    println!("{}: {}", l,
             if l.len() % 2 == 0 {
                 "functional"
             } else {
                 "imperative"
             });
}

Running this program with a list of programming languages is illuminating:

$ cargo run Lisp Scheme C C++ Fortran
   Compiling fragments v0.1.0 (file:///home/jimb/rust/book/fragments)
     Running `.../target/debug/fragments Lisp Scheme C C++ Fortran`
Lisp: functional
Scheme: functional
C: imperative
C++: imperative
Fortran: imperative
$

Finally, a satisfying definition for the term functional language.

Despite its fundamental role, Vec is an ordinary type defined in Rust, not built into the language. We’ll cover the techniques needed to implement such types in Chapter 21.

Building Vectors Element by Element

Building a vector one element at a time isn’t as bad as it might sound. Whenever a vector outgrows its buffer’s capacity, it chooses a new buffer twice as large as the old one. Suppose the vector starts with a buffer that can hold only one element: as it grows to its final capacity, it’ll have buffers of size 1, 2, 4, 8, and so on until it reaches its final size of 2n, for some n. If you think about how powers of two work, you’ll see that the total size of all the previous, smaller buffers put together is 2n–1, very close to the final buffer size. Since the number of actual elements is at least half the buffer size, the vector has always performed less than two copies per element!

What this means is that using Vec::with_capacity instead of Vec::new is a way to gain a constant factor improvement in speed, rather than an algorithmic improvement. For small vectors, avoiding a few calls to the heap allocator can make an observable difference in performance.

Slices

A slice, written [T] without specifying the length, is a region of an array or vector. Since a slice can be any length, slices can’t be stored directly in variables or passed as function arguments. Slices are always passed by reference.

A reference to a slice is a fat pointer: a two-word value comprising a pointer to the slice’s first element, and the number of elements in the slice.

Suppose you run the following code:

let v: Vec<f64> = vec![0.0,  0.707,  1.0,  0.707];
let a: [f64; 4] =     [0.0, -0.707, -1.0, -0.707];

let sv: &[f64] = &v;
let sa: &[f64] = &a;

On the last two lines, Rust automatically converts the &Vec<f64> reference and the &[f64; 4] reference to slice references that point directly to the data.

By the end, memory looks like Figure 3-2.

Figure 3-2. A vector v and an array a in memory, with slices sa and sv referring to each

Whereas an ordinary reference is a non-owning pointer to a single value, a reference to a slice is a non-owning pointer to several values. This makes slice references a good choice when you want to write a function that operates on any homogeneous data series, whether stored in an array, vector, stack, or heap. For example, here’s a function that prints a slice of numbers, one per line:

fn print(n: &[f64]) {
    for elt in n {
        println!("{}", elt);
    }
}

print(&v);  // works on vectors
print(&a);  // works on arrays

Because this function takes a slice reference as an argument, you can apply it to either a vector or an array, as shown. In fact, many methods you might think of as belonging to vectors or arrays are actually methods defined on slices: for example, the sort and reverse methods, which sort or reverse a sequence of elements in place, are actually methods on the slice type [T].

You can get a reference to a slice of an array or vector, or a slice of an existing slice, by indexing it with a range:

print(&v[0..2]);    // print the first two elements of v
print(&a[2..]);     // print elements of a starting with a[2]
print(&sv[1..3]);   // print v[1] and v[2]

As with ordinary array accesses, Rust checks that the indices are valid. Trying to borrow a slice that extends past the end of the data results in a panic.

We often use the term slice for reference types like &[T] or &str, but that is a bit of shorthand: those are properly called references to slices. Since slices almost always appear behind references, we use the shorter name for the more common concept.

String Types

Programmers familiar with C++ will recall that there are two string types in the language. String literals have the pointer type const char *. The standard library also offers a class, std::string, for dynamically creating strings at runtime.

Rust has a similar design. In this section, we’ll show all the ways to write string literals, then introduce Rust’s two string types. We provide more detail about strings and text handling in Chapter 17.

String Literals

String literals are enclosed in double quotes. They use the same backslash escape sequences as char literals:

let speech = "\"Ouch!\" said the well.\n";

In string literals, unlike char literals, single quotes don’t need a backslash escape, and double quotes do.

A string may span multiple lines:

println!("In the room the women come and go,
    Singing of Mount Abora");

The newline character in that string literal is included in the string, and therefore in the output. So are the spaces at the beginning of the second line.

If one line of a string ends with a backslash, then the newline character and the leading whitespace on the next line are dropped:

println!("It was a bright, cold day in April, and \
    there were four of us—\
    more or less.");

This prints a single line of text. The string contains a single space between “and” and “there”, because there is a space before the backslash in the program, and no space after the dash.

In a few cases, the need to double every backslash in a string is a nuisance. (The classic examples are regular expressions and Windows paths.) For these cases, Rust offers raw strings. A raw string is tagged with the lowercase letter r. All backslashes and whitespace characters inside a raw string are included verbatim in the string. No escape sequences are recognized.

let default_win_install_path = r"C:\Program Files\Gorillas";

let pattern = Regex::new(r"\d+(\.\d+)*");

You can’t include a double-quote character in a raw string simply by putting a backslash in front of it—remember, we said no escape sequences are recognized. However, there is a cure for that too. The start and end of a raw string can be marked with pound signs:

println!(r###"
    This raw string started with 'r###"'.
    Therefore it does not end until we reach a quote mark ('"')
    followed immediately by three pound signs ('###'):
"###);

You can add as few or as many pound signs as needed to make it clear where the raw string ends.

Byte Strings

A string literal with the b prefix is a byte string. Such a string is a slice of u8 values—that is, bytes—rather than Unicode text:

let method = b"GET";
assert_eq!(method, &[b'G', b'E', b'T']);

This combines with all the other string syntax we’ve shown: byte strings can span multiple lines, use escape sequences, and use backslashes to join lines. Raw byte strings start with br".

Byte strings can’t contain arbitrary Unicode characters. They must make do with ASCII and \xHH escape sequences.

The type of method shown here is &[u8; 3]: it’s a reference to an array of three bytes. It doesn’t have any of the string methods we’ll discuss in a minute. The most string-like thing about it is the syntax we used to write it.

Strings in Memory

Rust strings are sequences of Unicode characters, but they are not stored in memory as arrays of chars. Instead, they are stored using UTF-8, a variable-width encoding. Each ASCII character in a string is stored in one byte. Other characters take up multiple bytes.

Figure 3-3 shows the String and &str values created by the code:

let noodles = "noodles".to_string();
let oodles = &noodles[1..];
let poodles = "ಠ_ಠ";

Figure 3-3. String, &str, and str

A String has a resizable buffer holding UTF-8 text. The buffer is allocated on the heap, so it can resize its buffer as needed or requested. In the example, noodles is a String that owns an eight-byte buffer, of which seven are in use. You can think of a String as a Vec<u8> that is guaranteed to hold well-formed UTF-8; in fact, this is how String is implemented.

A &str (pronounced “stir” or “string slice”) is a reference to a run of UTF-8 text owned by someone else: it “borrows” the text. In the example, oodles is a &str referring to the last six bytes of the text belonging to noodles, so it represents the text “oodles”. Like other slice references, a &str is a fat pointer, containing both the address of the actual data and its length. You can think of a &str as being nothing more than a &[u8] that is guaranteed to hold well-formed UTF-8.

A string literal is a &str that refers to preallocated text, typically stored in read-only memory along with the program’s machine code. In the preceding example, poodles is a string literal, pointing to seven bytes that are created when the program begins execution, and that last until it exits.

A String or &str’s .len() method returns its length. The length is measured in bytes, not characters:

assert_eq!("ಠ_ಠ".len(), 7);
assert_eq!("ಠ_ಠ".chars().count(), 3);

It is impossible to modify a &str:

let mut s = "hello";
s[0] = 'c';    // error: the type `str` cannot be mutably indexed
s.push('\n');  // error: no method named `push` found for type `&str`

For creating new strings at run time, use String.

The type &mut str does exist, but it is not very useful, since almost any operation on UTF-8 can change its overall byte length, and a slice cannot reallocate its referent. In fact, the only operations available on &mut str are make_ascii_uppercase and make_ascii_lowercase, which modify the text in place and affect only single-byte characters, by definition.

String

&str is very much like &[T]: a fat pointer to some data. String is analogous to Vec<T>:

| Vec<T> | String
Automatically frees buffers | Yes | Yes
Growable | Yes | Yes
::new() and ::with_capacity() static methods | Yes | Yes
.reserve() and .capacity() methods | Yes | Yes
.push() and .pop() methods | Yes | Yes
Range syntax v[start..stop] | Yes, returns &[T] | Yes, returns &str
Automatic conversion | &Vec<T> to &[T] | &String to &str
Inherits methods | From &[T] | From &str

Like a Vec, each String has its own heap-allocated buffer that isn’t shared with any other String. When a String variable goes out of scope, the buffer is automatically freed, unless the String was moved.

There are several ways to create Strings:

The .to_string() method converts a &str to a String. This copies the string:

let error_message = "too many pets".to_string();

The format!() macro works just like println!(), except that it returns a new String instead of writing text to stdout, and it doesn’t automatically add a newline at the end.

assert_eq!(format!("{}°{:02}′{:02}″N", 24, 5, 23),
           "24°05′23″N".to_string());

Arrays, slices, and vectors of strings have two methods, .concat() and .join(sep), that form a new String from many strings.

let bits = vec!["veni", "vidi", "vici"];
assert_eq!(bits.concat(), "venividivici");
assert_eq!(bits.join(", "), "veni, vidi, vici");

The choice sometimes arises of which type to use: &str or String. Chapter 5 addresses this question in detail. For now it will suffice to point out that a &str can refer to any slice of any string, whether it is a string literal (stored in the executable) or a String (allocated and freed at run time). This means that &str is more appropriate for function arguments when the caller should be allowed to pass either kind of string.

Using Strings

Strings support the == and != operators. Two strings are equal if they contain the same characters in the same order (regardless of whether they point to the same location in memory).

assert!("ONE".to_lowercase() == "one");

Strings also support the comparison operators <, <=, >, and >=, as well as many useful methods and functions that you can find in the online documentation by searching for “str (primitive type)” or the “std::str” module (or just flip to Chapter 17). Here are a few examples:

assert!("peanut".contains("nut"));
assert_eq!("ಠ_ಠ".replace("ಠ", "■"), "■_■");
assert_eq!("    clean\n".trim(), "clean");

for word in "veni, vidi, vici".split(", ") {
    assert!(word.starts_with("v"));
}

Keep in mind that, given the nature of Unicode, simple char-by-char comparison does not always give the expected answers. For example, the Rust strings "th\u{e9}" and "the\u{301}" are both valid Unicode representations for thé, the French word for tea. Unicode says they should both be displayed and processed in the same way, but Rust treats them as two completely distinct strings. Similarly, Rust’s ordering operators like < use a simple lexicographical order based on character code point values. This ordering only sometimes resembles the ordering used for text in the user’s language and culture. We discuss these issues in more detail in Chapter 17.

Other String-Like Types

Rust guarantees that strings are valid UTF-8. Sometimes a program really needs to be able to deal with strings that are not valid Unicode. This usually happens when a Rust program has to interoperate with some other system that doesn’t enforce any such rules. For example, in most operating systems it’s easy to create a file with a filename that isn’t valid Unicode. What should happen when a Rust program comes across this sort of filename?

Rust’s solution is to offer a few string-like types for these situations:

Stick to String and &str for Unicode text.

When working with filenames, use std::path::PathBuf and &Path instead.

When working with binary data that isn’t character data at all, use Vec<u8> and &[u8].

When working with environment variable names and command-line arguments in the native form presented by the operating system, use OsString and &OsStr.

When interoperating with C libraries that use null-terminated strings, use std::ffi::CString and &CStr.

Beyond the Basics

Types are a central part of Rust. We’ll continue talking about types and introducing new ones throughout the book. In particular, Rust’s user-defined types give the language much of its flavor, because that’s where methods are defined. There are three kinds of user-defined types, and we’ll cover them in three successive chapters: structs in Chapter 9, enums in Chapter 10, and traits in Chapter 11.

Functions and closures have their own types, covered in Chapter 14. And the types that make up the standard library are covered throughout the book. For example, Chapter 16 presents the standard collection types.

All of that will have to wait, though. Before we move on, it’s time to tackle the concepts that are at the heart of Rust’s safety rules.

Chapter 4. Ownership

I’ve found that Rust has forced me to learn many of the things that I was slowly learning as ‘good practice’ in C/C++ before I could even compile my code. ...I want to stress that Rust isn’t the kind of language you can learn in a couple days and just deal with the hard/technical/good-practice stuff later. You will be forced to learn strict safety immediately and it will probably feel uncomfortable at first. However in my own experience, this has led me towards feeling like compiling my code actually means something to me again.

Mitchell Nordine

Rust makes the following pair of promises, both essential to a safe systems programming language:

You decide the lifetime of each value in your program. Rust frees memory and other resources belonging to a value promptly, at a point under your control.

Even so, your program will never use a pointer to an object after it has been freed. Using a dangling pointer is a common mistake in C and C++: if you’re lucky, your program crashes. If you’re unlucky, your program has a security hole. Rust catches these mistakes at compile time.

C and C++ keep the first promise: you can call free or delete on any object in the dynamically allocated heap you like, whenever you like. But in exchange, the second promise is set aside: it is entirely your responsibility to ensure that no pointer to the value you freed is ever used. There’s ample empirical evidence that this is a difficult responsibility to meet: pointer misuse has been a common culprit in public databases of reported security problems for as long as that data has been collected.

Plenty of languages fulfill the second promise using garbage collection, automatically freeing objects only when all reachable pointers to them are gone. But in exchange, you relinquish control to the collector over exactly when objects get freed. In general, garbage collectors are surprising beasts, and understanding why memory wasn’t freed when you expected can be a challenge. And if you’re working with objects that represent files, network connections, or other operating system resources, not being able to trust that they’ll be freed at the time you intended, and their underlying resources cleaned up along with them, is a disappointment.

None of these compromises are acceptable for Rust: the programmer should have control over values’ lifetimes, and the language should be safe. But this is a pretty well-explored area of language design. You can’t make major improvements without some fundamental changes.

Rust breaks the deadlock in a surprising way: by restricting how your programs can use pointers. This chapter and the next are devoted to explaining exactly what these restrictions are and why they work. For now, suffice it to say that some common structures you are accustomed to using may not fit within the rules, and you’ll need to look for alternatives. But the net effect of these restrictions is to bring just enough order to the chaos to allow Rust’s compile-time checks to verify that your program is free of memory safety errors: dangling pointers, double frees, using uninitialized memory, and so on. At runtime, your pointers are simple addresses in memory, just as they would be in C and C++. The difference is that your code has been proven to use them safely.

These same rules also form the basis of Rust’s support for safe concurrent programming. Using Rust’s carefully designed threading primitives, the rules that ensure your code uses memory correctly also serve to prove that it is free of data races. A bug in a Rust program cannot cause one thread to corrupt another’s data, introducing hard-to-reproduce failures in unrelated parts of the system. The nondeterministic behavior inherent in multithreaded code is isolated to those features designed to handle it—mutexes, message channels, atomic values, and so on—rather than appearing in ordinary memory references. Multithreaded code in C and C++ has earned its ugly reputation, but Rust rehabilitates it quite nicely.

Rust’s radical wager, the claim on which it stakes its success, and that forms the root of the language, is that even with these restrictions in place, you’ll find the language more than flexible enough for almost every task, and that the benefits—the elimination of broad classes of memory management and concurrency bugs—will justify the adaptations you’ll need to make to your style. The authors of this book are bullish on Rust exactly because of our extensive experience with C and C++. For us, Rust’s deal is a no-brainer.

Rust’s rules are probably unlike what you’ve seen in other programming languages. Learning how to work with them and turn them to your advantage is, in our opinion, the central challenge of learning Rust. In this chapter, we’ll first motivate Rust’s rules by showing how the same underlying issues play out in other languages. Then, we’ll explain Rust’s rules in detail. Finally, we’ll talk about some exceptions and almost-exceptions.

Ownership

If you’ve read much C or C++ code, you’ve probably come across a comment saying that an instance of some class owns some other object that it points to. This generally means that the owning object gets to decide when to free the owned object: when the owner is destroyed, it destroys its possessions along with it.

For example, suppose you write the following C++ code:

std::string s = "frayed knot";

The string s is usually represented in memory as shown in Figure 4-1.

Figure 4-1. A C++ std::string value on the stack, pointing to its heap-allocated buffer

Here, the actual std::string object itself is always exactly three words long, comprising a pointer to a heap-allocated buffer, the buffer’s overall capacity (that is, how large the text can grow before the string must allocate a larger buffer to hold it), and the length of the text it holds now. These are fields private to the std::string class, not accessible to the string’s users.

A std::string owns its buffer: when the program destroys the string, the string’s destructor frees the buffer. In the past, some C++ libraries shared a single buffer among several std::string values, using a reference count to decide when the buffer should be freed. Newer versions of the C++ specification effectively preclude that representation; all modern C++ libraries use the approach shown here. In these situations it’s generally understood that, although it’s fine for other code to create temporary pointers to the owned memory, it is that code’s responsibility to make sure its pointers are gone before the owner decides to destroy the owned object. You can create a pointer to a character living in a std::string’s buffer, but when the string is destroyed, your pointer becomes invalid, and it’s up to you to make sure you don’t use it anymore. The owner determines the lifetime of the owned, and everyone else must respect its decisions.

Rust takes this principle out of the comments and makes it explicit in the language. In Rust, every value has a single owner that determines its lifetime. When the owner is freed—dropped, in Rust terminology—the owned value is dropped too. These rules are meant to make it easy for you to find any given value’s lifetime simply by inspecting the code, giving you the control over its lifetime that a systems language should provide.

A variable owns its value. When control leaves the block in which the variable is declared, the variable is dropped, so its value is dropped along with it. For example:

fn print_padovan() {
    let mut padovan = vec![1,1,1];  // allocated here
    for i in 3..10 {
        let next = padovan[i-3] + padovan[i-2];
        padovan.push(next);
    }
    println!("P(1..10) = {:?}", padovan);
}                                   // dropped here

The type of the variable padovan is std::vec::Vec<i32>, a vector of 32-bit integers. In memory, the final value of padovan will look something like Figure 4-2.

Figure 4-2. A Vec 32 on the stack, pointing to its buffer in the heap

This is very similar to the C++ std::string we showed earlier, except that the elements in the buffer are 32-bit values, not characters. Note that the words holding padovan’s pointer, capacity, and length live directly in the stack frame of the print_padovan function; only the vector’s buffer is allocated on the heap.

As with the string s earlier, the vector owns the buffer holding its elements. When the variable padovan goes out of scope at the end of the function, the program drops the vector. And since the vector owns its buffer, the buffer goes with it.

Rust’s Box type serves as another example of ownership. A Box<T> is a pointer to a value of type T stored on the heap. Calling Box::new(v) allocates some heap space, moves the value v into it, and returns a Box pointing to the heap space. Since a Box owns the space it points to, when the Box is dropped, it frees the space too.

For example, you can allocate a tuple in the heap like so:

{
    let point = Box::new((0.625, 0.5));  // point allocated here
    let label = format!("{:?}", point);  // label allocated here
    assert_eq!(label, "(0.625, 0.5)");
}                                        // both dropped here

When the program calls Box::new, it allocates space for a tuple of two f64 values on the heap, moves its argument (0.625, 0.5) into that space, and returns a pointer to it. By the time control reaches the call to assert_eq!, the stack frame looks like Figure 4-3.

Figure 4-3. Two local variables, each owning memory in the heap

The stack frame itself holds the variables point and label, each of which refers to a heap allocation that it owns. When they are dropped, the allocations they own are freed along with them.

Just as variables own their values, structs own their fields; and tuples, arrays, and vectors own their elements:

struct Person { name: String, birth: i32 }

let mut composers = Vec::new();
composers.push(Person { name: "Palestrina".to_string(),
                        birth: 1525 });
composers.push(Person { name: "Dowland".to_string(),
                        birth: 1563 });
composers.push(Person { name: "Lully".to_string(),
                        birth: 1632 });
for composer in &composers {
    println!("{}, born {}", composer.name, composer.birth);
}

Here, composers is a Vec<Person>, a vector of structs, each of which holds a string and a number. In memory, the final value of composers looks like Figure 4-4.

Figure 4-4. A more complex tree of ownership

There are many ownership relationships here, but each one is pretty straightforward: composers owns a vector; the vector owns its elements, each of which is a Person structure; each structure owns its fields; and the string field owns its text. When control leaves the scope in which composers is declared, the program drops its value, and takes the entire arrangement with it. If there were other sorts of collections in the picture—a HashMap, perhaps, or a BTreeSet—the story would be the same.

At this point, take a step back and consider the consequences of the ownership relations we’ve presented so far. Every value has a single owner, making it easy to decide when to drop it. But a single value may own many other values: for example, the vector composers owns all of its elements. And those values may own other values in turn: each element of composers owns a string, which owns its text.

It follows that the owners and their owned values form trees: your owner is your parent, and the values you own are your children. And at the ultimate root of each tree is a variable; when that variable goes out of scope, the entire tree goes with it. We can see such an ownership tree in the diagram for composers: it’s not a “tree” in the sense of a search tree data structure, or an HTML document made from DOM elements. Rather, we have a tree built from a mixture of types, with Rust’s single-owner rule forbidding any rejoining of structure that could make the arrangement more complex than a tree. Every value in a Rust program is a member of some tree, rooted in some variable.

Rust programs don’t usually explicitly drop values at all, in the way C and C++ programs would use free and delete. The way to drop a value in Rust is to remove it from the ownership tree somehow: by leaving the scope of a variable, or deleting an element from a vector, or something of that sort. At that point, Rust ensures the value is properly dropped, along with everything it owns.

In a certain sense, Rust is less powerful than other languages: every other practical programming language lets you build arbitrary graphs of objects that point to each other in whatever way you see fit. But it is exactly because Rust is less powerful that the analyses the language can carry out on your programs can be more powerful. Rust’s safety guarantees are possible exactly because the relationships it may encounter in your code are more tractable. This is part of Rust’s “radical wager” we mentioned earlier: in practice, Rust claims, there is usually more than enough flexibility in how one goes about solving a problem to ensure that at least a few perfectly fine solutions fall within the restrictions the language imposes.

That said, the story we’ve told so far is still much too rigid to be usable. Rust extends this picture in several ways:

You can move values from one owner to another. This allows you to build, rearrange, and tear down the tree.

The standard library provides the reference-counted pointer types Rc and Arc, which allow values to have multiple owners, under some restrictions.

You can “borrow a reference” to a value; references are nonowning pointers, with limited lifetimes.

Each of these strategies contributes flexibility to the ownership model, while still upholding Rust’s promises. We’ll explain each one in turn, with references covered in the next chapter.

Moves

In Rust, for most types, operations like assigning a value to a variable, passing it to a function, or returning it from a function don’t copy the value: they move it. The source relinquishes ownership of the value to the destination, and becomes uninitialized; the destination now controls the value’s lifetime. Rust programs build up and tear down complex structures one value at a time, one move at a time.

You may be surprised that Rust would change the meaning of such fundamental operations; surely assignment is something that should be pretty well nailed down at this point in history. However, if you look closely at how different languages have chosen to handle assignment, you’ll see that there’s actually significant variation from one school to another. The comparison also makes the meaning and consequences of Rust’s choice easier to see.

Consider the following Python code:

s = ['udon', 'ramen', 'soba']
t = s
u = s

Each Python object carries a reference count, tracking the number of values that are currently referring to it. So after the assignment to s, the state of the program looks like Figure 4-5 (note that some fields are left out).

Figure 4-5. How Python represents a list of strings in memory

Since only s is pointing to the list, the list’s reference count is 1; and since the list is the only object pointing to the strings, each of their reference counts is also 1.

What happens when the program executes the assignments to t and u? Python implements assignment simply by making the destination point to the same object as the source, and incrementing the object’s reference count. So the final state of the program is something like Figure 4-6.

Figure 4-6. The result of assigning s to both t and u in Python

Python has copied the pointer from s into t and u, and updated the list’s reference count to 3. Assignment in Python is cheap, but because it creates a new reference to the object, we must maintain reference counts to know when we can free the value.

Now consider the analogous C++ code:

using namespace std;
vector<string> s = { "udon", "ramen", "soba" };
vector<string> t = s;
vector<string> u = s;

The original value of s looks like Figure 4-7 in memory.

Figure 4-7. How C++ represents a vector of strings in memory

What happens when the program assigns s to t and u? Assigning a std::vector produces a copy of the vector in C++; std::string behaves similarly. So by the time the program reaches the end of this code, it has actually allocated three vectors and nine strings (Figure 4-8).

Figure 4-8. The result of assigning s to both t and u in C++

Depending on the values involved, assignment in C++ can consume unbounded amounts of memory and processor time. The advantage, however, is that it’s easy for the program to decide when to free all this memory: when the variables go out of scope, everything allocated here gets cleaned up automatically.

In a sense, C++ and Python have chosen opposite trade-offs: Python makes assignment cheap, at the expense of requiring reference counting (and in the general case, garbage collection). C++ keeps the ownership of all the memory clear, at the expense of making assignment carry out a deep copy of the object. C++ programmers are often less than enthusiastic about this choice: deep copies can be expensive, and there are usually more practical alternatives.

So what would the analogous program do in Rust? Here’s the code:

let s = vec!["udon".to_string(), "ramen".to_string(), "soba".to_string()];
let t = s;
let u = s;

Like C and C++, Rust puts plain string literals like "udon" in read-only memory, so for a clearer comparison with the C++ and Python examples, we call to_string here to get heap-allocated String values.

After carrying out the initialization of s, since Rust and C++ use similar representations for vectors and strings, the situation looks just as it did in C++ (Figure 4-9).

Figure 4-9. How Rust represents a vector of strings in memory

But recall that, in Rust, assignments of most types move the value from the source to the destination, leaving the source uninitialized. So after initializing t, the program’s memory looks like Figure 4-10.

Figure 4-10. The result of assigning s to t in Rust

What has happened here? The initialization let t = s; moved the vector’s three header fields from s to t; now t owns the vector. The vector’s elements stayed just where they were, and nothing happened to the strings either. Every value still has a single owner, although one has changed hands. There were no reference counts to be adjusted. And the compiler now considers s uninitialized.

So what happens when we reach the initialization let u = s;? This would assign the uninitialized value s to u. Rust prudently prohibits using uninitialized values, so the compiler rejects this code with the following error:

error[E0382]: use of moved value: `s`
 --> ownership_double_move.rs:9:9
  |
8 |     let t = s;
  |         - value moved here
9 |     let u = s;
  |         ^ value used here after move
  |

Consider the consequences of Rust’s use of a move here. Like Python, the assignment is cheap: the program simply moves the three-word header of the vector from one spot to another. But like C++, ownership is always clear: the program doesn’t need reference counting or garbage collection to know when to free the vector elements and string contents.

The price you pay is that you must explicitly ask for copies when you want them. If you want to end up in the same state as the C++ program, with each variable holding an independent copy of the structure, you must call the vector’s clone method, which performs a deep copy of the vector and its elements:

let s = vec!["udon".to_string(), "ramen".to_string(), "soba".to_string()];
let t = s.clone();
let u = s.clone();

You could also re-create Python’s behavior by using Rust’s reference-counted pointer types; we’ll discuss those shortly in “Rc and Arc: Shared Ownership”.

More Operations That Move

In the examples thus far, we’ve shown initializations, providing values for variables as they come into scope in a let statement. Assigning to a variable is slightly different, in that if you move a value into a variable that was already initialized, Rust drops the variable’s prior value. For example:

let mut s = "Govinda".to_string();
s = "Siddhartha".to_string(); // value "Govinda" dropped here

In this code, when the program assigns the string "Siddhartha" to s, its prior value "Govinda" gets dropped first. But consider the following:

let mut s = "Govinda".to_string();
let t = s;
s = "Siddhartha".to_string(); // nothing is dropped here

This time, t has taken ownership of the original string from s, so that by the time we assign to s, it is uninitialized. In this scenario, no string is dropped.

We’ve used initializations and assignments in the examples here because they’re simple, but Rust applies move semantics to almost any use of a value. Passing arguments to functions moves ownership to the function’s parameters; returning a value from a function moves ownership to the caller. Building a tuple moves the values into the tuple. And so on.

You may now have a better insight into what’s really going on in the examples we offered in the previous section. For example, when we were constructing our vector of composers, we wrote:

struct Person { name: String, birth: i32 }

let mut composers = Vec::new();
composers.push(Person { name: "Palestrina".to_string(),
                        birth: 1525 });

This code shows several places at which moves occur, beyond initialization and assignment:

Returning values from a function

The call Vec::new() constructs a new vector, and returns, not a pointer to the vector, but the vector itself: its ownership moves from Vec::new to the variable composers. Similarly, the to_string call returns a fresh String instance.

Constructing new values

The name field of the new Person structure is initialized with the return value of to_string. The structure takes ownership of the string.

Passing values to a function

The entire Person structure, not just a pointer, is passed to the vector’s push method, which moves it onto the end of the structure. The vector takes ownership of the Person, and thus becomes the indirect owner of the name String as well.

Moving values around like this may sound inefficient, but there are two things to keep in mind. First, the moves always apply to the value proper, not the heap storage they own. For vectors and strings, the value proper is the three-word header alone; the potentially large element arrays and text buffers sit where they are in the heap. Second, the Rust compiler’s code generation is good at “seeing through” all these moves; in practice, the machine code often stores the value directly where it belongs.

Moves and Control Flow

The previous examples all have very simple control flow; how do moves interact with more complicated code? The general principle is that, if it’s possible for a variable to have had its value moved away, and it hasn’t definitely been given a new value since, it’s considered uninitialized. For example, if a variable still has a value after evaluating an if expression’s condition, then we can use it in both branches:

let x = vec![10, 20, 30];
if c {
    f(x); // ... ok to move from x here
} else {
    g(x); // ... and ok to also move from x here
}
h(x) // bad: x is uninitialized here if either path uses it

For similar reasons, moving from a variable in a loop is forbidden:

let x = vec![10, 20, 30];
while f() {
    g(x); // bad: x would be moved in first iteration,
          // uninitialized in second
}

That is, unless we’ve definitely given it a new value by the next iteration:

let mut x = vec![10, 20, 30];
while f() {
    g(x);           // move from x
    x = h();        // give x a fresh value
}
e(x);

Moves and Indexed Content

We’ve mentioned that a move leaves its source uninitialized, as the destination takes ownership of the value. But not every kind of value owner is prepared to become uninitialized. For example, consider the following code:

// Build a vector of the strings "101", "102", ... "105"
let mut v = Vec::new();
for i in 101 .. 106 {
    v.push(i.to_string());
}

// Pull out random elements from the vector.
let third = v[2];
let fifth = v[4];

For this to work, Rust would somehow need to remember that the third and fifth elements of the vector have become uninitialized, and track that information until the vector is dropped. In the most general case, vectors would need to carry around extra information with them to indicate which elements are live and which have become uninitialized. That is clearly not the right behavior for a systems programming language; a vector should be nothing but a vector. In fact, Rust rejects the preceding code with the following error:

error[E0507]: cannot move out of indexed content
  --> ownership_move_out_of_vector.rs:14:17
   |
14 |     let third = v[2];
   |                 ^^^^
   |                 |
   |                 help: consider using a reference instead `&v[2]`
   |                 cannot move out of indexed content

It also makes a similar complaint about the move to fifth. In the error message, Rust suggests using a reference, in case you want to access the element without moving it. This is often what you want. But what if you really do want to move an element out of a vector? You need to find a method that does so in a way that respects the limitations of the type. Here are three possibilities:

// Build a vector of the strings "101", "102", ... "105"
let mut v = Vec::new();
for i in 101 .. 106 {
    v.push(i.to_string());
}

// 1. Pop a value off the end of the vector:
let fifth = v.pop().unwrap();
assert_eq!(fifth, "105");

// 2. Move a value out of the middle of the vector, and move the last
// element into its spot:
let second = v.swap_remove(1);
assert_eq!(second, "102");

// 3. Swap in another value for the one we're taking out:
let third = std::mem::replace(&mut v[2], "substitute".to_string());
assert_eq!(third, "103");

// Let's see what's left of our vector.
assert_eq!(v, vec!["101", "104", "substitute"]);

Each one of these methods moves an element out of the vector, but does so in a way that leaves the vector in a state that is fully populated, if perhaps smaller.

Collection types like Vec also generally offer methods to consume all their elements in a loop:

let v = vec!["liberté".to_string(),
             "égalité".to_string(),
             "fraternité".to_string()];

for mut s in v {
    s.push('!');
    println!("{}", s);
}

When we pass the vector to the loop directly, as in for ... in v, this moves the vector out of v, leaving v uninitialized. The for loop’s internal machinery takes ownership of the vector, and dissects it into its elements. At each iteration, the loop moves another element to the variable s. Since s now owns the string, we’re able to modify it in the loop body before printing it. And since the vector itself is no longer visible to the code, nothing can observe it mid-loop in some partially emptied state.

If you do find yourself needing to move a value out of an owner that the compiler can’t track, you might consider changing the owner’s type to something that can dynamically track whether it has a value or not. For example, here’s a variant on the earlier example:

struct Person { name: Option<String>, birth: i32 }

let mut composers = Vec::new();
composers.push(Person { name: Some("Palestrina".to_string()),
                        birth: 1525 });

You can’t do this:

let first_name = composers[0].name;

That will just elicit the same “cannot move out of indexed content” error shown earlier. But because you’ve changed the type of the name field from String to Option<String>, that means that None is a legitimate value for the field to hold, so this works:

let first_name = std::mem::replace(&mut composers[0].name, None);
assert_eq!(first_name, Some("Palestrina".to_string()));
assert_eq!(composers[0].name, None);

The replace call moves out the value of composers[0].name, leaving None in its place, and passes ownership of the original value to its caller. In fact, using Option this way is common enough that the type provides a take method for this very purpose. You could write the preceding manipulation more legibly as follows:

let first_name = composers[0].name.take();

This call to take has the same effect as the earlier call to replace.

Copy Types: The Exception to Moves

The examples we’ve shown so far of values being moved involve vectors, strings, and other types that could potentially use a lot of memory and be expensive to copy. Moves keep ownership of such types clear and assignment cheap. But for simpler types like integers or characters, this sort of careful handling really isn’t necessary.

Compare what happens in memory when we assign a String with what happens when we assign an i32 value:

let str1 = "somnambulance".to_string();
let str2 = str1;

let num1: i32 = 36;
let num2 = num1;

After running this code, memory looks like Figure 4-11.

Figure 4-11. Assigning a string moves the value, whereas assigning an i32 copies it

As with the vectors earlier, assignment moves str1 to str2, so that we don’t end up with two strings responsible for freeing the same buffer. However, the situation with num1 and num2 is different. An i32 is simply a pattern of bits in memory; it doesn’t own any heap resources, or really depend on anything other than the bytes it comprises. By the time we’ve moved its bits to num2, we’ve made a completely independent copy of num1.

Moving a value leaves the source of the move uninitialized. But whereas it serves an essential purpose to treat str1 as valueless, treating num1 that way is pointless; no harm could result from continuing to use it. The advantages of a move don’t apply here, and it’s inconvenient.

Earlier we were careful to say that most types are moved; now we’ve come to the exceptions, the types Rust designates as Copy types. Assigning a value of a Copy type copies the value, rather than moving it. The source of the assignment remains initialized and usable, with the same value it had before. Passing Copy types to functions and constructors behaves similarly.

The standard Copy types include all the machine integer and floating-point numeric types, the char and bool types, and a few others. A tuple or fixed-size array of Copy types is itself a Copy type.

Only types for which a simple bit-for-bit copy suffices can be Copy. As we’ve already explained, String is not a Copy type, because it owns a heap-allocated buffer. For similar reasons, Box<T> is not Copy; it owns its heap-allocated referent. The File type, representing an operating system file handle, is not Copy; duplicating such a value would entail asking the operating system for another file handle. Similarly, the MutexGuard type, representing a locked mutex, isn’t Copy: this type isn’t meaningful to copy at all, as only one thread may hold a mutex at a time.

As a rule of thumb, any type that needs to do something special when a value is dropped cannot be Copy. A Vec needs to free its elements; a File needs to close its file handle; a MutexGuard needs to unlock its mutex. Bit-for-bit duplication of such types would leave it unclear which value was now responsible for the original’s resources.

What about types you define yourself? By default, struct and enum types are not Copy:

struct Label { number: u32 }

fn print(l: Label) { println!("STAMP: {}", l.number); }

let l = Label { number: 3 };
print(l);
println!("My label number is: {}", l.number);

This won’t compile; Rust complains:

error[E0382]: use of moved value: `l.number`
  --> ownership_struct.rs:12:40
   |
11 |     print(l);
   |           - value moved here
12 |     println!("My label number is: {}", l.number);
   |                                        ^^^^^^^^ value used here after move
   |
   = note: move occurs because `l` has type `main::Label`, which does not
           implement the `Copy` trait

Since Label is not Copy, passing it to print moved ownership of the value to the print function, which then dropped it before returning. But this is silly; a Label is nothing but an i32 with pretensions. There’s no reason passing l to print should move the value.

But user-defined types being non-Copy is only the default. If all the fields of your struct are themselves Copy, then you can make the type Copy as well by placing the attribute #[derive(Copy, Clone)] above the definition, like so:

#[derive(Copy, Clone)]
struct Label { number: u32 }

With this change, the preceding code compiles without complaint. However, if we try this on a type whose fields are not all Copy, it doesn’t work. Compiling the following code:

#[derive(Copy, Clone)]
struct StringLabel { name: String }

elicits this error:

error[E0204]: the trait `Copy` may not be implemented for this type
 --> ownership_string_label.rs:7:10
  |
7 | #[derive(Copy, Clone)]
  |          ^^^^
8 | struct StringLabel { name: String }
  |                      ------------ this field does not implement `Copy`

Why aren’t user-defined types automatically Copy, assuming they’re eligible? Whether a type is Copy or not has a big effect on how code is allowed to use it: Copy types are more flexible, since assignment and related operations don’t leave the original uninitialized. But for a type’s implementer, the opposite is true: Copy types are very limited in which types they can contain, whereas non-Copy types can use heap allocation and own other sorts of resources. So making a type Copy represents a serious commitment on the part of the implementer: if it’s necessary to change it to non-Copy later, much of the code that uses it will probably need to be adapted.

While C++ lets you overload assignment operators and define specialized copy and move constructors, Rust doesn’t permit this sort of customization. In Rust, every move is a byte-for-byte, shallow copy that leaves the source uninitialized. Copies are the same, except that the source remains initialized. This does mean that C++ classes can provide convenient interfaces that Rust types cannot, where ordinary-looking code implicitly adjusts reference counts, puts off expensive copies for later, or uses other sophisticated implementation tricks.

But the effect of this flexibility on C++ as a language is to make basic operations like assignment, passing parameters, and returning values from functions less predictable. For example, earlier in this chapter we showed how assigning one variable to another in C++ can require arbitrary amounts of memory and processor time. One of Rust’s principles is that costs should be apparent to the programmer. Basic operations must remain simple. Potentially expensive operations should be explicit, like the calls to clone in the earlier example that make deep copies of vectors and the strings they contain.

In this section, we’ve talked about Copy and Clone in vague terms as characteristics a type might have. They are actually examples of traits, Rust’s open-ended facility for categorizing types based on what you can do with them. We describe traits in general in Chapter 11, and Copy and Clone in particular in Chapter 13.

Rc and Arc: Shared Ownership

Although most values have unique owners in typical Rust code, in some cases it’s difficult to find every value a single owner that has the lifetime you need; you’d like the value to simply live until everyone’s done using it. For these cases, Rust provides the reference-counted pointer types Rc and Arc. As you would expect from Rust, these are entirely safe to use: you cannot forget to adjust the reference count, or create other pointers to the referent that Rust doesn’t notice, or stumble over any of the other sorts of problems that accompany reference-counted pointer types in C++.

The Rc and Arc types are very similar; the only difference between them is that an Arc is safe to share between threads directly—the name Arc is short for atomic reference count—whereas a plain Rc uses faster non-thread-safe code to update its reference count. If you don’t need to share the pointers between threads, there’s no reason to pay the performance penalty of an Arc, so you should use Rc; Rust will prevent you from accidentally passing one across a thread boundary. The two types are otherwise equivalent, so for the rest of this section, we’ll only talk about Rc.

Earlier in the chapter we showed how Python uses reference counts to manage its values’ lifetimes. You can use Rc to get a similar effect in Rust. Consider the following code:

use std::rc::Rc;

// Rust can infer all these types; written out for clarity
let s: Rc<String> = Rc::new("shirataki".to_string());
let t: Rc<String> = s.clone();
let u: Rc<String> = s.clone();

For any type T, an Rc<T> value is a pointer to a heap-allocated T that has had a reference count affixed to it. Cloning an Rc<T> value does not copy the T; instead, it simply creates another pointer to it, and increments the reference count. So the preceding code produces the situation illustrated in Figure 4-12 in memory.

Each of the three Rc<String> pointers is referring to the same block of memory, which holds a reference count and space for the String. The usual ownership rules apply to the Rc pointers themselves, and when the last extant Rc is dropped, Rust drops the String as well.

Figure 4-12. A reference-counted string, with three references

You can use any of String’s usual methods directly on an Rc<String>:

assert!(s.contains("shira"));
assert_eq!(t.find("taki"), Some(5));
println!("{} are quite chewy, almost bouncy, but lack flavor", u);

A value owned by an Rc pointer is immutable. If you try to add some text to the end of the string:

s.push_str(" noodles");

Rust will decline:

error: cannot borrow immutable borrowed content as mutable
  --> ownership_rc_mutability.rs:12:5
   |
12 |     s.push_str(" noodles");
   |     ^ cannot borrow as mutable

Rust’s memory and thread-safety guarantees depend on ensuring that no value is ever simultaneously shared and mutable. Rust assumes the referent of an Rc pointer might in general be shared, so it must not be mutable. We explain why this restriction is important in Chapter 5.

One well-known problem with using reference counts to manage memory is that, if there are ever two reference-counted values that point to each other, each will hold the other’s reference count above zero, so the values will never be freed (Figure 4-13).

Figure 4-13. A reference-counting loop; these objects will not be freed

It is possible to leak values in Rust this way, but such situations are rare. You cannot create a cycle without, at some point, making an older value point to a newer value. This obviously requires the older value to be mutable. Since Rc pointers hold their referents immutable, it’s not normally possible to create a cycle. However, Rust does provide ways to create mutable portions of otherwise immutable values; this is called interior mutability, and we cover it in “Interior Mutability”. If you combine those techniques with Rc pointers, you can create a cycle and leak memory.

You can sometimes avoid creating cycles of Rc pointers by using weak pointers, std::rc::Weak, for some of the links instead. However, we won’t cover those in this book; see the standard library’s documentation for details.

Moves and reference-counted pointers are two ways to relax the rigidity of the ownership tree. In the next chapter, we’ll look at a third way: borrowing references to values. Once you have become comfortable with both ownership and borrowing, you will have climbed the steepest part of Rust’s learning curve, and you’ll be ready to take advantage of Rust’s unique strengths.

Chapter 5. References

Libraries cannot provide new inabilities.

Mark Miller

All the pointer types we’ve seen so far—the simple Box<T> heap pointer, and the pointers internal to String and Vec values—are owning pointers: when the owner is dropped, the referent goes with it. Rust also has nonowning pointer types called references, which have no effect on their referents’ lifetimes.

In fact, it’s rather the opposite: references must never outlive their referents. You must make it apparent in your code that no reference can possibly outlive the value it points to. To emphasize this, Rust refers to creating a reference to some value as borrowing the value: what you have borrowed, you must eventually return to its owner.

If you felt a moment of skepticism when reading the phrase “You must make it apparent in your code,” you’re in excellent company. The references themselves are nothing special—under the hood, they’re just addresses. But the rules that keep them safe are novel to Rust; outside of research languages, you won’t have seen anything like them before. And although these rules are the part of Rust that requires the most effort to master, the breadth of classic, absolutely everyday bugs they prevent is surprising, and their effect on multithreaded programming is liberating. This is Rust’s radical wager, again.

As an example, let’s suppose we’re going to build a table of murderous Renaissance artists and the works they’re known for. Rust’s standard library includes a hash table type, so we can define our type like this:

use std::collections::HashMap;

type Table = HashMap<String, Vec<String>>;

In other words, this is a hash table that maps String values to Vec<String> values, taking the name of an artist to a list of the names of their works. You can iterate over the entries of a HashMap with a for loop, so we can write a function to print out a Table for debugging:

fn show(table: Table) {
    for (artist, works) in table {
        println!("works by {}:", artist);
        for work in works {
            println!("  {}", work);
        }
    }
}

Constructing and printing the table is straightforward:

fn main() {
    let mut table = Table::new();
    table.insert("Gesualdo".to_string(),
                 vec!["many madrigals".to_string(),
                      "Tenebrae Responsoria".to_string()]);
    table.insert("Caravaggio".to_string(),
                 vec!["The Musicians".to_string(),
                      "The Calling of St. Matthew".to_string()]);
    table.insert("Cellini".to_string(),
                 vec!["Perseus with the head of Medusa".to_string(),
                      "a salt cellar".to_string()]);

    show(table);
}

And it all works fine:

$ cargo run
     Running `/home/jimb/rust/book/fragments/target/debug/fragments`
works by Gesualdo:
  Tenebrae Responsoria
  many madrigals
works by Cellini:
  Perseus with the head of Medusa
  a salt cellar
works by Caravaggio:
  The Musicians
  The Calling of St. Matthew
$

But if you’ve read the previous chapter’s section on moves, this definition for show should raise a few questions. In particular, HashMap is not Copy—it can’t be, since it owns a dynamically allocated table. So when the program calls show(table), the whole structure gets moved to the function, leaving the variable table uninitialized. If the calling code tries to use table now, it’ll run into trouble:

...
show(table);
assert_eq!(table["Gesualdo"][0], "many madrigals");

Rust complains that table isn’t available anymore:

error[E0382]: use of moved value: `table`
  --> references_show_moves_table.rs:29:16
   |
28 |     show(table);
   |          ----- value moved here
29 |     assert_eq!(table["Gesualdo"][0], "many madrigals");
   |                ^^^^^ value used here after move
   |
   = note: move occurs because `table` has type `HashMap<String, Vec<String>>`,
           which does not implement the `Copy` trait

In fact, if we look into the definition of show, the outer for loop takes ownership of the hash table and consumes it entirely; and the inner for loop does the same to each of the vectors. (We saw this behavior earlier, in the “liberté, égalité, fraternité” example.) Because of move semantics, we’ve completely destroyed the entire structure simply by trying to print it out. Thanks, Rust!

The right way to handle this is to use references. A reference lets you access a value without affecting its ownership. References come in two kinds:

A shared reference lets you read but not modify its referent. However, you can have as many shared references to a particular value at a time as you like. The expression &e yields a shared reference to e’s value; if e has the type T, then &e has the type &T, pronounced “ref T”. Shared references are Copy.

If you have a mutable reference to a value, you may both read and modify the value. However, you may not have any other references of any sort to that value active at the same time. The expression &mut e yields a mutable reference to e’s value; you write its type as &mut T, which is pronounced “ref mute T”. Mutable references are not Copy.

You can think of the distinction between shared and mutable references as a way to enforce a multiple readers or single writer rule at compile time. In fact, this rule doesn’t apply only to references; it covers the borrowed value’s owner as well. As long as there are shared references to a value, not even its owner can modify it; the value is locked down. Nobody can modify table while show is working with it. Similarly, if there is a mutable reference to a value, it has exclusive access to the value; you can’t use the owner at all, until the mutable reference goes away. Keeping sharing and mutation fully separate turns out to be essential to memory safety, for reasons we’ll go into later in the chapter.

The printing function in our example doesn’t need to modify the table, just read its contents. So the caller should be able to pass it a shared reference to the table, as follows:

show(&table);

References are nonowning pointers, so the table variable remains the owner of the entire structure; show has just borrowed it for a bit. Naturally, we’ll need to adjust the definition of show to match, but you’ll have to look closely to see the difference:

fn show(table: &Table) {
    for (artist, works) in table {
        println!("works by {}:", artist);
        for work in works {
            println!("  {}", work);
        }
    }
}

The type of show’s parameter table has changed from Table to &Table: instead of passing the table by value (and hence moving ownership into the function), we’re now passing a shared reference. That’s the only textual change. But how does this play out as we work through the body?

Whereas our original outer for loop took ownership of the HashMap and consumed it, in our new version it receives a shared reference to the HashMap. Iterating over a shared reference to a HashMap is defined to produce shared references to each entry’s key and value: artist has changed from a String to a &String, and works from a Vec<String> to a &Vec<String>.

The inner loop is changed similarly. Iterating over a shared reference to a vector is defined to produce shared references to its elements, so work is now a &String. No ownership changes hands anywhere in this function; it’s just passing around nonowning references.

Now, if we wanted to write a function to alphabetize the works of each artist, a shared reference doesn’t suffice, since shared references don’t permit modification. Instead, the sorting function needs to take a mutable reference to the table:

fn sort_works(table: &mut Table) {
    for (_artist, works) in table {
        works.sort();
    }
}

And we need to pass it one:

sort_works(&mut table);

This mutable borrow grants sort_works the ability to read and modify our structure, as required by the vectors’ sort method.

When we pass a value to a function in a way that moves ownership of the value to the function, we say that we have passed it by value. If we instead pass the function a reference to the value, we say that we have passed the value by reference. For example, we fixed our show function by changing it to accept the table by reference, rather than by value. Many languages draw this distinction, but it’s especially important in Rust, because it spells out how ownership is affected.

References as Values

The preceding example shows a pretty typical use for references: allowing functions to access or manipulate a structure without taking ownership. But references are more flexible than that, so let’s look at some examples to get a more detailed view of what’s going on.

Rust References Versus C++ References

If you’re familiar with references in C++, they do have something in common with Rust references. Most importantly, they’re both just addresses at the machine level. But in practice, Rust’s references have a very different feel.

In C++, references are created implicitly by conversion, and dereferenced implicitly too:

// C++ code!
int x = 10;
int &r = x;             // initialization creates reference implicitly
assert(r == 10);        // implicitly dereference r to see x's value
r = 20;                 // stores 20 in x, r itself still points to x

In Rust, references are created explicitly with the & operator, and dereferenced explicitly with the * operator:

// Back to Rust code from this point onward.
let x = 10;
let r = &x;             // &x is a shared reference to x
assert!(*r == 10);      // explicitly dereference r

To create a mutable reference, use the &mut operator:

let mut y = 32;
let m = &mut y;        // &mut y is a mutable reference to y
*m += 32;              // explicitly dereference m to set y's value
assert!(*m == 64);     // and to see y's new value

But you might recall that, when we fixed the show function to take the table of artists by reference instead of by value, we never had to use the * operator. Why is that?

Since references are so widely used in Rust, the . operator implicitly dereferences its left operand, if needed:

struct Anime { name: &'static str, bechdel_pass: bool };
let aria = Anime { name: "Aria: The Animation", bechdel_pass: true };
let anime_ref = &aria;
assert_eq!(anime_ref.name, "Aria: The Animation");

// Equivalent to the above, but with the dereference written out:
assert_eq!((*anime_ref).name, "Aria: The Animation");

The println! macro used in the show function expands to code that uses the . operator, so it takes advantage of this implicit dereference as well.

The . operator can also implicitly borrow a reference to its left operand, if needed for a method call. For example, Vec’s sort method takes a mutable reference to the vector, so the two calls shown here are equivalent:

let mut v = vec![1973, 1968];
v.sort();           // implicitly borrows a mutable reference to v
(&mut v).sort();    // equivalent; much uglier

In a nutshell, whereas C++ converts implicitly between references and lvalues (that is, expressions referring to locations in memory), with these conversions appearing anywhere they’re needed, in Rust you use the & and * operators to create and follow references, with the exception of the . operator, which borrows and dereferences implicitly.

Assigning References

Assigning to a Rust reference makes it point at a new value:

let x = 10;
let y = 20;
let mut r = &x;

if b { r = &y; }

assert!(*r == 10 || *r == 20);

The reference r initially points to x. But if b is true, the code points it at y instead, as illustrated in Figure 5-1.

Figure 5-1. The reference r, now pointing to y instead of x

This is very different from C++, where assigning to a reference stores the value in its referent. There’s no way to point a C++ reference to a location other than the one it was initialized with.

References to References

Rust permits references to references:

struct Point { x: i32, y: i32 }
let point = Point { x: 1000, y: 729 };
let r: &Point = &point;
let rr: &&Point = &r;
let rrr: &&&Point = &rr;

(We’ve written out the reference types for clarity, but you could omit them; there’s nothing here Rust can’t infer for itself.) The . operator follows as many references as it takes to find its target:

assert_eq!(rrr.y, 729);

In memory, the references are arranged as shown in Figure 5-2.

Figure 5-2. A chain of references to references

So the expression rrr.y, guided by the type of rrr, actually traverses three references to get to the Point before fetching its y field.

Comparing References

Like the . operator, Rust’s comparison operators “see through” any number of references, as long as both operands have the same type:

let x = 10;
let y = 10;

let rx = &x;
let ry = &y;

let rrx = &rx;
let rry = &ry;

assert!(rrx <= rry);
assert!(rrx == rry);

The final assertion here succeeds, even though rrx and rry point at different values (namely, rx and ry), because the == operator follows all the references and performs the comparison on their final targets, x and y. This is almost always the behavior you want, especially when writing generic functions. If you actually want to know whether two references point to the same memory, you can use std::ptr::eq, which compares them as addresses:

assert!(rx == ry);              // their referents are equal
assert!(!std::ptr::eq(rx, ry)); // but occupy different addresses

References Are Never Null

Rust references are never null. There’s no analogue to C’s NULL or C++’s nullptr; there is no default initial value for a reference (you can’t use any variable until it’s been initialized, regardless of its type); and Rust won’t convert integers to references (outside of unsafe code), so you can’t convert zero into a reference.

C and C++ code often uses a null pointer to indicate the absence of a value: for example, the malloc function either returns a pointer to a new block of memory, or nullptr if there isn’t enough memory available to satisfy the request. In Rust, if you need a value that is either a reference to something or not, use the type Option<&T>. At the machine level, Rust represents None as a null pointer, and Some(r), where r is a &T value, as the nonzero address, so Option<&T> is just as efficient as a nullable pointer in C or C++, even though it’s safer: its type requires you to check whether it’s None before you can use it.

Borrowing References to Arbitrary Expressions

Whereas C and C++ only let you apply the & operator to certain kinds of expressions, Rust lets you borrow a reference to the value of any sort of expression at all:

fn factorial(n: usize) -> usize {
    (1..n+1).fold(1, |a, b| a * b)
}
let r = &factorial(6);
assert_eq!(r + &1009, 1729);

In situations like this, Rust simply creates an anonymous variable to hold the expression’s value, and makes the reference point to that. The lifetime of this anonymous variable depends on what you do with the reference:

If you immediately assign the reference to a variable in a let statement (or make it part of some struct or array that is being immediately assigned), then Rust makes the anonymous variable live as long as the variable the let initializes. In the preceding example, Rust would do this for the referent of r.

Otherwise, the anonymous variable lives to the end of the enclosing statement. In our example, the anonymous variable created to hold 1009 lasts only to the end of the assert_eq! statement.

If you’re used to C or C++, this may sound error-prone. But remember that Rust will never let you write code that would produce a dangling reference. If the reference could ever be used beyond the anonymous variable’s lifetime, Rust will always report the problem to you at compile time. You can then fix your code to keep the referent in a named variable with an appropriate lifetime.

References to Slices and Trait Objects

The references we’ve shown so far are all simple addresses. However, Rust also includes two kinds of fat pointers, two-word values carrying the address of some value, along with some further information necessary to put the value to use.

A reference to a slice is a fat pointer, carrying the starting address of the slice and its length. We described slices in detail in Chapter 3.

Rust’s other kind of fat pointer is a trait object, a reference to a value that implements a certain trait. A trait object carries a value’s address and a pointer to the trait’s implementation appropriate to that value, for invoking the trait’s methods. We’ll cover trait objects in detail in “Trait Objects”.

Aside from carrying this extra data, slice and trait object references behave just like the other sorts of references we’ve shown so far in this chapter: they don’t own their referents; they are not allowed to outlive their referents; they may be mutable or shared; and so on.

Reference Safety

As we’ve presented them so far, references look pretty much like ordinary pointers in C or C++. But those are unsafe; how does Rust keep its references under control? Perhaps the best way to see the rules in action is to try to break them. We’ll start with the simplest example possible, and then add in interesting complications and explain how they work out.

Borrowing a Local Variable

Here’s a pretty obvious case. You can’t borrow a reference to a local variable and take it out of the variable’s scope:

{
    let r;
    {
        let x = 1;
        r = &x;
    }
    assert_eq!(*r, 1);  // bad: reads memory `x` used to occupy
}

The Rust compiler rejects this program, with a detailed error message:

error: `x` does not live long enough
  --> references_dangling.rs:8:5
   |
7  |         r = &x;
   |              - borrow occurs here
8  |     }
   |     ^ `x` dropped here while still borrowed
9  |     assert_eq!(*r, 1);  // bad: reads memory `x` used to occupy
10 | }
   | - borrowed value needs to live until here

Rust’s complaint is that x lives only until the end of the inner block, whereas the reference remains alive until the end of the outer block, making it a dangling pointer, which is verboten.

While it’s obvious to a human reader that this program is broken, it’s worth looking at how Rust itself reached that conclusion. Even this simple example shows the logical tools Rust uses to check much more complex code.

Rust tries to assign each reference type in your program a lifetime that meets the constraints imposed by how it is used. A lifetime is some stretch of your program for which a reference could be safe to use: a lexical block, a statement, an expression, the scope of some variable, or the like. Lifetimes are entirely figments of Rust’s compile-time imagination. At runtime, a reference is nothing but an address; its lifetime is part of its type and has no runtime representation.

In this example, there are three lifetimes whose relationships we need to work out. The variables r and x each have a lifetime, extending from the point at which they’re initialized until the point that they go out of scope. The third lifetime is that of a reference type: the type of the reference we borrow to &x, and store in r.

Here’s one constraint that should seem pretty obvious: if you have a variable x, then a reference to x must not outlive x itself, as shown in Figure 5-3.

Figure 5-3. Permissible lifetimes for &x

Beyond the point where x goes out of scope, the reference would be a dangling pointer. We say that the variable’s lifetime must contain or enclose that of the reference borrowed from it.

Here’s another kind of constraint: if you store a reference in a variable r, the reference’s type must be good for the entire lifetime of the variable, from the point it is initialized to the point it goes out of scope, as shown in Figure 5-4.

Figure 5-4. Permissible lifetimes for reference stored in r

If the reference can’t live at least as long as the variable does, then at some point r will be a dangling pointer. We say that the reference’s lifetime must contain or enclose the variable’s.

The first kind of constraint limits how large a reference’s lifetime can be, while the second kind limits how small it can be. Rust simply tries to find a lifetime for each reference that satisfies all these constraints. In our example, however, there is no such lifetime, as shown in Figure 5-5.

Figure 5-5. A reference with contradictory constraints on its lifetime

Let’s now consider a different example where things do work out. We have the same kinds of constraints: the reference’s lifetime must be contained by x’s, but fully enclose r’s. But because r’s lifetime is smaller now, there is a lifetime that meets the constraints, as shown in Figure 5-6.

Figure 5-6. A reference with a lifetime enclosing r’s scope, but within x’s scope

These rules apply in a natural way when you borrow a reference to some part of some larger data structure, like an element of a vector:

let v = vec![1, 2, 3];
let r = &v[1];

Since v owns the vector, which owns its elements, the lifetime of v must enclose that of the reference type of &v[1]. Similarly, if you store a reference in some data structure, its lifetime must enclose that of the data structure. If you build a vector of references, say, all of them must have lifetimes enclosing that of the variable that owns the vector.

This is the essence of the process Rust uses for all code. Bringing more language features into the picture—data structures and function calls, say—introduces new sorts of constraints, but the principle remains the same: first, understand the constraints arising from the way the program uses references; then, find lifetimes that satisfy them. This is not so different from the process C and C++ programmers impose on themselves; the difference is that Rust knows the rules, and enforces them.

Receiving References as Parameters

When we pass a reference to a function, how does Rust make sure the function uses it safely? Suppose we have a function f that takes a reference and stores it in a global variable. We’ll need to make a few revisions to this, but here’s a first cut:

// This code has several problems, and doesn't compile.
static mut STASH: &i32;
fn f(p: &i32) { STASH = p; }

Rust’s equivalent of a global variable is called a static: it’s a value that’s created when the program starts and lasts until it terminates. (Like any other declaration, Rust’s module system controls where statics are visible, so they’re only “global” in their lifetime, not their visibility.) We cover statics in Chapter 8, but for now we’ll just call out a few rules that the code just shown doesn’t follow:

Every static must be initialized.

Mutable statics are inherently not thread-safe (after all, any thread can access a static at any time), and even in single-threaded programs, they can fall prey to other sorts of reentrancy problems. For these reasons, you may access a mutable static only within an unsafe block. In this example we’re not concerned with those particular problems, so we’ll just throw in an unsafe block and move on.

With those revisions made, we now have the following:

static mut STASH: &i32 = &128;
fn f(p: &i32) { // still not good enough
    unsafe {
        STASH = p;
    }
}

We’re almost done. To see the remaining problem, we need to write out a few things that Rust is helpfully letting us omit. The signature of f as written here is actually shorthand for the following:

fn f<'a>(p: &'a i32) { ... }

Here, the lifetime 'a (pronounced “tick A”) is a lifetime parameter of f. You can read <'a> as “for any lifetime 'a” so when we write fn f<'a>(p: &'a i32), we’re defining a function that takes a reference to an i32 with any given lifetime 'a.

Since we must allow 'a to be any lifetime, things had better work out if it’s the smallest possible lifetime: one just enclosing the call to f. This assignment then becomes a point of contention:

STASH = p;

Since STASH lives for the program’s entire execution, the reference type it holds must have a lifetime of the same length; Rust calls this the 'static lifetime. But the lifetime of p’s reference is some 'a, which could be anything, as long as it encloses the call to f. So, Rust rejects our code:

error[E0312]: lifetime of reference outlives lifetime of borrowed content...
 --> references_static.rs:6:17
  |
6 |         STASH = p;
  |                 ^
  |
  = note: ...the reference is valid for the static lifetime...
note: ...but the borrowed content is only valid for the anonymous lifetime #1
      defined on the function body at 4:0
 --> references_static.rs:4:1
  |
4 | / fn f(p: &i32) { // still not good enough
5 | |     unsafe {
6 | |         STASH = p;
7 | |     }
8 | | }
  | |_^

At this point, it’s clear that our function can’t accept just any reference as an argument. But it ought to be able to accept a reference that has a 'static lifetime: storing such a reference in STASH can’t create a dangling pointer. And indeed, the following code compiles just fine:

static mut STASH: &i32 = &10;

fn f(p: &'static i32) {
    unsafe {
        STASH = p;
    }
}

This time, f’s signature spells out that p must be a reference with lifetime 'static, so there’s no longer any problem storing that in STASH. We can only apply f to references to other statics, but that’s the only thing that’s certain not to leave STASH dangling anyway. So we can write:

static WORTH_POINTING_AT: i32 = 1000;
f(&WORTH_POINTING_AT);

Since WORTH_POINTING_AT is a static, the type of &WORTH_POINTING_AT is &'static i32, which is safe to pass to f.

Take a step back, though, and notice what happened to f’s signature as we amended our way to correctness: the original f(p: &i32) ended up as f(p: &'static i32). In other words, we were unable to write a function that stashed a reference in a global variable without reflecting that intention in the function’s signature. In Rust, a function’s signature always exposes the body’s behavior.

Conversely, if we do see a function with a signature like g(p: &i32) (or with the lifetimes written out, g<'a>(p: &'a i32)), we can tell that it does not stash its argument p anywhere that will outlive the call. There’s no need to look into g’s definition; the signature alone tells us what g can and can’t do with its argument. This fact ends up being very useful when you’re trying to establish the safety of a call to the function.

Passing References as Arguments

Now that we’ve shown how a function’s signature relates to its body, let’s examine how it relates to the function’s callers. Suppose you have the following code:

// This could be written more briefly: fn g(p: &i32),
// but let's write out the lifetimes for now.
fn g<'a>(p: &'a i32) { ... }

let x = 10;
g(&x);

From g’s signature alone, Rust knows it will not save p anywhere that might outlive the call: any lifetime that encloses the call must work for 'a. So Rust chooses the smallest possible lifetime for &x: that of the call to g. This meets all constraints: it doesn’t outlive x, and encloses the entire call to g. So this code passes muster.

Note that although g takes a lifetime parameter 'a, we didn’t need to mention it when calling g. You only need to worry about lifetime parameters when defining functions and types; when using them, Rust infers the lifetimes for you.

What if we tried to pass &x to our function f from earlier that stores its argument in a static?

fn f(p: &'static i32) { ... }

let x = 10;
f(&x);

This fails to compile: the reference &x must not outlive x, but by passing it to f, we constrain it to live at least as long as 'static. There’s no way to satisfy everyone here, so Rust rejects the code.

Returning References

It’s common for a function to take a reference to some data structure, and then return a reference into some part of that structure. For example, here’s a function that returns a reference to the smallest element of a slice:

// v should have at least one element.
fn smallest(v: &[i32]) -> &i32 {
    let mut s = &v[0];
    for r in &v[1..] {
        if *r < *s { s = r; }
    }
    s
}

We’ve omitted lifetimes from that function’s signature in the usual way. When a function takes a single reference as an argument, and returns a single reference, Rust assumes that the two must have the same lifetime. Writing this out explicitly would give us:

fn smallest<'a>(v: &'a [i32]) -> &'a i32 { ... }

Suppose we call smallest like this:

let s;
{
    let parabola = [9, 4, 1, 0, 1, 4, 9];
    s = smallest(&parabola);
}
assert_eq!(*s, 0); // bad: points to element of dropped array

From smallest’s signature, we can see that its argument and return value must have the same lifetime, 'a. In our call, the argument &parabola must not outlive parabola itself; yet smallest’s return value must live at least as long as s. There’s no possible lifetime 'a that can satisfy both constraints, so Rust rejects the code:

error: `parabola` does not live long enough
  --> references_lifetimes_propagated.rs:12:5
   |
11 |         s = smallest(&parabola);
   |                       -------- borrow occurs here
12 |     }
   |     ^ `parabola` dropped here while still borrowed
13 |     assert_eq!(*s, 0); // bad: points to element of dropped array
14 | }
   | - borrowed value needs to live until here

Moving s so that its lifetime is clearly contained within parabola’s fixes the problem:

{
    let parabola = [9, 4, 1, 0, 1, 4, 9];
    let s = smallest(&parabola);
    assert_eq!(*s, 0); // fine: parabola still alive
}

Lifetimes in function signatures let Rust assess the relationships between the references you pass to the function and those the function returns, and ensure they’re being used safely.

Structs Containing References

How does Rust handle references stored in data structures? Here’s the same erroneous program we looked at earlier, except that we’ve put the reference inside a structure:

// This does not compile.
struct S {
    r: &i32
}

let s;
{
    let x = 10;
    s = S { r: &x };
}
assert_eq!(*s.r, 10); // bad: reads from dropped `x`

The safety constraints Rust places on references can’t magically disappear just because we hid the reference inside a struct. Somehow, those constraints must end up applying to S as well. Indeed, Rust is skeptical:

error[E0106]: missing lifetime specifier
 --> references_in_struct.rs:7:12
  |
7 |         r: &i32
  |            ^ expected lifetime parameter

Whenever a reference type appears inside another type’s definition, you must write out its lifetime. You can write this:

struct S {
    r: &'static i32
}

This says that r can only refer to i32 values that will last for the lifetime of the program, which is rather limiting. The alternative is to give the type a lifetime parameter 'a, and use that for r:

struct S<'a> {
    r: &'a i32
}

Now the S type has a lifetime, just as reference types do. Each value you create of type S gets a fresh lifetime 'a, which becomes constrained by how you use the value. The lifetime of any reference you store in r had better enclose 'a, and 'a must outlast the lifetime of wherever you store the S.

Turning back to the preceding code, the expression S { r: &x } creates a fresh S value with some lifetime 'a. When you store &x in the r field, you constrain 'a to lie entirely within x’s lifetime.

The assignment s = S { ... } stores this S in a variable whose lifetime extends to the end of the example, constraining 'a to outlast the lifetime of s. And now Rust has arrived at the same contradictory constraints as before: 'a must not outlive x, yet must live at least as long as s. No satisfactory lifetime exists, and Rust rejects the code. Disaster averted!

How does a type with a lifetime parameter behave when placed inside some other type?

struct T {
    s: S  // not adequate
}

Rust is skeptical, just as it was when we tried placing a reference in S without specifying its lifetime:

error[E0106]: missing lifetime specifier
 --> references_in_nested_struct.rs:8:8
  |
8 |     s: S  // not adequate
  |        ^ expected lifetime parameter

We can’t leave off S’s lifetime parameter here: Rust needs to know how a T’s lifetime relates to that of the reference in its S, in order to apply the same checks to T that it does for S and plain references.

We could give s the 'static lifetime. This works:

struct T {
    s: S<'static>
}

With this definition, the s field may only borrow values that live for the entire execution of the program. That’s somewhat restrictive, but it does mean that a T can’t possibly borrow a local variable; there are no special constraints on a T’s lifetime.

The other approach would be to give T its own lifetime parameter, and pass that to S:

struct T<'a> {
    s: S<'a>
}

By taking a lifetime parameter 'a and using it in s’s type, we’ve allowed Rust to relate a T value’s lifetime to that of the reference its S holds.

We showed earlier how a function’s signature exposes what it does with the references we pass it. Now we’ve shown something similar about types: a type’s lifetime parameters always reveal whether it contains references with interesting (that is, non-'static) lifetimes, and what those lifetimes can be.

For example, suppose we have a parsing function that takes a slice of bytes, and returns a structure holding the results of the parse:

fn parse_record<'i>(input: &'i [u8]) -> Record<'i> { ... }

Without looking into the definition of the Record type at all, we can tell that, if we receive a Record from parse_record, whatever references it contains must point into the input buffer we passed in, and nowhere else (except perhaps at 'static values).

In fact, this exposure of internal behavior is the reason Rust requires types that contain references to take explicit lifetime parameters. There’s no reason Rust couldn’t simply make up a distinct lifetime for each reference in the struct, and save you the trouble of writing them out. Early versions of Rust actually behaved this way, but developers found it confusing: it is helpful to know when one value borrows something from another value, especially when working through errors.

It’s not just references and types like S that have lifetimes. Every type in Rust has a lifetime, including i32 and String. Most are simply 'static, meaning that values of those types can live for as long as you like; for example, a Vec<i32> is self-contained, and needn’t be dropped before any particular variable goes out of scope. But a type like Vec<&'a i32> has a lifetime that must be enclosed by 'a: it must be dropped while its referents are still alive.

Distinct Lifetime Parameters

Suppose you’ve defined a structure containing two references like this:

struct S<'a> {
    x: &'a i32,
    y: &'a i32
}

Both references use the same lifetime 'a. This could be a problem if your code wants to do something like this:

let x = 10;
let r;
{
    let y = 20;
    {
        let s = S { x: &x, y: &y };
        r = s.x;
    }
}

This code doesn’t create any dangling pointers. The reference to y stays in s, which goes out of scope before y does. The reference to x ends up in r, which doesn’t outlive x.

If you try to compile this, however, Rust will complain that y does not live long enough, even though it clearly does. Why is Rust worried? If you work through the code carefully, you can follow its reasoning:

Both fields of S are references with the same lifetime 'a, so Rust must find a single lifetime that works for both s.x and s.y.

We assign r = s.x, requiring 'a to enclose r’s lifetime.

We initialized s.y with &y, requiring 'a to be no longer than y’s lifetime.

These constraints are impossible to satisfy: no lifetime is shorter than y’s scope, but longer than r’s. Rust balks.

The problem arises because both references in S have the same lifetime 'a. Changing the definition of S to let each reference have a distinct lifetime fixes everything:

struct S<'a, 'b> {
    x: &'a i32,
    y: &'b i32
}

With this definition, s.x and s.y have independent lifetimes. What we do with s.x has no effect on what we store in s.y, so it’s easy to satisfy the constraints now: 'a can simply be r’s lifetime, and 'b can be s’s. (y’s lifetime would work too for 'b, but Rust tries to choose the smallest lifetime that works.) Everything ends up fine.

Function signatures can have similar effects. Suppose we have a function like this:

fn f<'a>(r: &'a i32, s: &'a i32) -> &'a i32 { r } // perhaps too tight

Here, both reference parameters use the same lifetime 'a, which can unnecessarily constrain the caller in the same way we’ve shown previously. If this is a problem, you can let parameters’ lifetimes vary independently:

fn f<'a, 'b>(r: &'a i32, s: &'b i32) -> &'a i32 { r } // looser

The downside to this is that adding lifetimes can make types and function signatures harder to read. Your authors tend to try the simplest possible definition first, and then loosen restrictions until the code compiles. Since Rust won’t permit the code to run unless it’s safe, simply waiting to be told when there’s a problem is a perfectly acceptable tactic.

Omitting Lifetime Parameters

We’ve shown plenty of functions so far in this book that return references or take them as parameters, but we’ve usually not needed to spell out which lifetime is which. The lifetimes are there; Rust is just letting us omit them when it’s reasonably obvious what they should be.

In the simplest case, if your function doesn’t return any references (or other types that require lifetime parameters), then you never need to write out lifetimes for your parameters. Rust just assigns a distinct lifetime to each spot that needs one. For example:

struct S<'a, 'b> {
    x: &'a i32,
    y: &'b i32
}

fn sum_r_xy(r: &i32, s: S) -> i32 {
    r + s.x + s.y
}

This function’s signature is shorthand for:

fn sum_r_xy<'a, 'b, 'c>(r: &'a i32, s: S<'b, 'c>) -> i32

If you do return references or other types with lifetime parameters, Rust still tries to make the unambiguous cases easy. If there’s only a single lifetime that appears among your function’s parameters, then Rust assumes any lifetimes in your return value must be that one:

fn first_third(point: &[i32; 3]) -> (&i32, &i32) {
    (&point[0], &point[2])
}

With all the lifetimes written out, the equivalent would be:

fn first_third<'a>(point: &'a [i32; 3]) -> (&'a i32, &'a i32)

If there are multiple lifetimes among your parameters, then there’s no natural reason to prefer one over the other for the return value, and Rust makes you spell out what’s going on.

But as one final shorthand, if your function is a method on some type and takes its self parameter by reference, then that breaks the tie: Rust assumes that self’s lifetime is the one to give everything in your return value. (A self parameter refers to the value the method is being called on, Rust’s equivalent of this in C++, Java, or JavaScript, or self in Python. We’ll cover methods in “Defining Methods with impl”.)

For example, you can write the following:

struct StringTable {
    elements: Vec<String>,
}

impl StringTable {
    fn find_by_prefix(&self, prefix: &str) -> Option<&String> {
        for i in 0 .. self.elements.len() {
            if self.elements[i].starts_with(prefix) {
                return Some(&self.elements[i]);
            }
        }
        None
    }
}

The find_by_prefix method’s signature is shorthand for:

fn find_by_prefix<'a, 'b>(&'a self, prefix: &'b str) -> Option<&'a String>

Rust assumes that whatever you’re borrowing, you’re borrowing from self.

Again, these are just abbreviations, meant to be helpful without introducing surprises. When they’re not what you want, you can always write the lifetimes out explicitly.

Sharing Versus Mutation

So far, we’ve discussed how Rust ensures no reference will ever point to a variable that has gone out of scope. But there are other ways to introduce dangling pointers. Here’s an easy case:

let v = vec![4, 8, 19, 27, 34, 10];
let r = &v;
let aside = v;  // move vector to aside
r[0];           // bad: uses `v`, which is now uninitialized

The assignment to aside moves the vector, leaving v uninitialized, turning r into a dangling pointer, as shown in Figure 5-7.

Figure 5-7. A reference to a vector that has been moved away

Although v stays in scope for r’s entire lifetime, the problem here is that v’s value gets moved elsewhere, leaving v uninitialized while r still refers to it. Naturally, Rust catches the error:

error[E0505]: cannot move out of `v` because it is borrowed
  --> references_sharing_vs_mutation_1.rs:10:9
   |
9  |     let r = &v;
   |              - borrow of `v` occurs here
10 |     let aside = v;  // move vector to aside
   |         ^^^^^ move out of `v` occurs here

Throughout its lifetime, a shared reference makes its referent read-only: you may not assign to the referent or move its value elsewhere. In this code, r’s lifetime contains the attempt to move the vector, so Rust rejects the program. If you change the program as shown here, there’s no problem:

let v = vec![4, 8, 19, 27, 34, 10];
{
    let r = &v;
    r[0];       // ok: vector is still there
}
let aside = v;

In this version, r goes out of scope earlier, the reference’s lifetime ends before v is moved aside, and all is well.

Here’s a different way to wreak havoc. Suppose we have a handy function to extend a vector with the elements of a slice:

fn extend(vec: &mut Vec<f64>, slice: &[f64]) {
    for elt in slice {
        vec.push(*elt);
    }
}

This is a less flexible (and much less optimized) version of the standard library’s extend_from_slice method on vectors. We can use it to build up a vector from slices of other vectors or arrays:

let mut wave = Vec::new();
let head = vec![0.0, 1.0];
let tail = [0.0, -1.0];

extend(&mut wave, &head);   // extend wave with another vector
extend(&mut wave, &tail);   // extend wave with an array

assert_eq!(wave, vec![0.0, 1.0, 0.0, -1.0]);

So we’ve built up one period of a sine wave here. If we want to add another undulation, can we append the vector to itself?

extend(&mut wave, &wave);
assert_eq!(wave, vec![0.0, 1.0, 0.0, -1.0,
                      0.0, 1.0, 0.0, -1.0]);

This may look fine on casual inspection. But remember that when we add an element to a vector, if its buffer is full, it must allocate a new buffer with more space. Suppose wave starts with space for four elements, and so must allocate a larger buffer when extend tries to add a fifth. Memory ends up looking like Figure 5-8.

Figure 5-8. A slice turned into a dangling pointer by a vector reallocation

The extend function’s vec argument borrows wave (owned by the caller), which has allocated itself a new buffer with space for eight elements. But slice continues to point to the old four-element buffer, which has been dropped.

This sort of problem isn’t unique to Rust: modifying collections while pointing into them is delicate territory in many languages. In C++, the std::vector specification cautions you that “reallocation [of the vector’s buffer] invalidates all the references, pointers, and iterators referring to the elements in the sequence.” Similarly, Java says, of modifying a java.util.Hashtable object:

[I]f the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException.

What’s especially difficult about this sort of bug is that it doesn’t happen all the time. In testing, your vector might always happen to have enough space, the buffer might never be reallocated, and the problem might never come to light.

Rust, however, reports the problem with our call to extend at compile time:

error[E0502]: cannot borrow `wave` as immutable because it is also borrowed as mutable
 --> references_sharing_vs_mutation_2.rs:9:24
  |
9 |     extend(&mut wave, &wave);
  |                 ----   ^^^^- mutable borrow ends here
  |                 |      |
  |                 |      immutable borrow occurs here
  |                 mutable borrow occurs here

In other words, we may borrow a mutable reference to the vector, and we may borrow a shared reference to its elements, but those two references’ lifetimes may not overlap. In our case, both references’ lifetimes contain the call to extend, so Rust rejects the code.

These errors both stem from violations of Rust’s rules for mutation and sharing:

Shared access is read-only access. Values borrowed by shared references are read-only. Across the lifetime of a shared reference, neither its referent, nor anything reachable from that referent, can be changed by anything. There exist no live mutable references to anything in that structure; its owner is held read-only; and so on. It’s really frozen.

Mutable access is exclusive access. A value borrowed by a mutable reference is reachable exclusively via that reference. Across the lifetime of a mutable reference, there is no other usable path to its referent, or to any value reachable from there. The only references whose lifetimes may overlap with a mutable reference are those you borrow from the mutable reference itself.

Rust reported the extend example as a violation of the second rule: since we’ve borrowed a mutable reference to wave, that mutable reference must be the only way to reach the vector or its elements. The shared reference to the slice is itself another way to reach the elements, violating the second rule.

But Rust could also have treated our bug as a violation of the first rule: since we’ve borrowed a shared reference to wave’s elements, the elements and the Vec itself are all read-only. You can’t borrow a mutable reference to a read-only value.

Each kind of reference affects what we can do with the values along the owning path to the referent, and the values reachable from the referent (Figure 5-9).

Figure 5-9. Borrowing a reference affects what you can do with other values in the same ownership tree

Note that in both cases, the path of ownership leading to the referent cannot be changed for the reference’s lifetime. For a shared borrow, the path is read-only; for a mutable borrow, it’s completely inaccessible. So there’s no way for the program to do anything that will invalidate the reference.

Paring these principles down to the simplest possible examples:

let mut x = 10;
let r1 = &x;
let r2 = &x;     // ok: multiple shared borrows permitted
x += 10;         // error: cannot assign to `x` because it is borrowed
let m = &mut x;  // error: cannot borrow `x` as mutable because it is
                 // also borrowed as immutable

let mut y = 20;
let m1 = &mut y;
let m2 = &mut y;  // error: cannot borrow as mutable more than once
let z = y;        // error: cannot use `y` because it was mutably borrowed

It is OK to reborrow a shared reference from a shared reference:

let mut w = (107, 109);
let r = &w;
let r0 = &r.0;         // ok: reborrowing shared as shared
let m1 = &mut r.1;     // error: can't reborrow shared as mutable

You can reborrow from a mutable reference:

let mut v = (136, 139);
let m = &mut v;
let m0 = &mut m.0;      // ok: reborrowing mutable from mutable
*m0 = 137;
let r1 = &m.1;          // ok: reborrowing shared from mutable,
                        // and doesn't overlap with m0
v.1;                    // error: access through other paths still forbidden

These restrictions are pretty tight. Turning back to our attempted call extend(&mut wave, &wave), there’s no quick and easy way to fix up the code to work the way we’d like. And Rust applies these rules everywhere: if we borrow, say, a shared reference to a key in a HashMap, we can’t borrow a mutable reference to the HashMap until the shared reference’s lifetime ends.

But there’s good justification for this: designing collections to support unrestricted, simultaneous iteration and modification is difficult, and often precludes simpler, more efficient implementations. Java’s Hashtable and C++’s vector don’t bother, and neither Python dictionaries nor JavaScript objects define exactly how such access behaves. Other collection types in JavaScript do, but require heavier implementations as a result. C++’s std::map promises that inserting new entries doesn’t invalidate pointers to other entries in the map, but by making that promise, the standard precludes more cache-efficient designs like Rust’s BTreeMap, which stores multiple entries in each node of the tree.

Here’s another example of the kind of bug these rules catch. Consider the following C++ code, meant to manage a file descriptor. To keep things simple, we’re only going to show a constructor and a copying assignment operator, and we’re going to omit error handling:

struct File {
  int descriptor;

  File(int d) : descriptor(d) { }

  File& operator=(const File &rhs) {
    close(descriptor);
    descriptor = dup(rhs.descriptor);
  }
};

The assignment operator is simple enough, but fails badly in a situation like this:

File f(open("foo.txt", ...));
...
f = f;

If we assign a File to itself, both rhs and *this are the same object, so operator= closes the very file descriptor it’s about to pass to dup. We destroy the same resource we were meant to copy.

In Rust, the analogous code would be:

struct File {
    descriptor: i32
}

fn new_file(d: i32) -> File {
    File { descriptor: d }
}

fn clone_from(this: &mut File, rhs: &File) {
    close(this.descriptor);
    this.descriptor = dup(rhs.descriptor);
}

(This is not idiomatic Rust. There are excellent ways to give Rust types their own constructor functions and methods, which we describe in Chapter 9, but the preceding definitions work for this example.)

If we write the Rust code corresponding to the use of File, we get:

let mut f = new_file(open("foo.txt", ...));
...
clone_from(&mut f, &f);

Rust, of course, refuses to even compile this code:

error[E0502]: cannot borrow `f` as immutable because it is also
borrowed as mutable
  --> references_self_assignment.rs:18:25
   |
18 |     clone_from(&mut f, &f);
   |                     -   ^- mutable borrow ends here
   |                     |   |
   |                     |   immutable borrow occurs here
   |                     mutable borrow occurs here

This should look familiar. It turns out that two classic C++ bugs—failure to cope with self-assignment, and using invalidated iterators—are the same underlying kind of bug! In both cases, code assumes it is modifying one value while consulting another, when in fact they’re both the same value. If you’ve ever accidentally let the source and destination of a call to memcpy or strcpy call overlap in C or C++, that’s yet another form the bug can take. By requiring mutable access to be exclusive, Rust has fended off a wide class of everyday mistakes.

The immiscibility of shared and mutable references really demonstrates its value when writing concurrent code. A data race is possible only when some value is both mutable and shared between threads—which is exactly what Rust’s reference rules eliminate. A concurrent Rust program that avoids unsafe code is free of data races by construction. We’ll cover this aspect in more detail when we talk about concurrency in Chapter 19, but in summary, concurrency is much easier to use in Rust than in most other languages.

Rust’s Shared References Versus C’s Pointers to const

On first inspection, Rust’s shared references seem to closely resemble C and C++’s pointers to const values. However, Rust’s rules for shared references are much stricter. For example, consider the following C code:

int x = 42;             // int variable, not const
const int *p = &x;      // pointer to const int
assert(*p == 42);
x++;                    // change variable directly
assert(*p == 43);       // “constant” referent's value has changed

The fact that p is a const int * means that you can’t modify its referent via p itself: (*p)++ is forbidden. But you can also get at the referent directly as x, which is not const, and change its value that way. The C family’s const keyword has its uses, but constant it is not.

In Rust, a shared reference forbids all modifications to its referent, until its lifetime ends:

let mut x = 42;         // nonconst i32 variable
let p = &x;             // shared reference to i32
assert_eq!(*p, 42);
x += 1;                 // error: cannot assign to x because it is borrowed
assert_eq!(*p, 42);     // if you take out the assignment, this is true

To ensure a value is constant, we need to keep track of all possible paths to that value, and make sure that they either don’t permit modification or cannot be used at all. C and C++ pointers are too unrestricted for the compiler to check this. Rust’s references are always tied to a particular lifetime, making it feasible to check them at compile time.

Taking Arms Against a Sea of Objects

Since the rise of automatic memory management in the 1990s, the default architecture of all programs has been the sea of objects, shown in Figure 5-10.

This is what happens if you have garbage collection and you start writing a program without designing anything. We’ve all built systems that look like this.

This architecture has many advantages that don’t show up in the diagram: initial progress is rapid, it’s easy to hack stuff in, and a few years down the road, you’ll have no difficulty justifying a complete rewrite. (Cue AC/DC’s “Highway to Hell.”)

Figure 5-10. A sea of objects

Of course, there are disadvantages too. When everything depends on everything else like this, it’s hard to test, evolve, or even think about any component in isolation.

One fascinating thing about Rust is that the ownership model puts a speed bump on the highway to hell. It takes a bit of effort to make a cycle in Rust—two values such that each one contains a reference pointing to the other. You have to use a smart pointer type, such as Rc, and interior mutability—a topic we haven’t even covered yet. Rust prefers for pointers, ownership, and data flow to pass through the system in one direction, as shown in Figure 5-11.

Figure 5-11. A tree of values

The reason we bring this up right now is that it would be natural, after reading this chapter, to want to run right out and create a “sea of structs,” all tied together with Rc smart pointers, and re-create all the object-oriented antipatterns you’re familiar with. This won’t work for you right away. Rust’s ownership model will give you some trouble. The cure is to do some up-front design and build a better program.

Rust is all about transferring the pain of understanding your program from the future to the present. It works unreasonably well: not only can Rust force you to understand why your program is thread-safe, it can even require some amount of high-level architectural design.

Chapter 6. Expressions

LISP programmers know the value of everything, but the cost of nothing.

Alan Perlis, epigram #55

In this chapter, we’ll cover the expressions of Rust, the building blocks that make up the body of Rust functions. A few concepts, such as closures and iterators, are deep enough that we will dedicate a whole chapter to them later on. For now, we aim to cover as much syntax as possible in a few pages.

An Expression Language

Rust visually resembles the C family of languages, but this is a bit of a ruse. In C, there is a sharp distinction between expressions, bits of code that look something like this:

5 * (fahr-32) / 9

and statements, which look more like this:

for (; begin != end; ++begin) {
    if (*begin == target)
        break;
}

Expressions have values. Statements don’t.

Rust is what is called an expression language. This means it follows an older tradition, dating back to Lisp, where expressions do all the work.

In C, if and switch are statements. They don’t produce a value, and they can’t be used in the middle of an expression. In Rust, if and match can produce values. We already saw a match expression that produces a numeric value in Chapter 2:

pixels[r * bounds.0 + c] =
    match escapes(Complex { re: point.0, im: point.1 }, 255) {
        None => 0,
        Some(count) => 255 - count as u8
    };

An if expression can be used to initialize a variable:

let status =
    if cpu.temperature <= MAX_TEMP {
        HttpStatus::Ok
    } else {
        HttpStatus::ServerError  // server melted
    };

A match expression can be passed as an argument to a function or macro:

println!("Inside the vat, you see {}.",
    match vat.contents {
        Some(brain) => brain.desc(),
        None => "nothing of interest"
    });

This explains why Rust does not have C’s ternary operator (expr1 ? expr2 : expr3). In C, it is a handy expression-level analogue to the if statement. It would be redundant in Rust: the if expression handles both cases.

Most of the control flow tools in C are statements. In Rust, they are all expressions.

Blocks and Semicolons

Blocks, too, are expressions. A block produces a value and can be used anywhere a value is needed:

let display_name = match post.author() {
    Some(author) => author.name(),
    None => {
        let network_info = post.get_network_metadata()?;
        let ip = network_info.client_address();
        ip.to_string()
    }
};

The code after Some(author) => is the simple expression author.name(). The code after None => is a block expression. It makes no difference to Rust. The value of the block is the value of its last expression, ip.to_string().

Note that there is no semicolon after that expression. Most lines of Rust code do end with either a semicolon or curly braces, just like C or Java. And if a block looks like C code, with semicolons in all the familiar places, then it will run just like a C block, and its value will be (). As we mentioned in Chapter 2, when you leave the semicolon off the last line of a block, you’re making that block produce a value—the value of the final expression.

In some languages, particularly JavaScript, you’re allowed to omit semicolons, and the language simply fills them in for you—a minor convenience. This is different. In Rust, the semicolon actually means something.

let msg = {
    // let-declaration: semicolon is always required
    let dandelion_control = puffball.open();

    // expression + semicolon: method is called, return value dropped
    dandelion_control.release_all_seeds(launch_codes);

    // expression with no semicolon: method is called,
    // return value stored in `msg`
    dandelion_control.get_status()
};

This ability of blocks to contain declarations and also produce a value at the end is a neat feature, one that quickly comes to feel natural. The one drawback is that it leads to an odd error message when you leave out a semicolon by accident.

...
if preferences.changed() {
    page.compute_size()  // oops, missing semicolon
}
...

If you made this mistake in a C or Java program, the compiler would simply point out that you’re missing a semicolon. Here’s what Rust says:

error[E0308]: mismatched types
  --> expressions_missing_semicolon.rs:19:9
   |
19 |         page.compute_size()  // oops, missing semicolon
   |         ^^^^^^^^^^^^^^^^^^^ expected (), found tuple
   |
   = note: expected type `()`
              found type `(u32, u32)`

Rust assumes you’ve omitted this semicolon on purpose; it doesn’t consider the possibility that it’s just a typo. A confused error message is the result. When you see expected type `()`, look for a missing semicolon first.

Empty statements are also allowed in blocks. An empty statement consists of a stray semicolon, all by itself:

loop {
    work();
    play();
    ;        // <-- empty statement
}

Rust follows the tradition of C in allowing this. Empty statements do nothing except convey a slight feeling of melancholy. We mention them only for completeness.

Declarations

In addition to expressions and semicolons, a block may contain any number of declarations. The most common are let declarations, which declare local variables:

let name: type = expr;

The type and initializer are optional. The semicolon is required.

A let declaration can declare a variable without initializing it. The variable can then be initialized with a later assignment. This is occasionally useful, because sometimes a variable should be initialized from the middle of some sort of control flow construct:

let name;
if user.has_nickname() {
    name = user.nickname();
} else {
    name = generate_unique_name();
    user.register(&name);
}

Here there are two different ways the local variable name might be initialized, but either way it will be initialized exactly once, so name does not need to be declared mut.

It’s an error to use a variable before it’s initialized. (This is closely related to the error of using a value after it’s been moved. Rust really wants you to use values only while they exist!)

You may occasionally see code that seems to redeclare an existing variable, like this:

for line in file.lines() {
    let line = line?;
    ...
}

This is equivalent to:

for line_result in file.lines() {
    let line = line_result?;
    ...
}

The let declaration creates a new, second variable, of a different type. The type of line_result is Result<String, io::Error>. The second variable, line, is a String. It’s legal to give the second variable the same name as the first. In this book, we’ll stick to using a _result suffix in such situations, so that all variables have distinct names.

A block can also contain item declarations. An item is simply any declaration that could appear globally in a program or module, such as a fn, struct, or use.

Later chapters will cover items in detail. For now, fn makes a sufficient example. Any block may contain a fn:

use std::io;
use std::cmp::Ordering;

fn show_files() -> io::Result<()> {
    let mut v = vec![];
    ...

    fn cmp_by_timestamp_then_name(a: &FileInfo, b: &FileInfo) -> Ordering {
        a.timestamp.cmp(&b.timestamp)   // first, compare timestamps
            .reverse()                  // newest file first
            .then(a.path.cmp(&b.path))  // compare paths to break ties
    }

    v.sort_by(cmp_by_timestamp_then_name);
    ...
}

When a fn is declared inside a block, its scope is the entire block—that is, it can be used throughout the enclosing block. But a nested fn cannot access local variables or arguments that happen to be in scope. For example, the function cmp_by_timestamp_then_name could not use v directly. (Rust also has closures, which do see into enclosing scopes. See Chapter 14.)

A block can even contain a whole module. This may seem a bit much—do we really need to be able to nest every piece of the language inside every other piece?—but programmers (and particularly programmers using macros) have a way of finding uses for every scrap of orthogonality the language provides.

if and match

The form of an if expression is familiar:

if condition1 {
    block1
} else if condition2 {
    block2
} else {
    block_n
}

Each condition must be an expression of type bool; true to form, Rust does not implicitly convert numbers or pointers to Boolean values.

Unlike C, parentheses are not required around conditions. In fact, rustc will emit a warning if unnecessary parentheses are present. The curly braces, however, are required.

The else if blocks, as well as the final else, are optional. An if expression with no else block behaves exactly as though it had an empty else block.

match expressions are something like the C switch statement, but more flexible. A simple example:

match code {
    0 => println!("OK"),
    1 => println!("Wires Tangled"),
    2 => println!("User Asleep"),
    _ => println!("Unrecognized Error {}", code)
}

This is something a switch statement could do. Exactly one of the four arms of this match expression will execute, depending on the value of code. The wildcard pattern _ matches everything, so it serves as the default: case.

The compiler can optimize this kind of match using a jump table, just like a switch statement in C++. A similar optimization is applied when each arm of a match produces a constant value. In that case, the compiler builds an array of those values, and the match is compiled into an array access. Apart from a bounds check, there is no branching at all in the compiled code.

The versatility of match stems from the variety of supported patterns that can be used to the left of => in each arm. Above, each pattern is simply a constant integer. We’ve also shown match expressions that distinguish the two kinds of Option value:

match params.get("name") {
    Some(name) => println!("Hello, {}!", name),
    None => println!("Greetings, stranger.")
}

This is only a hint of what patterns can do. A pattern can match a range of values. It can unpack tuples. It can match against individual fields of structs. It can chase references, borrow parts of a value, and more. Rust’s patterns are a mini-language of their own. We’ll dedicate several pages to them in Chapter 10.

The general form of a match expression is:

match value {
    pattern => expr,
    ...
}

The comma after an arm may be dropped if the expr is a block.

Rust checks the given value against each pattern in turn, starting with the first. When a pattern matches, the corresponding expr is evaluated and the match expression is complete; no further patterns are checked. At least one of the patterns must match. Rust prohibits match expressions that do not cover all possible values:

let score = match card.rank {
    Jack => 10,
    Queen => 10,
    Ace => 11
};  // error: nonexhaustive patterns

All blocks of an if expression must produce values of the same type:

let suggested_pet =
    if with_wings { Pet::Buzzard } else { Pet::Hyena };  // ok

let favorite_number =
    if user.is_hobbit() { "eleventy-one" } else { 9 };  // error

let best_sports_team =
    if is_hockey_season() { "Predators" };  // error

(The last example is an error because in July, the result would be ().)

Similarly, all arms of a match expression must have the same type:

let suggested_pet =
    match favorites.element {
        Fire => Pet::RedPanda,
        Air => Pet::Buffalo,
        Water => Pet::Orca,
        _ => None  // error: incompatible types
    };

if let

There is one more if form, the if let expression:

if let pattern = expr {
    block1
} else {
    block2
}

The given expr either matches the pattern, in which case block1 runs, or it doesn’t, and block2 runs. Sometimes this is a nice way to get data out of an Option or Result:

if let Some(cookie) = request.session_cookie {
    return restore_session(cookie);
}

if let Err(err) = present_cheesy_anti_robot_task() {
    log_robot_attempt(err);
    politely_accuse_user_of_being_a_robot();
} else {
    session.mark_as_human();
}

It’s never strictly necessary to use if let, because match can do everything if let can do. An if let expression is shorthand for a match with just one pattern:

match expr {
    pattern => { block1 }
    _ => { block2 }
}

Loops

There are four looping expressions:

while condition {
    block
}

while let pattern = expr {
    block
}

loop {
    block
}

for pattern in collection {
    block
}

Loops are expressions in Rust, but they don’t produce useful values. The value of a loop is ().

A while loop behaves exactly like the C equivalent, except that again, the condition must be of the exact type bool.

The while let loop is analogous to if let. At the beginning of each loop iteration, the value of expr either matches the given pattern, in which case the block runs, or it doesn’t, in which case the loop exits.

Use loop to write infinite loops. It executes the block repeatedly forever (or until a break or return is reached, or the thread panics).

A for loop evaluates the collection expression, then evaluates the block once for each value in the collection. Many collection types are supported. The standard C for loop:

for (int i = 0; i < 20; i++) {
    printf("%d\n", i);
}

is written like this in Rust:

for i in 0..20 {
    println!("{}", i);
}

As in C, the last number printed is 19.

The .. operator produces a range, a simple struct with two fields: start and end. 0..20 is the same as std::ops::Range { start: 0, end: 20 }. Ranges can be used with for loops because Range is an iterable type: it implements the std::iter::IntoIterator trait, which we’ll discuss in Chapter 15. The standard collections are all iterable, as are arrays and slices.

In keeping with Rust’s move semantics, a for loop over a value consumes the value:

let strings: Vec<String> = error_messages();
for s in strings {                  // each String is moved into s here...
    println!("{}", s);
}                                   // ...and dropped here
println!("{} error(s)", strings.len()); // error: use of moved value

This can be inconvenient. The easy remedy is to loop over a reference to the collection instead. The loop variable, then, will be a reference to each item in the collection:

for rs in &strings {
    println!("String {:?} is at address {:p}.", *rs, rs);
}

Here the type of &strings is &Vec<String> and the type of rs is &String.

Iterating over a mut reference provides a mut reference to each element:

for rs in &mut strings {  // the type of rs is &mut String
    rs.push('\n');  // add a newline to each string
}

Chapter 15 covers for loops in greater detail and shows many other ways to use iterators.

A break expression exits an enclosing loop. (In Rust, break works only in loops. It is not necessary in match expressions, which are unlike switch statements in this regard.)

A continue expression jumps to the next loop iteration:

// Read some data, one line at a time.
for line in input_lines {
    let trimmed = trim_comments_and_whitespace(line);
    if trimmed.is_empty() {
        // Jump back to the top of the loop and
        // move on to the next line of input.
        continue;
    }
    ...
}

In a for loop, continue advances to the next value in the collection. If there are no more values, the loop exits. Similarly, in a while loop, continue rechecks the loop condition. If it’s now false, the loop exits.

A loop can be labeled with a lifetime. In the following example, 'search: is a label for the outer for loop. Thus break 'search exits that loop, not the inner loop.

'search:
for room in apartment {
    for spot in room.hiding_spots() {
        if spot.contains(keys) {
            println!("Your keys are {} in the {}.", spot, room);
            break 'search;
        }
    }
}

Labels can also be used with continue.

return Expressions

A return expression exits the current function, returning a value to the caller.

return without a value is shorthand for return ():

fn f() {     // return type omitted: defaults to ()
    return;  // return value omitted: defaults to ()
}

Like a break expression, return can abandon work in progress. For example, back in Chapter 2, we used the ? operator to check for errors after calling a function that can fail:

let output = File::create(filename)?;

and we explained that this is shorthand for a match expression:

let output = match File::create(filename) {
    Ok(f) => f,
    Err(err) => return Err(err)
};

This code starts by calling File::create(filename). If that returns Ok(f), then the whole match expression evaluates to f, so f is stored in output and we continue with the next line of code following the match.

Otherwise, we’ll match Err(err) and hit the return expression. When that happens, it doesn’t matter that we’re in the middle of evaluating a match expression to determine the value of the variable output. We abandon all of that and exit the enclosing function, returning whatever error we got from File::create().

We’ll cover the ? operator more completely in “Propagating Errors”.

Why Rust Has loop

Several pieces of the Rust compiler analyze the flow of control through your program.

Rust checks that every path through a function returns a value of the expected return type. To do this correctly, it needs to know whether or not it’s possible to reach the end of the function.

Rust checks that local variables are never used uninitialized. This entails checking every path through a function to make sure there’s no way to reach a place where a variable is used without having already passed through code that initializes it.

Rust warns about unreachable code. Code is unreachable if no path through the function reaches it.

These are called flow-sensitive analyses. They are nothing new; Java has had a “definite assignment” analysis, similar to Rust’s, for years.

When enforcing this sort of rule, a language must strike a balance between simplicity, which makes it easier for programmers to figure out what the compiler is talking about sometimes—and cleverness, which can help eliminate false warnings and cases where the compiler rejects a perfectly safe program. Rust went for simplicity. Its flow-sensitive analyses do not examine loop conditions at all, instead simply assuming that any condition in a program can be either true or false.

This causes Rust to reject some safe programs:

fn wait_for_process(process: &mut Process) -> i32 {
    while true {
        if process.wait() {
            return process.exit_code();
        }
    }
}  // error: not all control paths return a value

The error here is bogus. It is not actually possible to reach the end of the function without returning a value.

The loop expression is offered as a “say-what-you-mean” solution to this problem.

Rust’s type system is affected by control flow, too. Earlier we said that all branches of an if expression must have the same type. But it would be silly to enforce this rule on blocks that end with a break or return expression, an infinite loop, or a call to panic!() or std::process:exit(). What all those expressions have in common is that they never finish in the usual way, producing a value. A break or return exits the current block abruptly; an infinite loop never finishes at all; and so on.

So in Rust, these expressions don’t have a normal type. Expressions that don’t finish normally are assigned the special type !, and they’re exempt from the rules about types having to match. You can see ! in the function signature of std::process::exit():

fn exit(code: i32) -> !

The ! means that exit() never returns. It’s a divergent function.

You can write divergent functions of your own using the same syntax, and this is perfectly natural in some cases:

fn serve_forever(socket: ServerSocket, handler: ServerHandler) -> ! {
    socket.listen();
    loop {
        let s = socket.accept();
        handler.handle(s);
    }
}

Of course, Rust then considers it an error if the function can return normally.

This concludes the part of this chapter that focuses on control flow. The rest covers Rust functions, methods, and operators.

Function and Method Calls

The syntax for calling functions and methods is the same in Rust as in many other languages:

let x = gcd(1302, 462);  // function call

let room = player.location();  // method call

In the second example here, player is a variable of the made-up type Player, which has a made-up .location() method. (We’ll show how to define your own methods when we start talking about user-defined types in Chapter 9.)

Rust usually makes a sharp distinction between references and the values they refer to. If you pass a &i32 to a function that expects an i32, that’s a type error. You’ll notice that the . operator relaxes those rules a bit. In the method call player.location(), player might be a Player, a reference of type &Player, or a smart pointer of type Box<Player> or Rc<Player>. The .location() method might take the player either by value or by reference. The same .location() syntax works in all cases, because Rust’s . operator automatically dereferences player or borrows a reference to it as needed.

A third syntax is used for calling static methods, like Vec::new().

let mut numbers = Vec::new();  // static method call

The difference between static and nonstatic methods is the same as in object-oriented languages: nonstatic methods are called on values (like my_vec.len()), and static methods are called on types (like Vec::new()).

Naturally, method calls can be chained:

Iron::new(router).http("localhost:3000").unwrap();

One quirk of Rust syntax is that in a function call or method call, the usual syntax for generic types, Vec<T>, does not work:

return Vec<i32>::with_capacity(1000);  // error: something about chained comparisons

let ramp = (0 .. n).collect<Vec<i32>>();  // same error

The problem is that in expressions, < is the less-than operator. The Rust compiler helpfully suggests writing ::<T> instead of <T> in this case, and that solves the problem:

return Vec::<i32>::with_capacity(1000);  // ok, using ::<

let ramp = (0 .. n).collect::<Vec<i32>>();  // ok, using ::<

The symbol ::<...> is affectionately known in the Rust community as the turbofish.

Alternatively, it is often possible to drop the type parameters and let Rust infer them:

return Vec::with_capacity(10);  // ok, if the fn return type is Vec<i32>

let ramp: Vec<i32> = (0 .. n).collect();  // ok, variable's type is given

It’s considered good style to omit the types whenever they can be inferred.

Fields and Elements

The fields of a struct are accessed using familiar syntax. Tuples are the same except that their fields have numbers rather than names:

game.black_pawns   // struct field
coords.1           // tuple element

If the value to the left of the dot is a reference or smart pointer type, it is automatically dereferenced, just as for method calls.

Square brackets access the elements of an array, a slice, or a vector:

pieces[i]          // array element

The value to the left of the brackets is automatically dereferenced.

Expressions like these three are called lvalues, because they can appear on the left side of an assignment:

game.black_pawns = 0x00ff0000_00000000_u64;
coords.1 = 0;
pieces[2] = Some(Piece::new(Black, Knight, coords));

Of course, this is permitted only if game, coords, and pieces are declared as mut variables.

Extracting a slice from an array or vector is straightforward:

let second_half = &game_moves[midpoint .. end];

Here game_moves may be either an array, a slice, or a vector; the result, regardless, is a borrowed slice of length end - midpoint. game_moves is considered borrowed for the lifetime of second_half.

The .. operator allows either operand to be omitted; it produces up to four different types of object depending on which operands are present:

..      // RangeFull
a ..    // RangeFrom { start: a }
.. b    // RangeTo { end: b }
a .. b  // Range { start: a, end: b }

Rust ranges are half-open: they include the start value, if any, but not the end value. The range 0 .. 4 includes the numbers 0, 1, 2, and 3.

Only ranges that include a start value are iterable, since a loop must have somewhere to start. But in array slicing, all four forms are useful. If the start or end of the range is omitted, it defaults to the start or end of the data being sliced.

So an implementation of quicksort, the classic divide-and-conquer sorting algorithm, might look, in part, like this:

fn quicksort<T: Ord>(slice: &mut [T]) {
    if slice.len() <= 1 {
        return;  // Nothing to sort.
    }

    // Partition the slice into two parts, front and back.
    let pivot_index = partition(slice);

    // Recursively sort the front half of `slice`.
    quicksort(&mut slice[.. pivot_index]);

    // And the back half.
    quicksort(&mut slice[pivot_index + 1 ..]);
}

Reference Operators

The address-of operators, & and &mut, are covered in Chapter 5.

The unary * operator is used to access the value pointed to by a reference. As we’ve seen, Rust automatically follows references when you use the . operator to access a field or method, so the * operator is necessary only when we want to read or write the entire value that the reference points to.

For example, sometimes an iterator produces references, but the program needs the underlying values:

let padovan: Vec<u64> = compute_padovan_sequence(n);
for elem in &padovan {
    draw_triangle(turtle, *elem);
}

In this example, the type of elem is &u64, so *elem is a u64.

Arithmetic, Bitwise, Comparison, and Logical Operators

Rust’s binary operators are like those in many other languages. To save time, we assume familiarity with one of those languages, and focus on the few points where Rust departs from tradition.

Rust has the usual arithmetic operators, +, -, *, /, and %. As mentioned in Chapter 3, integer overflow is detected, and causes a panic, in debug builds. The standard library provides methods like a.wrapping_add(b) for unchecked arithmetic.

Dividing an integer by zero triggers a panic even in release builds. Integers have a method a.checked_div(b) that returns an Option (None if b is zero) and never panics.

Unary - negates a number. It is supported for all the numeric types except unsigned integers. There is no unary + operator.

println!("{}", -100);     // -100
println!("{}", -100u32);  // error: can't apply unary `-` to type `u32`
println!("{}", +100);     // error: expected expression, found `+`

As in C, a % b computes the remainder, or modulus, of division. The result has the same sign as the lefthand operand. Note that % can be used on floating-point numbers as well as integers:

let x = 1234.567 % 10.0;  // approximately 4.567

Rust also inherits C’s bitwise integer operators, &, |, ^, <<, and >>. However, Rust uses ! instead of ~ for bitwise NOT:

let hi: u8 = 0xe0;
let lo = !hi;  // 0x1f

This means that !n can’t be used on an integer n to mean “n is zero.” For that, write n == 0.

Bit shifting is always sign-extending on signed integer types and zero-extending on unsigned integer types. Since Rust has unsigned integers, it does not need Java’s >>> operator.

Bitwise operations have higher precedence than comparisons, unlike C, so if you write x & BIT != 0, that means (x & BIT) != 0, as you probably intended. This is much more useful than C’s interpretation, x & (BIT != 0), which tests the wrong bit!

Rust’s comparison operators are ==, !=, <, <=, >, and >=. The two values being compared must have the same type.

Rust also has the two short-circuiting logical operators && and ||. Both operands must have the exact type bool.

Assignment

The = operator can be used to assign to mut variables and their fields or elements. But assignment is not as common in Rust as in other languages, since variables are immutable by default.

As described in Chapter 4, assignment moves values of noncopyable types, rather than implicitly copying them.

Compound assignment is supported:

total += item.price;

This is equivalent to total = total + item.price;. Other operators are supported too: –=, *=, and so forth. The full list is given in Table 6-1, at the end of this chapter.

Unlike C, Rust doesn’t support chaining assignment: you can’t write a = b = 3 to assign the value 3 to both a and b. Assignment is rare enough in Rust that you won’t miss this shorthand.

Rust does not have C’s increment and decrement operators ++ and --.

Type Casts

Converting a value from one type to another usually requires an explicit cast in Rust. Casts use the as keyword:

let x = 17;              // x is type i32
let index = x as usize;  // convert to usize

Several kinds of casts are permitted:

Numbers may be cast from any of the built-in numeric types to any other.

Casting an integer to another integer type is always well-defined. Converting to a narrower type results in truncation. A signed integer cast to a wider type is sign-extended; an unsigned integer is zero-extended; and so on. In short, there are no surprises.

However, as of this writing, casting a large floating-point value to an integer type that is too small to represent it can lead to undefined behavior. This can cause crashes even in safe Rust. It is a bug in the compiler, github.com/rust-lang/rust/issues/10184.

Values of type bool, char, or of a C-like enum type, may be cast to any integer type. (We’ll cover enums in Chapter 10.)

Casting in the other direction is not allowed, as bool, char, and enum types all have restrictions on their values that would have to be enforced with runtime checks. For example, casting a u16 to type char is banned because some u16 values, like 0xd800, correspond to Unicode surrogate code points and therefore would not make valid char values. There is a standard method, std::char::from_u32(), which performs the runtime check and returns an Option<char>; but more to the point, the need for this kind of conversion has grown rare. We typically convert whole strings or streams at once, and algorithms on Unicode text are often nontrivial and best left to libraries.

As an exception, a u8 may be cast to type char, since all integers from 0 to 255 are valid Unicode code points for char to hold.

Some casts involving unsafe pointer types are also allowed. See “Raw Pointers”.

We said that a conversion usually requires a cast. A few conversions involving reference types are so straightforward that the language performs them even without a cast. One trivial example is converting a mut reference to a non-mut reference.

Several more significant automatic conversions can happen, though:

Values of type &String auto-convert to type &str without a cast.

Values of type &Vec<i32> auto-convert to &[i32].

Values of type &Box<Chessboard> auto-convert to &Chessboard.

These are called deref coercions, because they apply to types that implement the Deref built-in trait. The purpose of Deref coercion is to make smart pointer types, like Box, behave as much like the underlying value as possible. Using a Box<Chessboard> is mostly just like using a plain Chessboard, thanks to Deref.

User-defined types can implement the Deref trait, too. When you need to write your own smart pointer type, see “Deref and DerefMut”.

Closures

Rust has closures, lightweight function-like values. A closure usually consists of an argument list, given between vertical bars, followed by an expression:

let is_even = |x| x % 2 == 0;

Rust infers the argument types and return type. You can also write them out explicitly, as you would for a function. If you do specify a return type, then the body of the closure must be a block, for the sake of syntactic sanity:

let is_even = |x: u64| -> bool x % 2 == 0;  // error

let is_even = |x: u64| -> bool { x % 2 == 0 };  // ok

Calling a closure uses the same syntax as calling a function:

assert_eq!(is_even(14), true);

Closures are one of Rust’s most delightful features, and there is a great deal more to be said about them. We shall say it in Chapter 14.

Precedence and Associativity

Table 6-1 gives a summary of Rust expression syntax. Operators are listed in order of precedence, from highest to lowest. (Like most programming languages, Rust has operator precedence to determine the order of operations when an expression contains multiple adjacent operators. For example, in limit < 2 * broom.size + 1, the . operator has the highest precedence, so the field access happens first.)

Table 6-1. Expressions

Expression type
Example
Related traits

Array literal
[1, 2, 3]

Repeat array literal
[0; 50]

Tuple
(6, "crullers")

Grouping
(2 + 2)

Block
{ f(); g() }

Control flow expressions
if ok { f() }

if ok { 1 } else { 0 }

if let Some(x) = f() { x } else { 0 }

match x { None => 0, _ => 1 }

for v in e { f(v); }
std::iter::IntoIterator

while ok { ok = f(); }

while let Some(x) = it.next() { f(x); }

loop { next_event(); }

break

continue

return 0

Macro invocation
println!("ok")

Path
std::f64::consts::PI

Struct literal
Point {x: 0, y: 0}

Tuple field access
pair.0
Deref, DerefMut

Struct field access
point.x
Deref, DerefMut

Method call
point.translate(50, 50)
Deref, DerefMut

Function call
stdin()

Fn(Arg0, ...) -> T,

FnMut(Arg0, ...) -> T,

FnOnce(Arg0, ...) -> T

Index
arr[0]

Index, IndexMut

Deref, DerefMut

Error check
create_dir("tmp")?

Logical/bitwise NOT
!ok
Not

Negation
-num
Neg

Dereference
*ptr
Deref, DerefMut

Borrow
&val

Type cast
x as u32

Multiplication
n * 2
Mul

Division
n / 2
Div

Remainder (modulus)
n % 2
Rem

Addition
n + 1
Add

Subtraction
n – 1
Sub

Left shift
n << 1
Shl

Right shift
n >> 1
Shr

Bitwise AND
n & 1
BitAnd

Bitwise exclusive OR
n ^ 1
BitXor

Bitwise OR
n | 1
BitOr

Less than
n < 1
std::cmp::PartialOrd

Less than or equal
n <= 1
std::cmp::PartialOrd

Greater than
n > 1
std::cmp::PartialOrd

Greater than or equal
n >= 1
std::cmp::PartialOrd

Equal
n == 1
std::cmp::PartialEq

Not equal
n != 1
std::cmp::PartialEq

Logical AND
x.ok && y.ok

Logical OR
x.ok || backup.ok

Range
start .. stop

Assignment
x = val

Compound assignment
x *= 1
MulAssign

x /= 1
DivAssign

x %= 1
RemAssign

x += 1
AddAssign

x –= 1
SubAssign

x <<= 1
ShlAssign

x >>= 1
ShrAssign

x &= 1
BitAndAssign

x ^= 1
BitXorAssign

x |= 1
BitOrAssign

Closure
|x, y| x + y

All of the operators that can usefully be chained are left-associative. That is, a chain of operations such as a – b – c is grouped as (a – b) – c, not a – (b – c). The operators that can be chained in this way are all the ones you might expect:

*   /   %   +   –   <<   >>   &   ^   |   &&   ||   as

The comparison operators, the assignment operators, and the range operator .. can’t be chained at all.

Onward

Expressions are what we think of as “running code.” They’re the part of a Rust program that compiles to machine instructions. Yet they are a small fraction of the whole language.

The same is true in most programming languages. The first job of a program is to run, but that’s not its only job. Programs have to communicate. They have to be testable. They have to stay organized and flexible, so that they can continue to evolve. They have to interoperate with code and services built by other teams. And even just to run, programs in a statically typed language like Rust need some more tools for organizing data than just tuples and arrays.

Coming up, we’ll spend several chapters talking about features in this area: modules and crates, which give your program structure, and then structs and enums, which do the same for your data.

First, we’ll dedicate a few pages to the important topic of what to do when things go wrong.

Chapter 7. Error Handling

I knew if I stayed around long enough, something like this would happen.

George Bernard Shaw on dying

Error handling in Rust is just different enough to warrant its own short chapter. There aren’t any difficult ideas here, just ideas that might be new to you. This chapter covers the two different kinds of error-handling in Rust: panic and Results.

Ordinary errors are handled using Results. These are typically caused by things outside the program, like erroneous input, a network outage, or a permissions problem. That such situations occur is not up to us; even a bug-free program will encounter them from time to time. Most of this chapter is dedicated to that kind of error. We’ll cover panic first, though, because it’s the simpler of the two.

Panic is for the other kind of error, the kind that should never happen.

Panic

A program panics when it encounters something so messed up that there must be a bug in the program itself. Something like:

Out-of-bounds array access

Integer division by zero

Calling .unwrap() on an Option that happens to be None

Assertion failure

(There’s also the macro panic!(), for cases where your own code discovers that it has gone wrong, and you therefore need to trigger a panic directly. panic!() accepts optional println!()-style arguments, for building an error message.)

What these conditions have in common is that they are all—not to put too fine a point on it—the programmer’s fault. A good rule of thumb is: “Don’t panic”.

But we all make mistakes. When these errors that shouldn’t happen, do happen—what then? Remarkably, Rust gives you a choice. Rust can either unwind the stack when a panic happens, or abort the process. Unwinding is the default.

Unwinding

When pirates divvy up the booty from a raid, the captain gets half of the loot. Ordinary crew members earn equal shares of the other half. (Pirates hate fractions, so if either division does not come out even, the result is rounded down, with the remainder going to the ship’s parrot.)

fn pirate_share(total: u64, crew_size: usize) -> u64 {
    let half = total / 2;
    half / crew_size as u64
}

This may work fine for centuries until one day it transpires that the captain is the sole survivor of a raid. If we pass a crew_size of zero to this function, it will divide by zero. In C++, this would be undefined behavior. In Rust, it triggers a panic, which typically proceeds as follows:

An error message is printed to the terminal:

thread 'main' panicked at 'attempt to divide by zero', pirates.rs:3780
note: Run with `RUST_BACKTRACE=1` for a backtrace.

If you set the RUST_BACKTRACE environment variable, as the messages suggests, Rust will also dump the stack at this point.

The stack is unwound. This is a lot like C++ exception handling.

Any temporary values, local variables, or arguments that the current function was using are dropped, in the reverse of the order they were created. Dropping a value simply means cleaning up after it: any Strings or Vecs the program was using are freed, any open Files are closed, and so on. User-defined drop methods are called too; see “Drop”. In the particular case of pirate_share(), there’s nothing to clean up.

Once the current function call is cleaned up, we move on to its caller, dropping its variables and arguments the same way. Then that function’s caller, and so on up the stack.

Finally, the thread exits. If the panicking thread was the main thread, then the whole process exits (with a nonzero exit code).

Perhaps panic is a misleading name for this orderly process. A panic is not a crash. It’s not undefined behavior. It’s more like a RuntimeException in Java or a std::logic_error in C++. The behavior is well-defined; it just shouldn’t be happening.

Panic is safe. It doesn’t violate any of Rust’s safety rules; even if you manage to panic in the middle of a standard library method, it will never leave a dangling pointer or a half-initialized value in memory. The idea is that Rust catches the invalid array access, or whatever it is, before anything bad happens. It would be unsafe to proceed, so Rust unwinds the stack. But the rest of the process can continue running.

Panic is per thread. One thread can be panicking while other threads are going on about their normal business. In Chapter 19, we’ll show how a parent thread can find out when a child thread panics and handle the error gracefully.

There is also a way to catch stack unwinding, allowing the thread to survive and continue running. The standard library function std::panic::catch_unwind() does this. We won’t cover how to use it, but this is the mechanism used by Rust’s test harness to recover when an assertion fails in a test. (It can also be necessary when writing Rust code that can be called from C or C++, because unwinding across non-Rust code is undefined behavior; see Chapter 21.)

Ideally, we would all have bug-free code that never panics. But nobody’s perfect. You can use threads and catch_unwind() to handle panic, making your program more robust. One important caveat is that these tools only catch panics that unwind the stack. Not every panic proceeds this way.

Aborting

Stack unwinding is the default panic behavior, but there are two circumstances in which Rust does not try to unwind the stack.

If a .drop() method triggers a second panic while Rust is still trying to clean up after the first, this is considered fatal. Rust stops unwinding and aborts the whole process.

Also, Rust’s panic behavior is customizable. If you compile with -C panic=abort, the first panic in your program immediately aborts the process. (With this option, Rust does not need to know how to unwind the stack, so this can reduce the size of your compiled code.)

This concludes our discussion of panic in Rust. There is not much to say, because ordinary Rust code has no obligation to handle panic. Even if you do use threads or catch_unwind(), all your panic-handling code will likely be concentrated in a few places. It’s unreasonable to expect every function in a program to anticipate and cope with bugs in its own code. Errors caused by other factors are another kettle of fish.

Result

Rust doesn’t have exceptions. Instead, functions that can fail have a return type that says so:

fn get_weather(location: LatLng) -> Result<WeatherReport, io::Error>

The Result type indicates possible failure. When we call the get_weather() function, it will return either a success result Ok(weather), where weather is a new WeatherReport value, or an error result Err(error_value), where error_value is an io::Error explaining what went wrong.

Rust requires us to write some kind of error handling whenever we call this function. We can’t get at the WeatherReport without doing something to the Result, and you’ll get a compiler warning if a Result value isn’t used.

In Chapter 10, we’ll see how the standard library defines Result and how you can define your own similar types. For now, we’ll take a “cookbook” approach and focus on how to use Results to get the error-handling behavior you want.

Catching Errors

The most thorough way of dealing with a Result is the way we showed in Chapter 2: use a match expression.

match get_weather(hometown) {
    Ok(report) => {
        display_weather(hometown, &report);
    }
    Err(err) => {
        println!("error querying the weather: {}", err);
        schedule_weather_retry();
    }
}

This is Rust’s equivalent of try/catch in other languages. It’s what you use when you want to handle errors head-on, not pass them on to your caller.

match is a bit verbose, so Result<T, E> offers a variety of methods that are useful in particular common cases. Each of these methods has a match expression in its implementation. (For the full list of Result methods, consult the online documentation. The methods listed here are the ones we use the most.)

result.is_ok() and result.is_err() return a bool telling if result is a success result or an error result.

result.ok() returns the success value, if any, as an Option<T>. If result is a success result, this returns Some(success_value); otherwise, it returns None, discarding the error value.

result.err() returns the error value, if any, as an Option<E>.

result.unwrap_or(fallback) returns the success value, if result is a success result. Otherwise, it returns fallback, discarding the error value.

// A fairly safe prediction for Southern California.
const THE_USUAL: WeatherReport = WeatherReport::Sunny(72);

// Get a real weather report, if possible.
// If not, fall back on the usual.
let report = get_weather(los_angeles).unwrap_or(THE_USUAL);
display_weather(los_angeles, &report);

This is a nice alternative to .ok() because the return type is T, not Option<T>. Of course, it only works when there’s an appropriate fallback value.

result.unwrap_or_else(fallback_fn) is the same, but instead of passing a fallback value directly, you pass a function or closure. This is for cases where it would be wasteful to compute a fallback value if you’re not going to use it. The fallback_fn is called only if we have an error result.

let report =
    get_weather(hometown)
    .unwrap_or_else(|_err| vague_prediction(hometown));

(Chapter 14 covers closures in detail.)

result.unwrap() also returns the success value, if result is a success result. However, if result is an error result, this method panics. This method has its uses; we’ll talk more about it later.

result.expect(message) is the same as .unwrap(), but lets you provide a message that it prints in case of panic.

Lastly, two methods for borrowing references to the value in a Result:

result.as_ref() converts a Result<T, E> to a Result<&T, &E>, borrowing a reference to the success or error value in the existing result.

result.as_mut() is the same, but borrows a mutable reference. The return type is Result<&mut T, &mut E>.

One reason these last two methods are useful is that all of the other methods listed here, except .is_ok() and .is_err(), consume the result they operate on. That is, they take the self argument by value. Sometimes it’s quite handy to access data inside a result without destroying it, and this is what .as_ref() and .as_mut() do for us. For example, suppose you’d like to call result.ok(), but you need result to be left intact. You can write result.as_ref().ok(), which merely borrows result, returning an Option<&T> rather than an Option<T>.

Result Type Aliases

Sometimes you’ll see Rust documentation that seems to omit the error type of a Result:

fn remove_file(path: &Path) -> Result<()>

This means that a Result type alias is being used.

A type alias is a kind of shorthand for type names. Modules often define a Result type alias to avoid having to repeat an error type that’s used consistently by almost every function in the module. For example, the standard library’s std::io module includes this line of code:

pub type Result<T> = result::Result<T, Error>;

This defines a public type std::io::Result<T>. It’s an alias for Result<T, E>, but hardcoding std::io::Error as the error type. In practical terms, this means that if you write use std::io; then Rust will understand io::Result<String> as shorthand for Result<String, io::Error>.

When something like Result<()> appears in the online documentation, you can click on the identifier Result to see which type alias is being used and learn the error type. In practice, it’s usually obvious from context.

Printing Errors

Sometimes the only way to handle an error is by dumping it to the terminal and moving on. We already showed one way to do this:

println!("error querying the weather: {}", err);

The standard library defines several error types with boring names: std::io::Error, std::fmt::Error, std::str::Utf8Error, and so on. All of them implement a common interface, the std::error::Error trait, which means they share the following features:

They’re all printable using println!(). Printing an error with the {} format specifier typically displays only a brief error message. Alternatively, you can print with the {:?} format specifier, to get a Debug view of the error. This is less user-friendly, but includes extra technical information.

// result of `println!("error: {}", err);`
error: failed to lookup address information: No address associated with
hostname

// result of `println!("error: {:?}", err);`
error: Error { repr: Custom(Custom { kind: Other, error: StringError(
"failed to lookup address information: No address associated with
hostname") }) }

err.description() returns an error message as a &str.

err.cause() returns an Option<&Error>: the underlying error, if any, that triggered err.

For example, a networking error might cause a banking transaction to fail, which could in turn cause your boat to be repossessed. If err.description() is "boat was repossessed", then err.cause() might return an error about the failed transaction; its .description() might be "failed to transfer $300 to United Yacht Supply", and its .cause() might be an io::Error with details about the specific network outage that caused all the fuss. That third error is the root cause, so its .cause() method would return None.

Since the standard library only includes rather low-level features, this is usually None for standard library errors.

Printing an error value does not also print out its cause. If you want to be sure to print all the available information, use this function:

use std::error::Error;
use std::io::{Write, stderr};

/// Dump an error message to `stderr`.
///
/// If another error happens while building the error message or
/// writing to `stderr`, it is ignored.
fn print_error(mut err: &Error) {
    let _ = writeln!(stderr(), "error: {}", err);
    while let Some(cause) = err.cause() {
        let _ = writeln!(stderr(), "caused by: {}", cause);
        err = cause;
    }
}

The standard library’s error types do not include a stack trace, but the error-chain crate makes it easy to define your own custom error type that supports grabbing a stack trace when it’s created. It uses the backtrace crate to capture the stack.

Propagating Errors

In most places where we try something that could fail, we don’t want to catch and handle the error immediately. It is simply too much code to use a 10-line match statement every place where something could go wrong.

Instead, if an error occurs, we usually want to let our caller deal with it. We want errors to propagate up the call stack.

Rust has a ? operator that does this. You can add a ? to any expression that produces a Result, such as the result of a function call:

let weather = get_weather(hometown)?;

The behavior of ? depends on whether this function returns a success result or an error result:

On success, it unwraps the Result to get the success value inside. The type of weather here is not Result<WeatherReport, io::Error> but simply WeatherReport.

On error, it immediately returns from the enclosing function, passing the error result up the call chain. To ensure that this works, ? can only be used in functions that have a Result return type.

There’s nothing magical about the ? operator. You can express the same thing using a match expression, although it’s much wordier:

let weather = match get_weather(hometown) {
    Ok(success_value) => success_value,
    Err(err) => return Err(err)
};

The only differences between this and the ? operator are some fine points involving types and conversions. We’ll cover those details in the next section.

In older code, you may see the try!() macro, which was the usual way to propagate errors until the ? operator was introduced in Rust 1.13.

let weather = try!(get_weather(hometown));

The macro expands to a match expression, like the one above.

It’s easy to forget just how pervasive the possibility of errors is in a program, particularly in code that interfaces with the operating system. The ? operator sometimes shows up on almost every line of a function:

use std::fs;
use std::io;
use std::path::Path;

fn move_all(src: &Path, dst: &Path) -> io::Result<()> {
    for entry_result in src.read_dir()? {  // opening dir could fail
        let entry = entry_result?;         // reading dir could fail
        let dst_file = dst.join(entry.file_name());
        fs::rename(entry.path(), dst_file)?;  // renaming could fail
    }
    Ok(())  // phew!
}

Working with Multiple Error Types

Often, more than one thing could go wrong. Suppose we are simply reading numbers from a text file.

use std::io::{self, BufRead};

/// Read integers from a text file.
/// The file should have one number on each line.
fn read_numbers(file: &mut BufRead) -> Result<Vec<i64>, io::Error> {
    let mut numbers = vec![];
    for line_result in file.lines() {
        let line = line_result?;         // reading lines can fail
        numbers.push(line.parse()?);     // parsing integers can fail
    }
    Ok(numbers)
}

Rust gives us a compiler error:

numbers.push(line.parse()?);     // parsing integers can fail
             ^^^^^^^^^^^^^ the trait `std::convert::From<std::num::ParseIntError>`
                           is not implemented for `std::io::Error`

The terms in this error message will make more sense when we reach Chapter 11, which covers traits. For now, just note that Rust is complaining that it can’t convert a std::num::ParseIntError value to the type std::io::Error.

The problem here is that reading a line from a file and parsing an integer produce two different potential error types. The type of line_result is Result<String, std::io::Error>. The type of line.parse() is Result<i64, std::num::Parse​IntError>. The return type of our read_numbers() function only accommodates io::Errors. Rust tries to cope with the ParseIntError by converting it to a io::Error, but there’s no such conversion, so we get a type error.

There are several ways of dealing with this. For example, the image crate that we used in Chapter 2 to create image files of the Mandelbrot set defines its own error type, ImageError, and implements conversions from io::Error and several other error types to ImageError. If you’d like to go this route, try the aforementioned error-chain crate, which is designed to help you define good error types with just a few lines of code.

A simpler approach is to use what’s built into Rust. All of the standard library error types can be converted to the type Box<std::error::Error>, which represents “any error.” So an easy way to handle multiple error types is to define these type aliases:

type GenError = Box<std::error::Error>;
type GenResult<T> = Result<T, GenError>;

Then, change the return type of read_numbers() to GenResult<Vec<i64>>. With this change, the function compiles. The ? operator automatically converts either type of error into a GenError as needed.

Incidentally, the ? operator does this automatic conversion using a standard method that you can use yourself. To convert any error to the GenError type, call GenError::from():

let io_error = io::Error::new(         // make our own io::Error
    io::ErrorKind::Other, "timed out");
return Err(GenError::from(io_error));  // manually convert to GenError

We’ll cover the From trait and its from() method fully in Chapter 13.

The downside of the GenError approach is that the return type no longer communicates precisely what kinds of errors the caller can expect. The caller must be ready for anything.

If you’re calling a function that returns a GenResult, and you want to handle one particular kind of error, but let all others propagate out, use the generic method error.downcast_ref::<ErrorType>(). It borrows a reference to the error, if it happens to be the particular type of error you’re looking for:

loop {
    match compile_project() {
        Ok(()) => return Ok(()),
        Err(err) => {
            if let Some(mse) = err.downcast_ref::<MissingSemicolonError>() {
                insert_semicolon_in_source_code(mse.file(), mse.line())?;
                continue;  // try again!
            }
            return Err(err);
        }
    }
}

Many languages have built-in syntax to do this, but it turns out to be rarely needed. Rust has a method for it instead.

Dealing with Errors That “Can’t Happen”

Sometimes we just know that an error can’t happen. For example, suppose we’re writing code to parse a configuration file, and at one point we find that the next thing in the file is a string of digits:

if next_char.is_digit(10) {
    let start = current_index;
    current_index = skip_digits(&line, current_index);
    let digits = &line[start..current_index];
    ...

We want to convert this string of digits to an actual number. There’s a standard method that does this:

let num = digits.parse::<u64>();

Now the problem: the str.parse::<u64>() method doesn’t return a u64. It returns a Result. It can fail, because some strings aren’t numeric.

"bleen".parse::<u64>()  // ParseIntError: invalid digit

But we happen to know that in this case, digits consists entirely of digits. What should we do?

If the code we’re writing already returns a GenResult, we can tack on a ? and forget about it. Otherwise, we face the irritating prospect of having to write error-handling code for an error that can’t happen. The best choice then would be to use .unwrap(), a Result method we mentioned earlier.

let num = digits.parse::<u64>().unwrap();

This is just like ? except that if we’re wrong about this error, if it can happen, then in that case we would panic.

In fact, we are wrong about this particular case. If the input contains a long enough string of digits, the number will be too big to fit in a u64.

"99999999999999999999".parse::<u64>()    // overflow error

Using .unwrap() in this particular case would therefore be a bug. Bogus input shouldn’t cause a panic.

That said, situations do come up where a Result value truly can’t be an error. For example, in Chapter 18, you’ll see that the Write trait defines a common set of methods (.write() and others) for text and binary output. All of those methods return io::Results, but if you happen to be writing to a Vec<u8>, they can’t fail. In such cases, it’s acceptable to use .unwrap() or .expect(message) to dispense with the Results.

These methods are also useful when an error would indicate a condition so severe or bizarre that panic is exactly how you want to handle it.

fn print_file_age(filename: &Path, last_modified: SystemTime) {
    let age = last_modified.elapsed().expect("system clock drift");
    ...
}

Here, the .elapsed() method can fail only if the system time is earlier than when the file was created. This can happen if the file was created recently, and the system clock was adjusted backward while our program was running. Depending on how this code is used, it’s a reasonable judgment call to panic in that case, rather than handle the error or propagate it to the caller.

Ignoring Errors

Occasionally we just want to ignore an error altogether. For example, in our print_error() function, we had to handle the unlikely situation where printing the error triggers another error. This could happen, for example, if stderr is piped to another process, and that process is killed. As there’s not much we can do about this kind of error, we just want to ignore it; but the Rust compiler warns about unused Result values:

writeln!(stderr(), "error: {}", err);  // warning: unused result

The idiom let _ = ... is used to silence this warning:

let _ = writeln!(stderr(), "error: {}", err);  // ok, ignore result

Handling Errors in main()

In most places where a Result is produced, letting the error bubble up to the caller is the right behavior. This is why ? is a single character in Rust. As we’ve seen, in some programs it’s used on many lines of code in a row.

But if you propagate an error long enough, eventually it reaches main(), and that’s where this approach has to stop. main() can’t use ? because its return type is not Result.

fn main() {
    calculate_tides()?;  // error: can't pass the buck any further
}

The simplest way to handle errors in main() is to use .expect().

fn main() {
    calculate_tides().expect("error");  // the buck stops here
}

If calculate_tides() returns an error result, the .expect() method panics. Panicking in the main thread prints an error message, then exits with a nonzero exit code, which is roughly the desired behavior. We use this all the time for tiny programs. It’s a start.

The error message is a little intimidating, though:

$ tidecalc --planet mercury
thread 'main' panicked at 'error: "moon not found"', /buildslave/rust-buildbot/s
lave/nightly-dist-rustc-linux/build/src/libcore/result.rs:837
note: Run with `RUST_BACKTRACE=1` for a backtrace.

The error message is lost in the noise. Also, RUST_BACKTRACE=1 is bad advice in this particular case. It pays to print the error message yourself:

fn main() {
    if let Err(err) = calculate_tides() {
        print_error(&err);
        std::process::exit(1);
    }
}

This code uses an if let expression to print the error message only if the call to calculate_tides() returns an error result. For details about if let expressions, see Chapter 10. The print_error function is listed in “Printing Errors”.

Now the output is nice and tidy:

$ tidecalc --planet mercury
error: moon not found

Declaring a Custom Error Type

Suppose you are writing a new JSON parser, and you want it to have its own error type. (We haven’t covered user-defined types yet; that’s coming up in a few chapters. But error types are handy, so we’ll include a bit of a sneak preview here.)

Approximately the minimum code you would write is:

// json/src/error.rs

#[derive(Debug, Clone)]
pub struct JsonError {
    pub message: String,
    pub line: usize,
    pub column: usize,
}

This struct will be called json::error::JsonError, and when you want to raise an error of this type, you can write:

return Err(JsonError {
    message: "expected ']' at end of array".to_string(),
    line: current_line,
    column: current_column
});

This will work fine. However, if you want your error type to work like the standard error types, as your library’s users will expect, then you have a bit more work to do:

use std;
use std::fmt;

// Errors should be printable.
impl fmt::Display for JsonError {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        write!(f, "{} ({}:{})", self.message, self.line, self.column)
    }
}

// Errors should implement the std::error::Error trait.
impl std::error::Error for JsonError {
    fn description(&self) -> &str {
        &self.message
    }
}

Again, the meaning of the impl keyword, self, and all the rest will be explained in the next few chapters.

Why Results?

Now we know enough to understand what Rust is getting at by choosing Results over exceptions. Here are the key points of the design:

Rust requires the programmer to make some sort of decision, and record it in the code, at every point where an error could occur. This is good because otherwise, it’s easy to get error handling wrong through neglect.

The most common decision is to allow errors to propagate, and that’s written with a single character, ‘?’. Thus error plumbing does not clutter up your code the way it does in C and Go. Yet it’s still visible: you can look at a chunk of code and see at a glance all places where errors are propagated.

Since the possibility of errors is part of every function’s return type, it’s clear which functions can fail and which can’t. If you change a function to be fallible, you’re changing its return type, so the compiler will make you update that function’s downstream users.

Rust checks that Result values are used, so you can’t accidentally let an error pass silently (a common mistake in C).

Since Result is a data type like any other, it’s easy to store success and error results in the same collection. This makes it easy to model partial success. For example, if you’re writing a program that loads millions of records from a text file, and you need a way to cope with the likely outcome that most will succeed, but some will fail, you can represent that situation in memory using a vector of Results.

The cost is that you’ll find yourself thinking about and engineering error handling more in Rust than you would in other languages. As in many other areas, Rust’s take on error handling is wound just a little tighter than what you’re used to. For systems programming, it’s worth it.

Chapter 8. Crates and Modules

This is one note in a Rust theme: systems programmers can have nice things.

Robert O’Callahan, “Random Thoughts on Rust: Crates.io and IDEs”

Suppose you’re writing a program that simulates the growth of ferns, from the level of individual cells on up. Your program, like a fern, will start out very simple, with all the code, perhaps, in a single file—just the spore of an idea. As it grows, it will start to have internal structure. Different pieces will have different purposes. It will branch out into multiple files. It may cover a whole directory tree. In time it may become a significant part of a whole software ecosystem.

This chapter covers the features of Rust that help keep your program organized: crates and modules. We’ll also cover a wide range of topics that come up naturally as your project grows, including how to document and test Rust code, how to silence unwanted compiler warnings, how to use Cargo to manage project dependencies and versioning, how to publish open source libraries on crates.io, and more.

Crates

Rust programs are made of crates. Each crate is a Rust project: all the source code for a single library or executable, plus any associated tests, examples, tools, configuration, and other junk. For your fern simulator, you might use third-party libraries for 3D graphics, bioinformatics, parallel computation, and so on. These libraries are distributed as crates (see Figure 8-1).

Figure 8-1. A crate and its dependencies

The easiest way to see what crates are and how they work together is to use cargo build with the --verbose flag to build an existing project that has some dependencies. We did this, using “A Concurrent Mandelbrot Program” as our example. The results are shown here:

$ cd mandelbrot
$ cargo clean    # delete previously compiled code
$ cargo build --verbose
    Updating registry `https://github.com/rust-lang/crates.io-index`
 Downloading image v0.6.1
 Downloading crossbeam v0.2.9
 Downloading gif v0.7.0
 Downloading png v0.4.2

 ... (downloading and compiling many more crates)

   Compiling png v0.4.2
     Running `rustc .../png-0.4.2/src/lib.rs
          --crate-name png
          --crate-type lib
          --extern num=.../libnum-a2e6e61627ca7fe5.rlib
          --extern inflate=.../libinflate-331fc425bf167339.rlib
          --extern flate2=.../libflate2-857dff75f2932d8a.rlib
          ...`
   Compiling image v0.6.1
     Running `rustc .../image-0.6.1/./src/lib.rs
          --crate-name image
          --crate-type lib
          --extern png=.../libpng-16c24f58491a5853.rlib
          ...`
   Compiling mandelbrot v0.1.0 (file://.../mandelbrot)
     Running `rustc src/main.rs
          --crate-name mandelbrot
          --crate-type bin
          --extern crossbeam=.../libcrossbeam-ba292320058da7df.rlib
          --extern image=.../libimage-254ec48c8f0684f2.rlib
          ...`
$

We reformatted the rustc command lines for readability, and we deleted a lot of compiler options that aren’t relevant to our discussion, replacing them with an ellipsis (...).

You might recall that by the time we were done, the Mandelbrot program’s main.rs contained three extern crate declarations:

extern crate num;
extern crate image;
extern crate crossbeam;

These lines simply tell Rust that num, image, and crossbeam are external libraries, not part of the Mandelbrot program itself.

We also specified in our Cargo.toml file which version of each crate we wanted:

[dependencies]
num = "0.1.27"
image = "0.6.1"
crossbeam = "0.2.8"

The word dependencies here just means other crates this project uses: code we’re depending on. We found these crates on crates.io, the Rust community’s site for open source crates. For example, we found out about the image library by going to crates.io and searching for an image library. Each crate’s page on crates.io provides links to documentation and source code, as well as a line of configuration like image = "0.6.1" that you can copy and add to your Cargo.toml. The version numbers shown here are simply the latest versions of these three packages at the time we wrote the program.

The Cargo transcript tells the story of how this information is used. When we run cargo build, Cargo starts by downloading source code for the specified versions of these crates from crates.io. Then, it reads those crates’ Cargo.toml files, downloads their dependencies, and so on recursively. For example, the source code for version 0.6.1 of the image crate contains a Cargo.toml file that includes this:

[dependencies]
byteorder = "0.4.0"
num = "0.1.27"
enum_primitive = "0.1.0"
glob = "0.2.10"

Seeing this, Cargo knows that before it can use image, it must fetch these crates as well. Later on, we’ll see how to tell Cargo to fetch source code from a Git repository or the local filesystem rather than crates.io.

Once it has obtained all the source code, Cargo compiles all the crates. It runs rustc, the Rust compiler, once for each crate in the project’s dependency graph. When compiling libraries, Cargo uses the --crate-type lib option. This tells rustc not to look for a main() function but instead to produce an .rlib file containing compiled code in a form that later rustc commands can use as input. When compiling a program, Cargo uses --crate-type bin, and the result is a binary executable for the target platform: mandelbrot.exe on Windows, for example.

With each rustc command, Cargo passes --extern options giving the filename of each library the crate will use. That way, when rustc sees a line of code like extern crate crossbeam;, it knows where to find that compiled crate on disk. The Rust compiler needs access to these .rlib files because they contain the compiled code of the library. Rust will statically link that code into the final executable. The .rlib also contains type information, so Rust can check that the library features we’re using in our code actually exist in the crate, and that we’re using them correctly. It also contains a copy of the crate’s public inline functions, generics, and macros, features that can’t be fully compiled to machine code until Rust sees how we use them.

cargo build supports all sorts of options, most of which are beyond the scope of this book, but we will mention one here: cargo build --release produces an optimized build. Release builds run faster, but they take longer to compile, they don’t check for integer overflow, they skip debug_assert!() assertions, and the stack traces they generate on panic are generally less reliable.

Build Profiles

There are several configuration settings you can put in your Cargo.toml file that affect the rustc command lines that cargo generates.

Command line | Cargo.toml section used
cargo build | [profile.debug]
cargo build --release | [profile.release]
cargo test | [profile.test]

The defaults are usually fine, but one exception we’ve found is when you want to use a profiler—a tool that measures where your program is spending its CPU time. To get the best data from a profiler, you need both optimizations (usually enabled only in release builds) and debug symbols (usually enabled only in debug builds). To enable both, add this to your Cargo.toml:

[profile.release]
debug = true  # enable debug symbols in release builds

The debug setting controls the -g option to rustc. With this configuration, when you type cargo build --release, you’ll get a binary with debug symbols. The optimization settings are unaffected.

The Cargo documentation lists many other settings you can adjust.

Modules

Modules are Rust’s namespaces. They’re containers for the functions, types, constants, and so on that make up your Rust program or library. Whereas crates are about code sharing between projects, modules are about code organization within a project. They look like this:

mod spores {
    use cells::Cell;

    /// A cell made by an adult fern. It disperses on the wind as part of
    /// the fern life cycle. A spore grows into a prothallus -- a whole
    /// separate organism, up to 5mm across -- which produces the zygote
    /// that grows into a new fern. (Plant sex is complicated.)
    pub struct Spore {
        ...
    }

    /// Simulate the production of a spore by meiosis.
    pub fn produce_spore(factory: &mut Sporangium) -> Spore {
        ...
    }

    /// Mix genes to prepare for meiosis (part of interphase).
    fn recombine(parent: &mut Cell) {
        ...
    }

    ...
}

A module is a collection of items, named features like the Spore struct and the two functions in this example. The pub keyword makes an item public, so it can be accessed from outside the module. Anything that isn’t marked pub is private.

let s = spores::produce_spore(&mut factory);  // ok

spores::recombine(&mut cell);  // error: `recombine` is private

Modules can nest, and it’s fairly common to see a module that’s just a collection of submodules:

mod plant_structures {
    pub mod roots {
        ...
    }
    pub mod stems {
        ...
    }
    pub mod leaves {
        ...
    }
}

In this way, we could write out a whole program, with a huge amount of code and a whole hierarchy of modules, all in a single source file. Actually working that way is a pain, though, so there’s an alternative.

Modules in Separate Files

A module can also be written like this:

mod spores;

Earlier, we included the body of the spores module, wrapped in curly braces. Here, we’re instead telling the Rust compiler that the spores module lives in a separate file, called spores.rs:

// spores.rs

/// A cell made by an adult fern...
pub struct Spore {
    ...
}

/// Simulate the production of a spore by meiosis.
pub fn produce_spore(factory: &mut Sporangium) -> Spore {
    ...
}

/// Mix genes to prepare for meiosis (part of interphase).
fn recombine(parent: &mut Cell) {
    ...
}

spores.rs contains only the items that make up the module. It doesn’t need any kind of boilerplate to declare that it’s a module.

The location of the code is the only difference between this spores module and the version we showed in the previous section. The rules about what’s public and what’s private are exactly the same either way. And Rust never compiles modules separately, even if they’re in separate files: when you build a Rust crate, you’re recompiling all of its modules.

A module can have its own directory. When Rust sees mod spores;, it checks for both spores.rs and spores/mod.rs; if neither file exists, or both exist, that’s an error. For this example, we used spores.rs, because the spores module did not have any submodules. But consider the plant_structures module we wrote out earlier. If we decide to split that module and its three submodules into their own files, the resulting project would look like this:

fern_sim/
├── Cargo.toml
└── src/
    ├── main.rs
    ├── spores.rs
    └── plant_structures/
        ├── mod.rs
        ├── leaves.rs
        ├── roots.rs
        └── stems.rs

In main.rs, we declare the plant_structures module:

pub mod plant_structures;

This causes Rust to load plant_structures/mod.rs, which declares the three submodules:

// in plant_structures/mod.rs
pub mod roots;
pub mod stems;
pub mod leaves;

The content of those three modules is stored in separate files named leaves.rs, roots.rs, and stems.rs, located alongside mod.rs in the plant_structures directory.

Paths and Imports

The :: operator is used to access features of a module. Code anywhere in your project can refer to any standard library feature by writing out its absolute path:

if s1 > s2 {
    ::std::mem::swap(&mut s1, &mut s2);
}

This function name, ::std::mem::swap, is an absolute path, because it starts with a double colon. The path ::std refers to the top-level module of the standard library. ::std::mem is a submodule within the standard library, and ::std::mem::swap is a public function in that module.

You could write all your code this way, spelling out ::std::f64::consts::PI and ::std::collections::HashMap::new every time you want a circle or a dictionary, but it would be tedious to type and hard to read. The alternative is to import features into the modules where they’re used:

use std::mem;

if s1 > s2 {
    mem::swap(&mut s1, &mut s2);
}

The use declaration causes the name mem to be a local alias for ::std::mem throughout the enclosing block or module. Paths in use declarations are automatically absolute paths, so there is no need for a leading ::.

We could write use std::mem::swap; to import the swap function itself instead of the mem module. However, what we did above is generally considered the best style: import types, traits, and modules (like std::mem), then use relative paths to access the functions, constants, and other members within.

Several names can be imported at once:

use std::collections::{HashMap, HashSet};  // import both

use std::io::prelude::*;  // import everything

This is just shorthand for writing out all the individual imports:

use std::collections::HashMap;
use std::collections::HashSet;

// all the public items in std::io::prelude:
use std::io::prelude::Read;
use std::io::prelude::Write;
use std::io::prelude::BufRead;
use std::io::prelude::Seek;

Modules do not automatically inherit names from their parent modules. For example, suppose we have this in our proteins/mod.rs:

// proteins/mod.rs
pub enum AminoAcid { ... }
pub mod synthesis;

Then the code in synthesis.rs does not automatically see the type AminoAcid:

// proteins/synthesis.rs
pub fn synthesize(seq: &[AminoAcid])  // error: can't find type `AminoAcid`
    ...

Instead, each module starts with a blank slate and must import the names it uses:

// proteins/synthesis.rs
use super::AminoAcid;  // explicitly import from parent

pub fn synthesize(seq: &[AminoAcid])  // ok
    ...

The keyword super has a special meaning in imports: it’s an alias for the parent module. Similarly, self is an alias for the current module.

// in proteins/mod.rs

// import from a submodule
use self::synthesis::synthesize;

// import names from an enum,
// so we can write `Lys` for lysine, rather than `AminoAcid::Lys`
use self::AminoAcid::*;

While paths in imports are treated as absolute paths by default, self and super let you override that and import from relative paths.

(The AminoAcid example here is, of course, a departure from the style rule we mentioned earlier about only importing types, traits, and modules. If our program includes long amino acid sequences, this is justified under Orwell’s Sixth Rule: “Break any of these rules sooner than say anything outright barbarous.”)

Submodules can access private items in their parent modules, but they have to import each one by name. use super::*; only imports items that are marked pub.

Modules aren’t the same thing as files, but there is a natural analogy between modules and the files and directories of a Unix filesystem. The use keyword creates aliases, just as the ln command creates links. Paths, like filenames, come in absolute and relative forms. self and super are like the . and .. special directories. And extern crate grafts another crate’s root module into your project. It is a lot like mounting a filesystem.

The Standard Prelude

We said a moment ago that each module starts with a “blank slate,” as far as imported names are concerned. But the slate is not completely blank.

For one thing, the standard library std is automatically linked with every project. It’s as though your lib.rs or main.rs contained an invisible declaration for it:

extern crate std;

Furthermore, a few particularly handy names, like Vec and Result, are included in the standard prelude and automatically imported. Rust behaves as though every module, including the root module, started with the following import:

use std::prelude::v1::*;

The standard prelude contains a few dozen commonly used traits and types. It does not contain std. So if your module refers to std, you’ll have to import it explicitly, like this:

use std;

Usually, it makes more sense to import the particular feature of std that you’re using.

In Chapter 2, we mentioned that libraries sometimes provide modules named prelude. But std::prelude::v1 is the only prelude that is ever imported automatically. Naming a module prelude is just a convention that tells users it’s meant to be imported using *.

Items, the Building Blocks of Rust

A module is made up of items. There are several kinds of item, and the list is really a list of the language’s major features:

Functions

We have seen a great many of these already.

Types

User-defined types are introduced using the struct, enum, and trait keywords. We’ll dedicate a chapter to each of them, in good time; a simple struct looks like this:

pub struct Fern {
    pub roots: RootSet,
    pub stems: StemSet
}

A struct’s fields, even private fields, are accessible throughout the module where the struct is declared. Outside the module, only public fields are accessible.

It turns out that enforcing access control by module, rather than by class as in Java or C++, is surprisingly helpful for software design. It cuts down on boilerplate “getter” and “setter” methods, and it largely eliminates the need for anything like C++ friend declarations. A single module can define several types that work closely together, such as perhaps frond::LeafMap and frond::LeafMapIter, accessing each other’s private fields as needed, while still hiding those implementation details from the rest of your program.

Type aliases

As we’ve seen, the type keyword can be used like typedef in C++, to declare a new name for an existing type:

type Table = HashMap<String, Vec<String>>;

The type Table that we’re declaring here is shorthand for this particular kind of HashMap.

fn show(table: &Table) {
    ...
}

impl blocks

Methods are attached to types using impl blocks:

impl Cell {
    pub fn distance_from_origin(&self) -> f64 {
        f64::hypot(self.x, self.y)
    }
}

The syntax is explained in Chapter 9. An impl block can’t be marked pub. Instead, individual methods are marked pub to make them visible outside the current module.

Private methods, like private struct fields, are visible throughout the module where they’re declared.

Constants

The const keyword introduces a constant. The syntax is just like let except that it may be marked pub, and the type is required. Also, UPPERCASE_NAMES are conventional for constants:

pub const ROOM_TEMPERATURE: f64 = 20.0;  // degrees Celsius

The static keyword introduces a static item, which is nearly the same thing:

pub static ROOM_TEMPERATURE: f64 = 68.0;  // degrees Fahrenheit

A constant is a bit like a C++ #define: the value is compiled into your code every place it’s used. A static is a variable that’s set up before your program starts running and lasts until it exits. Use constants for magic numbers and strings in your code. Use statics for larger amounts of data, or any time you’ll need to borrow a reference to the constant value.

There are no mut constants. Statics can be marked mut, but as discussed in Chapter 5, Rust has no way to enforce its rules about exclusive access on mut statics. They are, therefore, inherently non-thread-safe, and safe code can’t use them at all:

static mut PACKETS_SERVED: usize = 0;

println!("{} served", PACKETS_SERVED);  // error: use of mutable static

Rust discourages global mutable state. For a discussion of the alternatives, see “Global Variables”.

Modules

We’ve already talked about these quite a bit. As we’ve seen, a module can contain submodules, which can be public or private, like any other named item.

Imports

use and extern crate declarations are items too. Even though they’re just aliases, they can be public:

// in plant_structures/mod.rs
...
pub use self::leaves::Leaf;
pub use self::roots::Root;

This means that Leaf and Root are public items of the plant_structures module. They’re still simple aliases for plant_structures::leaves::Leaf and plant_structures::roots::Root.

The standard prelude is written as just such a series of pub imports.

extern blocks

These declare a collection of functions written in some other language (typically C or C++), so that your Rust code can call them. We’ll cover extern blocks in Chapter 21.

Rust warns about items that are declared, but never used:

warning: function is never used: `is_square`
  --> src/crates_unused_items.rs:23:9
   |
23 | /         pub fn is_square(root: &Root) -> bool {
24 | |             root.cross_section_shape().is_square()
25 | |         }
   | |_________^
   |

This warning can be puzzling, because there are two very different possible causes. Perhaps this function really is dead code at the moment. Or, maybe you meant to use it in other crates. In that case, you need to mark it and all enclosing modules as public.

Turning a Program into a Library

As your fern simulator starts to take off, you decide you need more than a single program. Suppose you’ve got one command-line program that runs the simulation and saves results in a file. Now, you want to write other programs for performing scientific analysis of the saved results, displaying 3D renderings of the growing plants in real time, rendering photorealistic pictures, and so on. All these programs need to share the basic fern simulation code. You need to make a library.

The first step is to factor your existing project into two parts: a library crate, which contains all the shared code, and an executable, which contains the code that’s only needed for your existing command-line program.

To show how you can do this, let’s use a grossly simplified example program:

struct Fern {
    size: f64,
    growth_rate: f64
}

impl Fern {
    /// Simulate a fern growing for one day.
    fn grow(&mut self) {
        self.size *= 1.0 + self.growth_rate;
    }
}

/// Run a fern simulation for some number of days.
fn run_simulation(fern: &mut Fern, days: usize) {
    for _ in 0 .. days {
        fern.grow();
    }
}

fn main() {
    let mut fern = Fern {
        size: 1.0,
        growth_rate: 0.001
    };
    run_simulation(&mut fern, 1000);
    println!("final fern size: {}", fern.size);
}

We’ll assume that this program has a trivial Cargo.toml file:

[package]
name = "fern_sim"
version = "0.1.0"
authors = ["You <you@example.com>"]

Turning this program into a library is easy. Here are the steps:

Rename the file src/main.rs to src/lib.rs.

Add the pub keyword to items in src/lib.rs that will be public features of our library.

Move the main function to a temporary file somewhere. We’ll come back to it in a minute.

The resulting src/lib.rs file looks like this:

pub struct Fern {
    pub size: f64,
    pub growth_rate: f64
}

impl Fern {
    /// Simulate a fern growing for one day.
    pub fn grow(&mut self) {
        self.size *= 1.0 + self.growth_rate;
    }
}

/// Run a fern simulation for some number of days.
pub fn run_simulation(fern: &mut Fern, days: usize) {
    for _ in 0 .. days {
        fern.grow();
    }
}

Note that we didn’t need to change anything in Cargo.toml. This is because our minimal Cargo.toml file leaves Cargo to its default behavior. By default, cargo build looks at the files in our source directory and figures out what to build. When it sees the file src/lib.rs, it knows to build a library.

The code in src/lib.rs forms the root module of the library. Other crates that use our library can only access the public items of this root module.

The src/bin Directory

Getting the original command-line fern_sim program working again is also straightforward: Cargo has some built-in support for small programs that live in the same codebase as a library.

In fact, Cargo itself is written this way. The bulk of the code is in a Rust library. The cargo command-line program that we’ve been using throughout this book is a thin wrapper program that calls out to the library for all the heavy lifting. Both the library and the command-line program live in the same source repository.

We can put our program and our library in the same codebase, too. Put this code into a file named src/bin/efern.rs:

extern crate fern_sim;
use fern_sim::{Fern, run_simulation};

fn main() {
    let mut fern = Fern {
        size: 1.0,
        growth_rate: 0.001
    };
    run_simulation(&mut fern, 1000);
    println!("final fern size: {}", fern.size);
}

The main function is the one we set aside earlier. We’ve added an extern crate declaration, since this program will use the fern_sim library crate, and we’re importing Fern and run_simulation from the library.

Because we’ve put this file into src/bin, Cargo will compile both the fern_sim library and this program the next time we run cargo build. We can run the efern program using cargo run --bin efern. Here’s what it looks like, using --verbose to show the commands Cargo is running:

$ cargo build --verbose
   Compiling fern_sim v0.1.0 (file:///.../fern_sim)
     Running `rustc src/lib.rs --crate-name fern_sim --crate-type lib ...`
     Running `rustc src/bin/efern.rs --crate-name efern --crate-type bin ...`
$ cargo run --bin efern --verbose
       Fresh fern_sim v0.1.0 (file:///.../fern_sim)
     Running `target/debug/efern`
final fern size: 2.7169239322355985

We still didn’t have to make any changes to Cargo.toml, because again, Cargo’s default is to look at your source files and figure things out. It automatically treats .rs files in src/bin as extra programs to build.

Of course, now that fern_sim is a library, we also have another option. We could have put this program in its own isolated project, in a completely separate directory, with its own Cargo.toml listing fern_sim as a dependency:

[dependencies]
fern_sim = { path = "../fern_sim" }

Perhaps that is what you’ll do for other fern-simulating programs down the road. The src/bin directory is just right for a simple program like efern.

Attributes

Any item in a Rust program can be decorated with attributes. Attributes are Rust’s catch-all syntax for writing miscellaneous instructions and advice to the compiler. For example, suppose you’re getting this warning:

libgit2.rs: warning: type `git_revspec` should have a camel case name
    such as `GitRevspec`, #[warn(non_camel_case_types)] on by default

But you chose this name for a reason, and you wish Rust would shut up about it. You can disable the warning by adding an #[allow] attribute on the type:

#[allow(non_camel_case_types)]
pub struct git_revspec {
    ...
}

Conditional compilation is another feature that’s written using an attribute, the #[cfg] attribute:

// Only include this module in the project if we're building for Android.
#[cfg(target_os = "android")]
mod mobile;

The full syntax of #[cfg] is specified in the Rust Reference; the most commonly used options are listed here:

#[cfg(...)] option | Enabled when
test | Tests are enabled (compiling with cargo test or rustc --test).
debug_assertions | Debug assertions are enabled (typically in nonoptimized builds).
unix | Compiling for Unix, including macOS.
windows | Compiling for Windows.
target_pointer_width = "64" | Targeting a 64-bit platform. The other possible value is "32".
target_arch = "x86_64" | Targeting x86-64 in particular. Other values: "x86", "arm", "aarch64", "powerpc", "powerpc64", "mips".
target_os = "macos" | Compiling for macOS. Other values: "windows", "ios", "android", "linux", "openbsd", "netbsd", "dragonfly", "bitrig".
feature = "robots" | The user-defined feature named "robots" is enabled (compiling with cargo build --feature robots or rustc --cfg feature='"robots"'). Features are declared in the [features] section of Cargo.toml.
not(A) | A is not satisfied. To provide two different implementations of a function, mark one with #[cfg(X)] and the other with #[cfg(not(X))].
all(A,B) | Both A and B are satisfied (the equivalent of &&).
any(A,B) | Either A or B is satisfied (the equivalent of ||).

Occasionally, we need to micromanage the inline expansion of functions, an optimization that we’re usually happy to leave to the compiler. We can use the #[inline] attribute for that:

/// Adjust levels of ions etc. in two adjacent cells
/// due to osmosis between them.
#[inline]
fn do_osmosis(c1: &mut Cell, c2: &mut Cell) {
    ...
}

There’s one situation where inlining won’t happen without #[inline]. When a function or method defined in one crate is called in another crate, Rust won’t inline it unless it’s generic (it has type parameters) or it’s explicitly marked #[inline].

Otherwise, the compiler treats #[inline] as a suggestion. Rust also supports the more insistent #[inline(always)], to request that a function be expanded inline at every call site, and #[inline(never)], to ask that a function never be inlined.

Some attributes, like #[cfg] and #[allow], can be attached to a whole module and apply to everything in it. Others, like #[test] and #[inline], must be attached to individual items. As you might expect for a catch-all feature, each attribute is custom-made and has its own set of supported arguments. The Rust Reference documents the full set of supported attributes in detail.

To attach an attribute to a whole crate, add it at the top of the main.rs or lib.rs file, before any items, and write #! instead of #, like this:

// libgit2_sys/lib.rs
#![allow(non_camel_case_types)]

pub struct git_revspec {
    ...
}

pub struct git_error {
    ...
}

The #! tells Rust to attach an attribute to the enclosing item rather than whatever comes next: in this case, the #![allow] attribute attaches to the whole libgit2_sys crate, not just struct git_revspec.

#! can also be used inside functions, structs, and so on, but it’s only typically used at the beginning of a file, to attach an attribute to the whole module or crate. Some attributes always use the #! syntax because they can only be applied to a whole crate.

For example, the #![feature] attribute is used to turn on unstable features of the Rust language and libraries, features that are experimental, and therefore might have bugs or might be changed or removed in the future. For instance, as we’re writing this, Rust has experimental support for 128-bit integer types i128 and u128; but since these types are experimental, you can only use them by (1) installing the Nightly version of Rust and (2) explicitly declaring that your crate uses them:

#![feature(i128_type)]

fn main() {
    // Do my math homework, Rust!
    println!("{}", 9204093811595833589_u128 * 19973810893143440503_u128);
}

Over time, the Rust team sometimes stabilizes an experimental feature, so that it becomes a standard part of the language. The #![feature] attribute then becomes superfluous, and Rust generates a warning advising you to remove it.

Tests and Documentation

As we saw in “Writing and Running Unit Tests”, a simple unit testing framework is built into Rust. Tests are ordinary functions marked with the #[test] attribute.

#[test]
fn math_works() {
    let x: i32 = 1;
    assert!(x.is_positive());
    assert_eq!(x + 1, 2);
}

cargo test runs all the tests in your project.

$ cargo test
   Compiling math_test v0.1.0 (file:///.../math_test)
     Running target/release/math_test-e31ed91ae51ebf22

running 1 test
test math_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

(You’ll also see some output about “doc-tests,” which we’ll get to in a minute.)

This works the same whether your crate is an executable or a library. You can run specific tests by passing arguments to Cargo: cargo test math runs all tests that contain math somewhere in their name.

Tests commonly use the assert! and assert_eq! macros from the Rust standard library. assert!(expr) succeeds if expr is true. Otherwise, it panics, which causes the test to fail. assert_eq!(v1, v2) is just like assert!(v1 == v2) except that if the assertion fails, the error message shows both values.

You can use these macros in ordinary code, to check invariants, but note that assert! and assert_eq! are included even in release builds. Use debug_assert! and debug_assert_eq! instead to write assertions that are checked only in debug builds.

To test error cases, add the #[should_panic] attribute to your test:

/// This test passes only if division by zero causes a panic,
/// as we claimed in the previous chapter.
#[test]
#[should_panic(expected="divide by zero")]
fn test_divide_by_zero_error() {
    1 / 0;  // should panic!
}

Functions marked with #[test] are conditionally compiled. When you run cargo test, Cargo builds a copy of your program with your tests and the test harness enabled. A plain cargo build or cargo build --release skips the testing code. This means your unit tests can live right alongside the code they test, accessing internal implementation details if they need to, and yet there’s no runtime cost. However, it can result in some warnings. For example:

fn roughly_equal(a: f64, b: f64) -> bool {
    (a - b).abs() < 1e-6
}

#[test]
fn trig_works() {
    use std::f64::consts::PI;
    assert!(roughly_equal(PI.sin(), 0.0));
}

In a testing build, this is fine. In a nontesting build, roughly_equal is unused, and Rust will complain:

$ cargo build
   Compiling math_test v0.1.0 (file:///.../math_test)
warning: function is never used: `roughly_equal`
 --> src/crates_unused_testing_function.rs:7:1
  |
7 | / fn roughly_equal(a: f64, b: f64) -> bool {
8 | |     (a - b).abs() < 1e-6
9 | | }
  | |_^
  |
   = note: #[warn(dead_code)] on by default

So the convention, when your tests get substantial enough to require support code, is to put them in a tests module and declare the whole module to be testing-only using the #[cfg] attribute:

#[cfg(test)]   // include this module only when testing
mod tests {
    fn roughly_equal(a: f64, b: f64) -> bool {
        (a - b).abs() < 1e-6
    }

    #[test]
    fn trig_works() {
        use std::f64::consts::PI;
        assert!(roughly_equal(PI.sin(), 0.0));
    }
}

Rust’s test harness uses multiple threads to run several tests at a time, a nice side benefit of your Rust code being thread-safe by default. (To disable this, either run a single test, cargo test testname; or set the environment variable RUST_TEST_THREADS to 1.) This means that, technically, the Mandelbrot program we showed in Chapter 2 was not the second multithreaded program in that chapter, but the third! The cargo test run in “Writing and Running Unit Tests” was the first.

Integration Tests

Your fern simulator continues to grow. You’ve decided to put all the major functionality into a library that can be used by multiple executables. It would be nice to have some tests that link with the library the way an end user would, using fern_sim.rlib as an external crate. Also, you have some tests that start by loading a saved simulation from a binary file, and it is awkward having those large test files in your src directory. Integration tests help with these two problems.

Integration tests are .rs files that live in a tests directory alongside your project’s src directory. When you run cargo test, Cargo compiles each integration test as a separate, standalone crate, linked with your library and the Rust test harness. Here is an example:

// tests/unfurl.rs - Fiddleheads unfurl in sunlight

extern crate fern_sim;
use fern_sim::Terrarium;
use std::time::Duration;

#[test]
fn test_fiddlehead_unfurling() {
    let mut world = Terrarium::load("tests/unfurl_files/fiddlehead.tm");
    assert!(world.fern(0).is_furled());
    let one_hour = Duration::from_secs(60 * 60);
    world.apply_sunlight(one_hour);
    assert!(world.fern(0).is_fully_unfurled());
}

Note that the integration test includes an extern crate declaration, since it uses fern_sim as a library. The point of integration tests is that they see your crate from the outside, just as a user would. They test the crate’s public API.

cargo test runs both unit tests and integration tests. To run only the integration tests in a particular file—for example, tests/unfurl.rs—use the command cargo test --test unfurl.

Documentation

The command cargo doc creates HTML documentation for your library:

$ cargo doc --no-deps --open
 Documenting fern_sim v0.1.0 (file:///.../fern_sim)

The --no-deps option tells Cargo to generate documentation only for fern_sim itself, and not for all the crates it depends on.

The --open option tells Cargo to open the documentation in your browser afterward.

You can see the result in Figure 8-2. Cargo saves the new documentation files in target/doc. The starting page is target/doc/fern_sim/index.html.

Figure 8-2. Example of documentation generated by rustdoc

The documentation is generated from the pub features of your library, plus any doc comments you’ve attached to them. We’ve seen a few doc comments in this chapter already. They look like comments:

/// Simulate the production of a spore by meiosis.
pub fn produce_spore(factory: &mut Sporangium) -> Spore {
    ...
}

But when Rust sees comments that start with three slashes, it treats them as a #[doc] attribute instead. Rust treats the preceding example exactly the same as this:

#[doc = "Simulate the production of a spore by meiosis."]
pub fn produce_spore(factory: &mut Sporangium) -> Spore {
    ...
}

When you compile or test a library, these attributes are ignored. When you generate documentation, doc comments on public features are included in the output.

Likewise, comments starting with //! are treated as #![doc] attributes, and are attached to the enclosing feature, typically a module or crate. For example, your fern_sim/src/lib.rs file might begin like this:

//! Simulate the growth of ferns, from the level of
//! individual cells on up.

The content of a doc comment is treated as Markdown, a shorthand notation for simple HTML formatting. Asterisks are used for *italics* and **bold type**, a blank line is treated as a paragraph break, and so on. However, you can also fall back on HTML; any HTML tags in your doc comments are copied through verbatim into the documentation.

You can use `backticks` to set off bits of code in the middle of running text. In the output, these snippets will be formatted in a fixed-width font. Larger code samples can be added by indenting four spaces.

/// A block of code in a doc comment:
///
///     if everything().works() {
///         println!("ok");
///     }

You can also use Markdown fenced code blocks. This has exactly the same effect.

/// Another snippet, the same code, but written differently:
///
/// ```
/// if everything().works() {
///     println!("ok");
/// }
/// ```

Whichever format you use, an interesting thing happens when you include a block of code in a doc comment. Rust automatically turns it into a test.

Doc-Tests

When you run tests in a Rust library crate, Rust checks that all the code that appears in your documentation actually runs and works. It does this by taking each block of code that appears in a doc comment, compiling it as a separate executable crate, linking it with your library, and running it.

Here is a standalone example of a doc-test. Create a new project by running cargo new ranges and put this code in ranges/src/lib.rs:

use std::ops::Range;

/// Return true if two ranges overlap.
///
///     assert_eq!(ranges::overlap(0..7, 3..10), true);
///     assert_eq!(ranges::overlap(1..5, 101..105), false);
///
/// If either range is empty, they don't count as overlapping.
///
///     assert_eq!(ranges::overlap(0..0, 0..10), false);
///
pub fn overlap(r1: Range<usize>, r2: Range<usize>) -> bool {
    r1.start < r1.end && r2.start < r2.end &&
        r1.start < r2.end && r2.start < r1.end
}

The two small blocks of code in the doc comment appear in the documentation generated by cargo doc, as shown in Figure 8-3.

Figure 8-3. Documentation showing some doc-tests

They also become two separate tests:

$ cargo test
   Compiling ranges v0.1.0 (file:///.../ranges)
...
   Doc-tests ranges

running 2 tests
test overlap_0 ... ok
test overlap_1 ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured

If you pass the --verbose flag to Cargo, you’ll see that it’s using rustdoc --test to run these two tests. Rustdoc stores each code sample in a separate file, adding a few lines of boilerplate code, to produce two programs. Here’s the first:

extern crate ranges;
fn main() {
    assert_eq!(ranges::overlap(0..7, 3..10), true);
    assert_eq!(ranges::overlap(1..5, 101..105), false);
}

And here’s the second:

extern crate ranges;
fn main() {
    assert_eq!(ranges::overlap(0..0, 0..10), false);
}

The tests pass if these programs compile and run successfully.

These two code samples contain assertions, but that’s just because in this case, the assertions make decent documentation. The idea behind doc-tests is not to put all your tests into comments. Rather, you write the best possible documentation, and Rust makes sure the code samples in your documentation actually compile and run.

Very often a minimal working example includes some details, such as imports or setup code, that are necessary to make the code compile, but just aren’t important enough to show in the documentation. To hide a line of a code sample, put a # followed by a space at the beginning of that line:

/// Let the sun shine in and run the simulation for a given
/// amount of time.
///
///     # use fern_sim::Terrarium;
///     # use std::time::Duration;
///     # let mut tm = Terrarium::new();
///     tm.apply_sunlight(Duration::from_secs(60));
///
pub fn apply_sunlight(&mut self, time: Duration) {
    ...
}

Sometimes it’s helpful to show a complete sample program in documentation, including a main function and an extern crate declaration. Obviously, if those pieces of code appear in your code sample, you do not also want Rustdoc to add them automatically. The result wouldn’t compile. Rustdoc therefore treats any code block containing the exact string fn main as a complete program, and doesn’t add anything to it.

Testing can be disabled for specific blocks of code. To tell Rust to compile your example, but stop short of actually running it, use a fenced code block with the no_run annotation:

/// Upload all local terrariums to the online gallery.
///
/// ```no_run
/// let mut session = fern_sim::connect();
/// session.upload_all();
/// ```
pub fn upload_all(&mut self) {
    ...
}

If the code isn’t even expected to compile, use ignore instead of no_run. If the code block isn’t Rust code at all, use the name of the language, like c++ or sh, or text for plain text. rustdoc doesn’t know the names of hundreds of programming languages; rather, it treats any annotation it doesn’t recognize as indicating that the code block isn’t Rust. This disables code highlighting as well as doc-testing.

Specifying Dependencies

We’ve seen one way of telling Cargo where to get source code for crates your project depends on: by version number.

image = "0.6.1"

There are several ways to specify dependencies, and some rather nuanced things you might want to say about which versions to use, so it’s worth spending a few pages on this.

First of all, you may want to use dependencies that aren’t published on crates.io at all. One way to do this is by specifying a Git repository URL and revision:

image = { git = "https://github.com/Piston/image.git", rev = "528f19c" }

This particular crate is open source, hosted on GitHub, but you could just as easily point to a private Git repository hosted on your corporate network. As shown here, you can specify the particular rev, tag, or branch to use. (These are all ways of telling Git which revision of the source code to check out.)

Another alternative is to specify a directory that contains the crate’s source code:

image = { path = "vendor/image" }

This is convenient when your team has a single version control repository that contains source code for several crates, or perhaps the entire dependency graph. Each crate can specify its dependencies using relative paths.

Having this level of control over your dependencies is powerful. If you ever decide that any of the open source crates you use isn’t exactly to your liking, you can trivially fork it: just hit the Fork button on GitHub and change one line in your Cargo.toml file. Your next cargo build will seamlessly use your fork of the crate instead of the official version.

Versions

When you write something like image = "0.6.1" in your Cargo.toml file, Cargo interprets this rather loosely. It uses the most recent version of image that is considered compatible with version 0.6.1.

The compatibility rules are adapted from Semantic Versioning.

A version number that starts with 0.0 is so raw that Cargo never assumes it’s compatible with any other version.

A version number that starts with 0.x, where x is nonzero, is considered compatible with other point releases in the 0.x series. We specified image version 0.6.1, but Cargo would use 0.6.3 if available. (This is not what the Semantic Versioning standard says about 0.x version numbers, but the rule proved too useful to leave out.)

Once a project reaches 1.0, only new major versions break compatibility. So if you ask for version 2.0.1, Cargo might use 2.17.99 instead, but not 3.0.

Version numbers are flexible by default because otherwise the problem of which version to use would quickly become overconstrained. Suppose one library, libA, used num = "0.1.31" while another, libB, used num = "0.1.29". If version numbers required exact matches, no project would be able to use those two libraries together. Allowing Cargo to use any compatible version is a much more practical default.

Still, different projects have different needs when it comes to dependencies and versioning. You can specify an exact version or range of versions by using operators:

Cargo.toml line | Meaning
image = "=0.10.0" | Use only the exact version 0.10.0
image = ">=1.0.5" | Use 1.0.5 or any higher version (even 2.9, if it’s available)
image = ">1.0.5 <1.1.9" | Use a version that’s higher than 1.0.5, but lower than 1.1.9
image = "<=2.7.10" | use any version up to 2.7.10

Another version specification you’ll occasionally see is the wildcard *. This tells Cargo that any version will do. Unless some other Cargo.toml file contains a more specific constraint, Cargo will use the latest available version. The Cargo documentation at doc.crates.io covers version specifications in even more detail.

Note that the compatibility rules mean that version numbers can’t be chosen purely for marketing reasons. They actually mean something. They’re a contract between a crate’s maintainers and its users. If you maintain a crate that’s at version 1.7, and you decide to remove a function or make any other change that isn’t fully backward compatible, you must bump your version number to 2.0. If you were to call it 1.8, you’d be claiming that the new version is compatible with 1.7, and your users might find themselves with broken builds.

Cargo.lock

The version numbers in Cargo.toml are deliberately flexible, yet we don’t want Cargo to upgrade us to the latest library versions every time we build. Imagine being in the middle of an intense debugging session when suddenly cargo build upgrades you to a new version of a library. This could be incredibly disruptive. Anything changing in the middle of debugging is bad. In fact, when it comes to libraries, there’s never a good time for an unexpected change.

Cargo therefore has a built-in mechanism to prevent this. The first time you build a project, Cargo outputs a Cargo.lock file that records the exact version of every crate it used. Later builds will consult this file and continue to use the same versions. Cargo upgrades to newer versions only when you tell it to, either by manually bumping up the version number in your Cargo.toml file, or by running cargo update:

$ cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
    Updating libc v0.2.7 -> v0.2.11
    Updating png v0.4.2 -> v0.4.3

cargo update only upgrades to the latest versions that are compatible with what you’ve specified in Cargo.toml. If you’ve specified image = "0.6.1", and you want to upgrade to version 0.10.0, you’ll have to change that in Cargo.toml. The next time you build, Cargo will update to the new version of the image library and store the new version number in Cargo.lock.

The preceding example shows Cargo updating two crates that are hosted on crates.io. Something very similar happens for dependencies that are stored in Git. Suppose our Cargo.toml file contains this:

image = { git = "https://github.com/Piston/image.git", branch = "master" }

cargo build will not pull new changes from the Git repository if it sees that we’ve got a Cargo.lock file. Instead, it reads Cargo.lock and uses the same revision as last time. But cargo update will pull from master, so that our next build uses the latest revision.

Cargo.lock is automatically generated for you, and you normally won’t edit it by hand. Nonetheless, if your project is an executable, you should commit Cargo.lock to version control. That way, everyone who builds your project will consistently get the same versions. The history of your Cargo.lock file will record your dependency updates.

If your project is an ordinary Rust library, don’t bother committing Cargo.lock. Your library’s downstream users will have Cargo.lock files that contain version information for their entire dependency graph; they will ignore your library’s Cargo.lock file. In the rare case that your project is a shared library (i.e., the output is a .dll, .dylib, or .so file), there is no such downstream cargo user, and you should therefore commit Cargo.lock.

Cargo.toml’s flexible version specifiers make it easy to use Rust libraries in your project and maximize compatibility among libraries. Cargo.lock’s bookkeeping supports consistent, reproducible builds across machines. Together, they go a long way toward helping you avoid dependency hell.

Publishing Crates to crates.io

You’ve decided to publish your fern-simulating library as open source software. Congratulations! This part is easy.

First, make sure Cargo can pack the crate for you.

$ cargo package
warning: manifest has no description, license, license-file, documentation,
homepage or repository. See http://doc.crates.io/manifest.html#package-metadata
for more info.
   Packaging fern_sim v0.1.0 (file:///.../fern_sim)
   Verifying fern_sim v0.1.0 (file:///.../fern_sim)
   Compiling fern_sim v0.1.0 (file:///.../fern_sim/target/package/fern_sim-0.1.0)

The cargo package command creates a file (in this case, target/package/fern_sim-0.1.0.crate) containing all your library’s source files, including Cargo.toml. This is the file that you’ll upload to crates.io to share with the world. (You can use cargo package --list to see which files are included.) Cargo then double-checks its work by building your library from the .crate file, just as your eventual users will.

Cargo warns that the [package] section of Cargo.toml is missing some information that will be important to downstream users, such as the license under which you’re distributing the code. The URL in the warning is an excellent resource, so we won’t explain all the fields in detail here. In short, you can fix the warning by adding a few lines to Cargo.toml:

[package]
name = "fern_sim"
version = "0.1.0"
authors = ["You <you@example.com>"]
license = "MIT"
homepage = "https://fernsim.example.com/"
repository = "https://gitlair.com/sporeador/fern_sim"
documentation = "http://fernsim.example.com/docs"
description = """
Fern simulation, from the cellular level up.
"""

Note

Once you publish this crate on crates.io, anyone who downloads your crate can see the Cargo.toml file. So if the authors field contains an email address that you’d rather keep private, now’s the time to change it.

Another problem that sometimes arises at this stage is that your Cargo.toml file might be specifying the location of other crates by path, as shown in “Specifying Dependencies”:

image = { path = "vendor/image" }

For you and your team, this might work fine. But naturally, when other people download the fern_sim library, they will not have the same files and directories on their computer that you have. Cargo therefore ignores the path key in automatically downloaded libraries, and this can cause build errors. The fix, however, is straightforward: if your library is going to be published on crates.io, its dependencies should be on crates.io too. Specify a version number instead of a path:

image = "0.6.1"

If you prefer, you can specify both a path, which takes precedence for your own local builds, and a version for all other users:

image = { path = "vendor/image", version = "0.6.1" }

Of course, in that case it’s your responsibility to make sure that the two stay in sync.

Lastly, before publishing a crate, you’ll need to log in to crates.io and get an API key. This step is straightforward: once you have an account on crates.io, your “Account Settings” page will show a cargo login command, like this one:

$ cargo login 5j0dV54BjlXBpUUbfIj7G9DvNl1vsWW1

Cargo saves the key in a configuration file, and the API key should be kept secret, like a password. So run this command only on a computer you control.

That done, the final step is to run cargo publish:

$ cargo publish
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Uploading fern_sim v0.1.0 (file:///.../fern_sim)

With this, your library joins thousands of others on crates.io.

Workspaces

As your project continues to grow, you end up writing many crates. They live side by side in a single source repository:

fernsoft/
├── .git/...
├── fern_sim/
│   ├── Cargo.toml
│   ├── Cargo.lock
│   ├── src/...
│   └── target/...
├── fern_img/
│   ├── Cargo.toml
│   ├── Cargo.lock
│   ├── src/...
│   └── target/...
└── fern_video/
    ├── Cargo.toml
    ├── Cargo.lock
    ├── src/...
    └── target/...

The way Cargo works, each crate has its own build directory, target, which contains a separate build of all that crate’s dependencies. These build directories are completely independent. Even if two crates have a common dependency, they can’t share any compiled code. This is wasteful.

You can save compilation time and disk space by using a Cargo workspace, a collection of crates that share a common build directory and Cargo.lock file.

All you need to do is create a Cargo.toml file in your repository’s root directory and put these lines in it:

[workspace]
members = ["fern_sim", "fern_img", "fern_video"]

where fern_sim etc. are the names of the subdirectories containing your crates. Delete any leftover Cargo.lock files and target directories that exist in those subdirectories.

Once you’ve done this, cargo build in any crate will automatically create and use a shared build directory under the root directory (in this case, fernsoft/target). The command cargo build --all builds all crates in the current workspace. cargo test and cargo doc accept the --all option as well.

More Nice Things

In case you’re not delighted yet, the Rust community has a few more odds and ends for you:

When you publish an open source crate on crates.io, your documentation is automatically rendered and hosted on docs.rs thanks to Onur Aslan.

If your project is on GitHub, Travis CI can build and test your code on every push. It’s surprisingly easy to set up; see travis-ci.org for details. If you’re already familiar with Travis, this .travis.yml file will get you started:

language: rust
rust:
  - stable

You can generate a README.md file from your crate’s top-level doc-comment. This feature is offered as a third-party Cargo plugin by Livio Ribeiro. Run cargo install readme to install the plugin, then cargo readme --help to learn how to use it.

We could go on.

Rust is new, but it’s designed to support large, ambitious projects. It has great tools and an active community. System programmers can have nice things.

Chapter 9. Structs

Long ago, when shepherds wanted to see if two herds of sheep were isomorphic, they would look for an explicit isomorphism.

John C. Baez and James Dolan, “Categorification”

Rust structs, sometimes called structures, resemble struct types in C and C++, classes in Python, and objects in JavaScript. A struct assembles several values of assorted types together into a single value, so you can deal with them as a unit. Given a struct, you can read and modify its individual components. And a struct can have methods associated with it that operate on its components.

Rust has three kinds of struct types, named-field, tuple-like, and unit-like, which differ in how you refer to their components: a named-field struct gives a name to each component, whereas a tuple-like struct identifies them by the order in which they appear. Unit-like structs have no components at all; these are not common, but more useful than you might think.

In this chapter, we’ll explain each kind in detail, and show what they look like in memory. We’ll cover how to add methods to them, how to define generic struct types that work with many different component types, and how to ask Rust to generate implementations of common handy traits for your structs.

Named-Field Structs

The definition of a named-field struct type looks like this:

/// A rectangle of eight-bit grayscale pixels.
struct GrayscaleMap {
    pixels: Vec<u8>,
    size: (usize, usize)
}

This declares a type GrayscaleMap with two fields named pixels and size, of the given types. The convention in Rust is for all types, structs included, to have names that capitalize the first letter of each word, like GrayscaleMap, a convention called CamelCase. Fields and methods are lowercase, with words separated by underscores. This is called snake_case.

You can construct a value of this type with a struct expression, like this:

let width = 1024;
let height = 576;
let image = GrayscaleMap {
    pixels: vec![0; width * height],
    size: (width, height)
};

A struct expression starts with the type name (GrayscaleMap), and lists the name and value of each field, all enclosed in curly braces. There’s also shorthand for populating fields from local variables or arguments with the same name:

fn new_map(size: (usize, usize), pixels: Vec<u8>) -> GrayscaleMap {
    assert_eq!(pixels.len(), size.0 * size.1);
    GrayscaleMap { pixels, size }
}

The struct expression GrayscaleMap { pixels, size } is short for GrayscaleMap { pixels: pixels, size: size }. You can use key: value syntax for some fields and shorthand for others in the same struct expression.

To access a struct’s fields, use the familiar . operator:

assert_eq!(image.size, (1024, 576));
assert_eq!(image.pixels.len(), 1024 * 576);

Like all other items, structs are private by default, visible only in the module where they’re declared. You can make a struct visible outside its module by prefixing its definition with pub. The same goes for each of its fields, which are also private by default:

/// A rectangle of eight-bit grayscale pixels.
pub struct GrayscaleMap {
    pub pixels: Vec<u8>,
    pub size: (usize, usize)
}

Even if a struct is declared pub, its fields can be private:

/// A rectangle of eight-bit grayscale pixels.
pub struct GrayscaleMap {
    pixels: Vec<u8>,
    size: (usize, usize)
}

Other modules can use this struct and any public methods it might have, but can’t access the private fields by name or use struct expressions to create new GrayscaleMap values. That is, creating a struct value requires all the struct’s fields to be visible. This is why you can’t write a struct expression to create a new String or Vec. These standard types are structs, but all their fields are private. To create one, you must use public methods like Vec::new().

When creating a named-field struct value, you can use another struct of the same type to supply values for fields you omit. In a struct expression, if the named fields are followed by .. EXPR, then any fields not mentioned take their values from EXPR, which must be another value of the same struct type. Suppose we have a struct representing a monster in a game:

struct Broom {
    name: String,
    height: u32,
    health: u32,
    position: (f32, f32, f32),
    intent: BroomIntent
}

/// Two possible alternatives for what a `Broom` could be working on.
#[derive(Copy, Clone)]
enum BroomIntent { FetchWater, DumpWater }

The best fairy tale for programmers is The Sorcerer’s Apprentice: a novice magician enchants a broom to do his work for him, but doesn’t know how to stop it when the job is done. Chopping the broom in half with an axe just produces two brooms, each of half the size, but continuing the task with the same blind dedication as the original:

// Receive the input Broom by value, taking ownership.
fn chop(b: Broom) -> (Broom, Broom) {
    // Initialize `broom1` mostly from `b`, changing only `height`. Since
    // `String` is not `Copy`, `broom1` takes ownership of `b`'s name.
    let mut broom1 = Broom { height: b.height / 2, .. b };

    // Initialize `broom2` mostly from `broom1`. Since `String` is not
    // `Copy`, we must clone `name` explicitly.
    let mut broom2 = Broom { name: broom1.name.clone(), .. broom1 };

    // Give each fragment a distinct name.
    broom1.name.push_str(" I");
    broom2.name.push_str(" II");

    (broom1, broom2)
}

With that definition in place, we can create a broom, chop it in two, and see what we get:

let hokey = Broom {
    name: "Hokey".to_string(),
    height: 60,
    health: 100,
    position: (100.0, 200.0, 0.0),
    intent: BroomIntent::FetchWater
};

let (hokey1, hokey2) = chop(hokey);
assert_eq!(hokey1.name, "Hokey I");
assert_eq!(hokey1.health, 100);

assert_eq!(hokey2.name, "Hokey II");
assert_eq!(hokey2.health, 100);

Tuple-Like Structs

The second kind of struct type is called a tuple-like struct, because it resembles a tuple:

struct Bounds(usize, usize);

You construct a value of this type much as you would construct a tuple, except that you must include the struct name:

let image_bounds = Bounds(1024, 768);

The values held by a tuple-like struct are called elements, just as the values of a tuple are. You access them just as you would a tuple’s:

assert_eq!(image_bounds.0 * image_bounds.1, 786432);

Individual elements of a tuple-like struct may be public or not:

pub struct Bounds(pub usize, pub usize);

The expression Bounds(1024, 768) looks like a function call, and in fact it is: defining the type also implicitly defines a function:

fn Bounds(elem0: usize, elem1: usize) -> Bounds { ... }

At the most fundamental level, named-field and tuple-like structs are very similar. The choice of which to use comes down to questions of legibility, ambiguity, and brevity. If you will use the . operator to get at a value’s components much at all, identifying fields by name provides the reader more information, and is probably more robust against typos. If you will usually use pattern matching to find the elements, tuple-like structs can work nicely.

Tuple-like structs are good for newtypes, structs with a single component that you define to get stricter type checking. For example, if you are working with ASCII-only text, you might define a newtype like this:

struct Ascii(Vec<u8>);

Using this type for your ASCII strings is much better than simply passing around Vec<u8> buffers and explaining what they are in the comments. The newtype helps Rust catch mistakes where some other byte buffer is passed to a function expecting ASCII text. We’ll give an example of using newtypes for efficient type conversions in Chapter 21.

Unit-Like Structs

The third kind of struct is a little obscure: it declares a struct type with no elements at all:

struct Onesuch;

A value of such a type occupies no memory, much like the unit type (). Rust doesn’t bother actually storing unit-like struct values in memory or generating code to operate on them, because it can tell everything it might need to know about the value from its type alone. But logically, an empty struct is a type with values like any other—or more precisely, a type of which there is only a single value:

let o = Onesuch;

You’ve already encountered a unit-like struct when reading about “Fields and Elements”. Whereas an expression like 3..5 is shorthand for the struct value Range { start: 3, end: 5 }, the expression .., a range omitting both endpoints, is shorthand for the unit-like struct value RangeFull.

Unit-like structs can also be useful when working with traits, which we’ll describe in Chapter 11.

Struct Layout

In memory, both named-field and tuple-like structs are the same thing: a collection of values, of possibly mixed types, laid out in a particular way in memory. For example, earlier in the chapter we defined this struct:

struct GrayscaleMap {
    pixels: Vec<u8>,
    size: (usize, usize)
}

A GrayscaleMap value is laid out in memory as diagrammed in Figure 9-1.

Figure 9-1. A GrayscaleMap structure in memory

Unlike C and C++, Rust doesn’t make specific promises about how it will order a struct’s fields or elements in memory; this diagram shows only one possible arrangement. However, Rust does promise to store fields’ values directly in the struct’s block of memory. Whereas JavaScript, Python, and Java would put the pixels and size values each in their own heap-allocated blocks and have GrayscaleMap’s fields point at them, Rust embeds pixels and size directly in the GrayscaleMap value. Only the heap-allocated buffer owned by the pixels vector remains in its own block.

You can ask Rust to lay out structures in a way compatible with C and C++, using the #[repr(C)] attribute. We’ll cover this in detail in Chapter 21.

Defining Methods with impl

Throughout the book we’ve been calling methods on all sorts of values. We’ve pushed elements onto vectors with v.push(e), fetched their length with v.len(), checked Result values for errors with r.expect("msg"), and so on.

You can define methods on any struct type you define. Rather than appearing inside the struct definition, as in C++ or Java, Rust methods appear in a separate impl block. For example:

/// A last-in, first-out queue of characters.
pub struct Queue {
    older: Vec<char>,   // older elements, eldest last.
    younger: Vec<char>  // younger elements, youngest last.
}

impl Queue {
    /// Push a character onto the back of a queue.
    pub fn push(&mut self, c: char) {
        self.younger.push(c);
    }

    /// Pop a character off the front of a queue. Return `Some(c)` if there
    /// was a character to pop, or `None` if the queue was empty.
    pub fn pop(&mut self) -> Option<char> {
        if self.older.is_empty() {
            if self.younger.is_empty() {
                return None;
            }

            // Bring the elements in younger over to older, and put them in
            // the promised order.
            use std::mem::swap;
            swap(&mut self.older, &mut self.younger);
            self.older.reverse();
        }

        // Now older is guaranteed to have something. Vec's pop method
        // already returns an Option, so we're set.
        self.older.pop()
    }
}

An impl block is simply a collection of fn definitions, each of which becomes a method on the struct type named at the top of the block. Here we’ve defined a public struct Queue, and then given it two public methods, push and pop.

Methods are also known as associated functions, since they’re associated with a specific type. The opposite of an associated function is a free function, one that is not defined as an impl block’s item.

Rust passes a method the value it’s being called on as its first argument, which must have the special name self. Since self’s type is obviously the one named at the top of the impl block, or a reference to that, Rust lets you omit the type, and write self, &self or &mut self as shorthand for self: Queue, self: &Queue or self: &mut Queue. You can use the longhand forms if you like, but almost all Rust code uses the shorthand, as shown before.

In our example, the push and pop methods refer to the Queue’s fields as self.older and self.younger. Unlike C++ and Java, where the members of the “this” object are directly visible in method bodies as unqualified identifiers, a Rust method must explicitly use self to refer to the value it was called on, similar to the way Python methods use self, and the way JavaScript methods use this.

Since push and pop need to modify the Queue, they both take &mut self. However, when you call a method, you don’t need to borrow the mutable reference yourself; the ordinary method call syntax takes care of that implicitly. So with these definitions in place, you can use Queue like this:

let mut q = Queue { older: Vec::new(), younger: Vec::new() };

q.push('0');
q.push('1');
assert_eq!(q.pop(), Some('0'));

q.push('∞');
assert_eq!(q.pop(), Some('1'));
assert_eq!(q.pop(), Some('∞'));
assert_eq!(q.pop(), None);

Simply writing q.push(...) borrows a mutable reference to q, as if you had written (&mut q).push(...), since that’s what the push method’s self requires.

If a method doesn’t need to modify its self, then you can define it to take a shared reference instead. For example:

impl Queue {
    pub fn is_empty(&self) -> bool {
        self.older.is_empty() && self.younger.is_empty()
    }
}

Again, the method call expression knows which sort of reference to borrow:

assert!(q.is_empty());
q.push('☉');
assert!(!q.is_empty());

Or, if a method wants to take ownership of self, it can take self by value:

impl Queue {
    pub fn split(self) -> (Vec<char>, Vec<char>) {
        (self.older, self.younger)
    }
}

Calling this split method looks like the other method calls:

let mut q = Queue { older: Vec::new(), younger: Vec::new() };

q.push('P');
q.push('D');
assert_eq!(q.pop(), Some('P'));
q.push('X');

let (older, younger) = q.split();
// q is now uninitialized.
assert_eq!(older, vec!['D']);
assert_eq!(younger, vec!['X']);

But note that, since split takes its self by value, this moves the Queue out of q, leaving q uninitialized. Since split’s self now owns the queue, it’s able to move the individual vectors out of it, and return them to the caller.

You can also define methods that don’t take self as an argument at all. These become functions associated with the struct type itself, not with any specific value of the type. Following the tradition established by C++ and Java, Rust calls these static methods. They’re often used to provide constructor functions, like this:

impl Queue {
    pub fn new() -> Queue {
        Queue { older: Vec::new(), younger: Vec::new() }
    }
}

To use this method, we refer to it as Queue::new: the type name, a double colon, and then the method name. Now our example code becomes a bit more svelte:

let mut q = Queue::new();

q.push('*');
...

It’s conventional in Rust for constructor functions to be named new; we’ve already seen Vec::new, Box::new, HashMap::new, and others. But there’s nothing special about the name new. It’s not a keyword, and types often have other static methods that serve as constructors, like Vec::with_capacity.

Although you can have many separate impl blocks for a single type, they must all be in the same crate that defines that type. However, Rust does let you attach your own methods to other types; we’ll explain how in Chapter 11.

If you’re used to C++ or Java, separating a type’s methods from its definition may seem unusual, but there are several advantages to doing so:

It’s always easy to find a type’s data members. In large C++ class definitions, you might need to skim hundreds of lines of member function definitions to be sure you haven’t missed any of the class’s data members; in Rust, they’re all in one place.

Although one can imagine fitting methods into the syntax for named-field structs, it’s not so neat for tuple-like and unit-like structs. Pulling methods out into an impl block allows a single syntax for all three. In fact, Rust uses this same syntax for defining methods on types that are not structs at all, such as enum types and primitive types like i32. (The fact that any type can have methods is one reason Rust doesn’t use the term object much, preferring to call everything a value.)

The same impl syntax also serves neatly for implementing traits, which we’ll go into in Chapter 11.

Generic Structs

Our earlier definition of Queue is unsatisfying: it is written to store characters, but there’s nothing about its structure or methods that is specific to characters at all. If we were to define another struct that held, say, String values, the code could be identical, except that char would be replaced with String. That would be a waste of time.

Fortunately, Rust structs can be generic, meaning that their definition is a template into which you can plug whatever types you like. For example, here’s a definition for Queue that can hold values of any type:

pub struct Queue<T> {
    older: Vec<T>,
    younger: Vec<T>
}

You can read the <T> in Queue<T> as “for any element type T...”. So this definition reads, “For any type T, a Queue<T> is two fields of type Vec<T>.” For example, in Queue<String>, T is String, so older and younger have type Vec<String>. In Queue<char>, T is char, and we get a struct identical to the char-specific definition we started with. In fact, Vec itself is a generic struct, defined in just this way.

In generic struct definitions, the type names used in <angle brackets> are called type parameters. An impl block for a generic struct looks like this:

impl<T> Queue<T> {
    pub fn new() -> Queue<T> {
        Queue { older: Vec::new(), younger: Vec::new() }
    }

    pub fn push(&mut self, t: T) {
        self.younger.push(t);
    }

    pub fn is_empty(&self) -> bool {
        self.older.is_empty() && self.younger.is_empty()
    }

    ...
}

You can read the line impl<T> Queue<T> as something like, “for any type T, here are some methods available on Queue<T>.” Then, you can use the type parameter T as a type in the method definitions.

We’ve used Rust’s shorthand for self parameters in the preceding code; writing out Queue<T> everywhere becomes a mouthful and a distraction. As another shorthand, every impl block, generic or not, defines the special type parameter Self (note the CamelCase name) to be whatever type we’re adding methods to. In the preceding code, Self would be Queue<T>, so we can abbreviate Queue::new’s definition a bit further:

pub fn new() -> Self {
    Queue { older: Vec::new(), younger: Vec::new() }
}

You might have noticed that, in the body of new, we didn’t need to write the type parameter in the construction expression; simply writing Queue { ... } was good enough. This is Rust’s type inference at work: since there’s only one type that works for that function’s return value—namely, Queue<T>—Rust supplies the parameter for us. However, you’ll always need to supply type parameters in function signatures and type definitions. Rust doesn’t infer those; instead, it uses those explicit types as the basis from which it infers types within function bodies.

For static method calls, you can supply the type parameter explicitly using the turbofish ::<> notation:

let mut q = Queue::<char>::new();

But in practice, you can usually just let Rust figure it out for you:

let mut q = Queue::new();
let mut r = Queue::new();

q.push("CAD");  // apparently a Queue<&'static str>
r.push(0.74);   // apparently a Queue<f64>

q.push("BTC");  // Bitcoins per USD, 2017-5
r.push(2737.7); // Rust fails to detect irrational exuberance

In fact, this is exactly what we’ve been doing with Vec, another generic struct type, throughout the book.

It’s not just structs that can be generic. Enums can take type parameters as well, with a very similar syntax. We’ll show that in detail in “Enums”.

Structs with Lifetime Parameters

As we discussed in “Structs Containing References”, if a struct type contains references, you must name those references’ lifetimes. For example, here’s a structure that might hold references to the greatest and least elements of some slice:

struct Extrema<'elt> {
    greatest: &'elt i32,
    least: &'elt i32
}

Earlier, we invited you to think of a declaration like struct Queue<T> as meaning that, given any specific type T, you can make a Queue<T> that holds that type. Similarly, you can think of struct Extrema<'elt> as meaning that, given any specific lifetime 'elt, you can make an Extrema<'elt> that holds references with that lifetime.

Here’s a function to scan a slice and return an Extrema value whose fields refer to its elements:

fn find_extrema<'s>(slice: &'s [i32]) -> Extrema<'s> {
    let mut greatest = &slice[0];
    let mut least = &slice[0];

    for i in 1..slice.len() {
        if slice[i] < *least    { least    = &slice[i]; }
        if slice[i] > *greatest { greatest = &slice[i]; }
    }
    Extrema { greatest, least }
}

Here, since find_extrema borrows elements of slice, which has lifetime 's, the Extrema struct we return also uses 's as the lifetime of its references. Rust always infers lifetime parameters for calls, so calls to find_extrema needn’t mention them:

let a = [0, -3, 0, 15, 48];
let e = find_extrema(&a);
assert_eq!(*e.least, -3);
assert_eq!(*e.greatest, 48);

Because it’s so common for the return type to use the same lifetime as an argument, Rust lets us omit the lifetimes when there’s one obvious candidate. We could also have written find_extrema’s signature like this, with no change in meaning:

fn find_extrema(slice: &[i32]) -> Extrema {
    ...
}

Granted, we might have meant Extrema<'static>, but that’s pretty unusual. Rust provides a shorthand for the common case.

Deriving Common Traits for Struct Types

Structs can be very easy to write:

struct Point {
    x: f64,
    y: f64
}

However, if you were to start using this Point type, you would quickly notice that it’s a bit of a pain. As written, Point is not copyable or cloneable. You can’t print it with println!("{:?}", point); and it does not support the == and != operators.

Each of these features has a name in Rust—Copy, Clone, Debug, and PartialEq. They are called traits. In Chapter 11, we’ll show how to implement traits by hand for your own structs. But in the case of these standard traits, and several others, you don’t need to implement them by hand unless you want some kind of custom behavior. Rust can automatically implement them for you, with mechanical accuracy. Just add a #[derive] attribute to the struct:

#[derive(Copy, Clone, Debug, PartialEq)]
struct Point {
    x: f64,
    y: f64
}

Each of these traits can be implemented automatically for a struct, provided that each of its fields implements the trait. We can ask Rust to derive PartialEq for Point because its two fields are both of type f64, which already implements PartialEq.

Rust can also derive PartialCmp, which would add support for the comparison operators <, >, <=, and >=. We haven’t done so here, because comparing two points to see if one is “less than” the other is actually a pretty weird thing to do. There’s no one conventional order on points. So we choose not to support those operators for Point values. Cases like this are one reason that Rust makes us write the #[derive] attribute rather than automatically deriving every trait it can. Another reason is that implementing a trait is automatically a public feature, so copyability, cloneability, and so forth are all part of your struct’s public API and should be chosen deliberately.

We’ll describe Rust’s standard traits in detail, and tell which ones are #[derive]able, in Chapter 13.

Interior Mutability

Mutability is like anything else: in excess, it causes problems, but you often want just a little bit of it. For example, say your spider robot control system has a central struct, SpiderRobot, that contains settings and I/O handles. It’s set up when the robot boots, and the values never change:

pub struct SpiderRobot {
    species: String,
    web_enabled: bool,
    leg_devices: [fd::FileDesc; 8],
    ...
}

Every major system of the robot is handled by a different struct, and each one has a pointer back to the SpiderRobot:

use std::rc::Rc;

pub struct SpiderSenses {
    robot: Rc<SpiderRobot>,  // <-- pointer to settings and I/O
    eyes: [Camera; 32],
    motion: Accelerometer,
    ...
}

The structs for web construction, predation, venom flow control, and so forth also each have an Rc<SpiderRobot> smart pointer. Recall that Rc stands for reference counting, and a value in an Rc box is always shared and therefore always immutable.

Now suppose you want to add a little logging to the SpiderRobot struct, using the standard File type. There’s a problem: a File has to be mut. All the methods for writing to it require a mut reference.

This sort of situation comes up fairly often. What we need is a little bit of mutable data (a File) inside an otherwise immutable value (the SpiderRobot struct). This is called interior mutability. Rust offers several flavors of it; in this section, we’ll discuss the two most straightforward types: Cell<T> and RefCell<T>, both in the std::cell module.

A Cell<T> is a struct that contains a single private value of type T. The only special thing about a Cell is that you can get and set the field even if you don’t have mut access to the Cell itself:

Cell::new(value) creates a new Cell, moving the given value into it.

cell.get() returns a copy of the value in the cell.

cell.set(value) stores the given value in the cell, dropping the previously stored value.

This method takes self as a non-mut reference:

fn set(&self, value: T)    // note: not `&mut self`

This is, of course, unusual for methods named set. By now, Rust has trained us to expect that we need mut access if we want to make changes to data. But by the same token, this one unusual detail is the whole point of Cells. They’re simply a safe way of bending the rules on immutability—no more, no less.

Cells also have a few other methods, which you can read about in the documentation.

A Cell would be handy if you were adding a simple counter to your SpiderRobot. You could write:

use std::cell::Cell;

pub struct SpiderRobot {
    ...
    hardware_error_count: Cell<u32>,
    ...
}

and then even non-mut methods of SpiderRobot can access that u32, using the .get() and .set() methods:

impl SpiderRobot {
    /// Increase the error count by 1.
    pub fn add_hardware_error(&self) {
        let n = self.hardware_error_count.get();
        self.hardware_error_count.set(n + 1);
    }

    /// True if any hardware errors have been reported.
    pub fn has_hardware_errors(&self) -> bool {
        self.hardware_error_count.get() > 0
    }
}

This is easy enough, but it doesn’t solve our logging problem. Cell does not let you call mut methods on a shared value. The .get() method returns a copy of the value in the cell, so it works only if T implements the Copy trait. For logging, we need a mutable File, and File isn’t copyable.

The right tool in this case is a RefCell. Like Cell<T>, RefCell<T> is a generic type that contains a single value of type T. Unlike Cell, RefCell supports borrowing references to its T value:

RefCell::new(value) creates a new RefCell, moving value into it.

ref_cell.borrow() returns a Ref<T>, which is essentially just a shared reference to the value stored in ref_cell.

This method panics if the value is already mutably borrowed; see details to follow.

ref_cell.borrow_mut() returns a RefMut<T>, essentially a mutable reference to the value in ref_cell.

This method panics if the value is already borrowed; see details to follow.

Again, RefCell has a few other methods, which you can find in the documentation.

The two borrow methods panic only if you try to break the Rust rule that mut references are exclusive references. For example, this would panic:

let ref_cell: RefCell<String> = RefCell::new("hello".to_string());

let r = ref_cell.borrow();      // ok, returns a Ref<String>
let count = r.len();            // ok, returns "hello".len()
assert_eq!(count, 5);

let mut w = ref_cell.borrow_mut();  // panic: already borrowed
w.push_str(" world");

To avoid panicking, you could put these two borrows into separate blocks. That way, r would be dropped before you try to borrow w.

This is a lot like how normal references work. The only difference is that normally, when you borrow a reference to a variable, Rust checks at compile time to ensure that you’re using the reference safely. If the checks fail, you get a compiler error. RefCell enforces the same rule using runtime checks. So if you’re breaking the rules, you get a panic.

Now we’re ready to put RefCell to work in our SpiderRobot type:

pub struct SpiderRobot {
    ...
    log_file: RefCell<File>,
    ...
}

impl SpiderRobot {
    /// Write a line to the log file.
    pub fn log(&self, message: &str) {
        let mut file = self.log_file.borrow_mut();
        writeln!(file, "{}", message).unwrap();
    }
}

The variable file has type RefMut<File>. It can be used just like a mutable reference to a File. For details about writing to files, see Chapter 18.

Cells are easy to use. Having to call .get() and .set() or .borrow() and .borrow_mut() is slightly awkward, but that’s just the price we pay for bending the rules. The other drawback is less obvious and more serious: cells—and any types that contain them—are not thread-safe. Rust therefore will not allow multiple threads to access them at once. We’ll describe thread-safe flavors of interior mutability in Chapter 19, when we discuss “Mutex<T>”, “Atomics”, and “Global Variables”.

Whether a struct has named fields or is tuple-like, it is an aggregation of other values: if I have a SpiderSenses struct, then I have an Rc pointer to a shared SpiderRobot struct, and I have eyes, and I have an accelerometer, and so on. So the essence of a struct is the word “and”: I have an X and a Y. But what if there were another kind of type built around the word “or”? That is, when you have a value of such a type, you’d have either an X or a Y? Such types turn out to be so useful that they’re ubiquitous in Rust, and they are the subject of the next chapter.

Chapter 10. Enums and Patterns

Surprising how much computer stuff makes sense viewed as tragic deprivation of sum types (cf. deprivation of lambdas)

Graydon Hoare

The first topic of this chapter is potent, as old as the hills, happy to help you get a lot done in short order (for a price), and known by many names in many cultures. But it’s not the devil. It’s a kind of user-defined data type, long known to ML and Haskell hackers as sum types, discriminated unions, or algebraic data types. In Rust, they are called enumerations, or simply enums. Unlike the devil, they are quite safe, and the price they ask is no great privation.

C++ and C# have enums; you can use them to define your own type whose values are a set of named constants. For example, you might define a type named Color with values Red, Orange, Yellow, and so on. This kind of enum works in Rust, too. But Rust takes enums much further. A Rust enum can also contain data, even data of varying types. For example, Rust’s Result<String, io::Error> type is an enum; such a value is either an Ok value containing a String, or an Err value containing an io::Error. This is beyond what C++ and C# enums can do. It’s more like a C union—but unlike unions, Rust enums are type-safe.

Enums are useful whenever a value might be either one thing or another. The “price” of using them is that you must access the data safely, using pattern matching, our topic for the second half of this chapter.

Patterns, too, may be familiar if you’ve used unpacking in Python or destructuring in JavaScript, but Rust takes patterns further. Rust patterns are a little like regular expressions for all your data. They’re used to test whether or not a value has a particular desired shape. They can extract several fields from a struct or tuple into local variables all at once. And like regular expressions, they are concise, typically doing it all in a single line of code.

Enums

Simple, C-style enums are straightforward:

enum Ordering {
    Less,
    Equal,
    Greater
}

This declares a type Ordering with three possible values, called variants or constructors: Ordering::Less, Ordering::Equal, and Ordering::Greater. This particular enum is part of the standard library, so Rust code can import it, either by itself:

use std::cmp::Ordering;

fn compare(n: i32, m: i32) -> Ordering {
    if n < m {
        Ordering::Less
    } else if n > m {
        Ordering::Greater
    } else {
        Ordering::Equal
    }
}

or with all its constructors:

use std::cmp::Ordering;
use std::cmp::Ordering::*;    // `*` to import all children

fn compare(n: i32, m: i32) -> Ordering {
    if n < m {
        Less
    } else if n > m {
        Greater
    } else {
        Equal
    }
}

After importing the constructors, we can write Less instead of Ordering::Less, and so on, but because this is less explicit, it’s generally considered better style not to import them except when it makes your code much more readable.

To import the constructors of an enum declared in the current module, use a self import:

enum Pet {
    Orca,
    Giraffe,
    ...
}

use self::Pet::*;

In memory, values of C-style enums are stored as integers. Occasionally it’s useful to tell Rust which integers to use:

enum HttpStatus {
    Ok = 200,
    NotModified = 304,
    NotFound = 404,
    ...
}

Otherwise Rust will assign the numbers for you, starting at 0.

By default, Rust stores C-style enums using the smallest built-in integer type that can accommodate them. Most fit in a single byte.

use std::mem::size_of;
assert_eq!(size_of::<Ordering>(), 1);
assert_eq!(size_of::<HttpStatus>(), 2);  // 404 doesn't fit in a u8

You can override Rust’s choice of in-memory representation by adding a #[repr] attribute to the enum. For details, see Chapter 21.

Casting a C-style enum to an integer is allowed:

assert_eq!(HttpStatus::Ok as i32, 200);

However, casting in the other direction, from the integer to the enum, is not. Unlike C and C++, Rust guarantees that an enum value is only ever one of the values spelled out in the enum declaration. An unchecked cast from an integer type to an enum type could break this guarantee, so it’s not allowed. You can either write your own checked conversion:

fn http_status_from_u32(n: u32) -> Option<HttpStatus> {
    match n {
        200 => Some(HttpStatus::Ok),
        304 => Some(HttpStatus::NotModified),
        404 => Some(HttpStatus::NotFound),
        ...
        _ => None
    }
}

or use the enum_primitive crate. It contains a macro that autogenerates this kind of conversion code for you.

As with structs, the compiler will implement features like the == operator for you, but you have to ask.

#[derive(Copy, Clone, Debug, PartialEq)]
enum TimeUnit {
    Seconds, Minutes, Hours, Days, Months, Years
}

Enums can have methods, just like structs:

impl TimeUnit {
    /// Return the plural noun for this time unit.
    fn plural(self) -> &'static str {
        match self {
            TimeUnit::Seconds => "seconds",
            TimeUnit::Minutes => "minutes",
            TimeUnit::Hours => "hours",
            TimeUnit::Days => "days",
            TimeUnit::Months => "months",
            TimeUnit::Years => "years"
        }
    }

    /// Return the singular noun for this time unit.
    fn singular(self) -> &'static str {
        self.plural().trim_right_matches('s')
    }
}

So much for C-style enums. The more interesting sort of Rust enum is the kind that contains data.

Enums with Data

Some programs always need to display full dates and times down to the millisecond, but for most applications, it’s more user-friendly to use a rough approximation, like “two months ago.” We can write an enum to help with that:

/// A timestamp that has been deliberately rounded off, so our program
/// says "6 months ago" instead of "February 9, 2016, at 9:49 AM".
#[derive(Copy, Clone, Debug, PartialEq)]
enum RoughTime {
    InThePast(TimeUnit, u32),
    JustNow,
    InTheFuture(TimeUnit, u32)
}

Two of the variants in this enum, InThePast and InTheFuture, take arguments. These are called tuple variants. Like tuple structs, these constructors are functions that create new RoughTime values.

let four_score_and_seven_years_ago =
    RoughTime::InThePast(TimeUnit::Years, 4*20 + 7);

let three_hours_from_now =
    RoughTime::InTheFuture(TimeUnit::Hours, 3);

Enums can also have struct variants, which contain named fields, just like ordinary structs:

enum Shape {
    Sphere { center: Point3d, radius: f32 },
    Cuboid { corner1: Point3d, corner2: Point3d }
}

let unit_sphere = Shape::Sphere { center: ORIGIN, radius: 1.0 };

In all, Rust has three kinds of enum variant, echoing the three kinds of struct we showed in the previous chapter. Variants with no data correspond to unit-like structs. Tuple variants look and function just like tuple structs. Struct variants have curly braces and named fields. A single enum can have variants of all three kinds.

enum RelationshipStatus {
    Single,
    InARelationship,
    ItsComplicated(Option<String>),
    ItsExtremelyComplicated {
        car: DifferentialEquation,
        cdr: EarlyModernistPoem
    }
}

All constructors and fields of a public enum are automatically public.

Enums in Memory

In memory, enums with data are stored as a small integer tag, plus enough memory to hold all the fields of the largest variant. The tag field is for Rust’s internal use. It tells which constructor created the value, and therefore which fields it has.

As of Rust 1.17, RoughTime fits in 8 bytes, as shown in Figure 10-1.

Figure 10-1. RoughTime values in memory

Rust makes no promises about enum layout, however, in order to leave the door open for future optimizations. In some cases, it would be possible to pack an enum more efficiently than the figure suggests. We’ll show later in this chapter how Rust can already optimize away the tag field for some enums.

Rich Data Structures Using Enums

Enums are also useful for quickly implementing tree-like data structures. For example, suppose a Rust program needs to work with arbitrary JSON data. In memory, any JSON document can be represented as a value of this Rust type:

enum Json {
    Null,
    Boolean(bool),
    Number(f64),
    String(String),
    Array(Vec<Json>),
    Object(Box<HashMap<String, Json>>)
}

The explanation of this data structure in English can’t improve much upon the Rust code. The JSON standard specifies the various data types that can appear in a JSON document: null, Boolean values, numbers, strings, arrays of JSON values, and objects with string keys and JSON values. The Json enum simply spells out these types.

This is not a hypothetical example. A very similar enum can be found in serde_json, a serialization library for Rust structs that is one of the most-downloaded crates on crates.io.

The Box around the HashMap that represents an Object serves only to make all Json values more compact. In memory, values of type Json take up four machine words. String and Vec values are three words, and Rust adds a tag byte. Null and Boolean values don’t have enough data in them to use up all that space, but all Json values must be the same size. The extra space goes unused. Figure 10-2 shows some examples of how Json values actually look in memory.

A HashMap is larger still. If we had to leave room for it in every Json value, they would be quite large, eight words or so. But a Box<HashMap> is a single word: it’s just a pointer to heap-allocated data. We could make Json even more compact by boxing more fields.

Figure 10-2. Json values in memory

What’s remarkable here is how easy it was to set this up. In C++, one might write a class for this:

class JSON {
private:
    enum Tag {
        Null, Boolean, Number, String, Array, Object
    };
    union Data {
        bool boolean;
        double number;
        shared_ptr<string> str;
        shared_ptr<vector<JSON>> array;
        shared_ptr<unordered_map<string, JSON>> object;

        Data() {}
        ~Data() {}
        ...
    };

    Tag tag;
    Data data;

public:
    bool is_null() const { return tag == Null; }
    bool is_boolean() const { return tag == Boolean; }
    bool get_boolean() const {
        assert(is_boolean());
        return data.boolean;
    }
    void set_boolean(bool value) {
        this->~JSON();  // clean up string/array/object value
        tag = Boolean;
        data.boolean = value;
    }
    ...
};

At 30 lines of code, we have barely begun the work. This class will need constructors, a destructor, and an assignment operator. An alternative would be to create a class hierarchy with a base class JSON and subclasses JSONBoolean, JSONString, and so on. Either way, when it’s done, our C++ JSON library will have more than a dozen methods. It will take a bit of reading for other programmers to pick it up and use it. The entire Rust enum is eight lines of code.

Generic Enums

Enums can be generic. Two examples from the standard library are among the most-used data types in the language:

enum Option<T> {
    None,
    Some(T)
}

enum Result<T, E> {
    Ok(T),
    Err(E)
}

These types are familiar enough by now, and the syntax for generic enums is the same as for generic structs. One unobvious detail is that Rust can eliminate the tag field of Option<T> when the type T is a Box or some other smart pointer type. An Option<Box<i32>> is stored in memory as a single machine word, 0 for None and nonzero for Some boxed value.

Generic data structures can be built with just a few lines of code:

// An ordered collection of `T`s.
enum BinaryTree<T> {
    Empty,
    NonEmpty(Box<TreeNode<T>>)
}

// A part of a BinaryTree.
struct TreeNode<T> {
    element: T,
    left: BinaryTree<T>,
    right: BinaryTree<T>
}

These few lines of code define a BinaryTree type that can store any number of values of type T.

A great deal of information is packed into these two definitions, so we will take the time to translate the code word for word into English. Each BinaryTree value is either Empty or NonEmpty. If it’s Empty, then it contains no data at all. If NonEmpty, then it has a Box, a pointer to a heap-allocated TreeNode.

Each TreeNode value contains one actual element, as well as two more BinaryTree values. This means a tree can contain subtrees, and thus a NonEmpty tree can have any number of descendants.

A sketch of a value of type BinaryTree<&str> is shown in Figure 10-3. As with Option<Box<T>>, Rust eliminates the tag field, so a BinaryTree value is just one machine word.

Figure 10-3. A BinaryTree containing six strings

Building any particular node in this tree is straightforward:

use self::BinaryTree::*;
let jupiter_tree = NonEmpty(Box::new(TreeNode {
    element: "Jupiter",
    left: Empty,
    right: Empty
}));

Larger trees can be built from smaller ones:

let mars_tree = NonEmpty(Box::new(TreeNode {
    element: "Mars",
    left: jupiter_tree,
    right: mercury_tree
}));

Naturally, this assignment transfers ownership of jupiter_node and mercury_node to their new parent node.

The remaining parts of the tree follow the same patterns. The root node is no different from the others:

let tree = NonEmpty(Box::new(TreeNode {
    element: "Saturn",
    left: mars_tree,
    right: uranus_tree
}));

Later in this chapter, we’ll show how to implement an add method on the BinaryTree type, so that we can instead write:

let mut tree = BinaryTree::Empty;
for planet in planets {
    tree.add(planet);
}

No matter what language you’re coming from, creating data structures like BinaryTree in Rust will likely take some practice. It won’t be obvious at first where to put the Boxes. One way to find a design that will work is to draw a picture like Figure 10-3 that shows how you want things laid out in memory. Then work backward from the picture to the code. Each collection of rectangles is a struct or tuple; each arrow is a Box or other smart pointer. Figuring out the type of each field is a bit of a puzzle, but a manageable one. The reward for solving the puzzle is control over your program’s memory usage.

Now comes the “price” we mentioned in the introduction. The tag field of an enum costs a little memory, up to 8 bytes in the worst case, but that is usually negligible. The real downside to enums (if it can be called that) is that Rust code cannot throw caution to the wind and try to access fields regardless of whether they are actually present in the value:

let r = shape.radius;  // error: no field `radius` on type `Shape`

The only way to access the data in an enum is the safe way: using patterns.

Patterns

Recall the definition of our RoughTime type from earlier in this chapter:

enum RoughTime {
    InThePast(TimeUnit, u32),
    JustNow,
    InTheFuture(TimeUnit, u32)
}

Suppose you have a RoughTime value and you’d like to display it on a web page. You need to access the TimeUnit and u32 fields inside the value. Rust doesn’t let you access them directly, by writing rough_time.0 and rough_time.1, because after all, the value might be RoughTime::JustNow, which has no fields. But then, how can you get the data out?

You need a match expression:

 1  fn rough_time_to_english(rt: RoughTime) -> String {
 2      match rt {
 3          RoughTime::InThePast(units, count) =>
 4              format!("{} {} ago", count, units.plural()),
 5          RoughTime::JustNow =>
 6              format!("just now"),
 7          RoughTime::InTheFuture(units, count) =>
 8              format!("{} {} from now", count, units.plural())
 9      }
10  }

match performs pattern matching; in this example, the patterns are the parts that appear before the => symbol on lines 3, 5, and 7. Patterns that match RoughTime values look just like the expressions used to create RoughTime values. This is no coincidence. Expressions produce values; patterns consume values. The two use a lot of the same syntax.

Let’s step through what happens when this match expression runs. Suppose rt is the value RoughTime::InTheFuture(TimeUnit::Months, 1). Rust first tries to match this value against the pattern on line 3. As you can see in Figure 10-4, it doesn’t match.

Figure 10-4. A RoughTime value and pattern that do not match

Pattern matching on an enum, struct, or tuple works as though Rust is doing a simple left-to-right scan, checking each component of the pattern to see if the value matches it. If it doesn’t, Rust moves on to the next pattern.

The patterns on lines 3 and 5 fail to match. But the pattern on line 7 succeeds (Figure 10-5).

Figure 10-5. A successful match

When a pattern contains simple identifiers like units and count, those become local variables in the code following the pattern. Whatever is present in the value is copied or moved into the new variables. Rust stores TimeUnit::Months in units and 1 in count, runs line 8, and returns the string "1 months from now".

That output has a minor grammatical issue, which can be fixed by adding another arm to the match:

RoughTime::InTheFuture(unit, 1) =>
    format!("a {} from now", unit.singular()),

This arm matches only if the count field is exactly 1. Note that this new code must be added before line 7. If we add it at the end, Rust will never get to it, because the pattern on line 7 matches all InTheFuture values. The Rust compiler will warn about an “unreachable pattern” if you make this kind of mistake.

Unfortunately, even with the new code, there is still a problem with RoughTime::InTheFuture(TimeUnit::Hours, 1): the result "a hour from now" is not quite right. Such is the English language. This too can be fixed by adding another arm to the match.

As this example shows, pattern matching works hand in hand with enums and can even test the data they contain, making match a powerful, flexible replacement for C’s switch statement.

So far, we’ve only seen patterns that match enum values. There’s more to it than that. Rust patterns are their own little language, summarized in Table 10-1. We’ll spend most of the rest of the chapter on the features shown in this table.

Table 10-1. Patterns

Pattern type | Example | Notes
Literal | 100 "name" | Matches an exact value; the name of a const is also allowed
Range | 0 ... 100 'a' ... 'k' | Matches any value in range, including the end value
Wildcard | _ | Matches any value and ignores it
Variable | name mut count | Like _ but moves or copies the value into a new local variable
ref variable | ref field ref mut field | Borrows a reference to the matched value instead of moving or copying it
Binding with subpattern | val @ 0 ... 99 ref circle @ Shape::Circle { .. } | Matches the pattern to the right of @, using the variable name to the left
Enum pattern | Some(value) None Pet::Orca |
Tuple pattern | (key, value) (r, g, b) |
Struct pattern | Color(r, g, b) Point { x, y } Card { suit: Clubs, rank: n } Account { id, name, .. } |
Reference | &value &(k, v) | Matches only reference values
Multiple patterns | 'a' | 'A' | In match only (not valid in let, etc.)
Guard expression | x if x * x <= r2 | In match only (not valid in let, etc.)

Literals, Variables, and Wildcards in Patterns

So far, we’ve shown match expressions working with enums. Other types can be matched too. When you need something like a C switch statement, use match with an integer value. Integer literals like 0 and 1 can serve as patterns:

match meadow.count_rabbits() {
    0 => {}  // nothing to say
    1 => println!("A rabbit is nosing around in the clover."),
    n => println!("There are {} rabbits hopping about in the meadow", n)
}

The pattern 0 matches if there are no rabbits in the meadow. 1 matches if there is just one. If there are two or more rabbits, we reach the third pattern, n. This pattern is just a variable name. It can match any value, and the matched value is moved or copied into a new local variable. So in this case, the value of meadow.count_rabbits() is stored in a new local variable n, which we then print.

Other literals can be used as patterns too, including Booleans, characters, and even strings:

let calendar =
    match settings.get_string("calendar") {
        "gregorian" => Calendar::Gregorian,
        "chinese" => Calendar::Chinese,
        "ethiopian" => Calendar::Ethiopian,
        other => return parse_error("calendar", other)
    };

In this example, other serves as a catch-all pattern, like n in the previous example. These patterns play the same role as a default case in a switch statement, matching values that don’t match any of the other patterns.

If you need a catch-all pattern, but you don’t care about the matched value, you can use a single underscore _ as a pattern, the wildcard pattern:

let caption =
    match photo.tagged_pet() {
        Pet::Tyrannosaur => "RRRAAAAAHHHHHH",
        Pet::Samoyed => "*dog thoughts*",
        _ => "I'm cute, love me"  // generic caption, works for any pet
    };

The wildcard pattern matches any value, but without storing it anywhere. Since Rust requires every match expression to handle all possible values, a wildcard is often required at the end. Even if you’re very sure the remaining cases can’t occur, you must at least add a fallback arm that panics:

// There are many Shapes, but we only support "selecting"
// either some text, or everything in a rectangular area.
// You can't select an ellipse or trapezoid.
match document.selection() {
    Shape::TextSpan(start, end) => paint_text_selection(start, end),
    Shape::Rectangle(rect) => paint_rect_selection(rect),
    _ => panic!("unexpected selection type")
}

It’s worth noting that existing variables can’t be used in patterns. Suppose we’re implementing a board game with hexagonal spaces, and the player just clicked to move a piece. To confirm that the click was valid, we might try something like this:

fn check_move(current_hex: Hex, click: Point) -> game::Result<Hex> {
    match point_to_hex(click) {
        None =>
            Err("That's not a game space."),
        Some(current_hex) =>  // try to match if user clicked the current_hex
                              // (it doesn't work: see explanation below)
            Err("You are already there! You must click somewhere else."),
        Some(other_hex) =>
            Ok(other_hex)
    }
}

This fails because identifiers in patterns introduce new variables. The pattern Some(current_hex) here creates a new local variable current_hex, shadowing the argument current_hex. Rust emits several warnings about this code—in particular, the last arm of the match is unreachable. To fix it, use an if expression:

Some(hex) =>
    if hex == current_hex {
        Err("You are already there! You must click somewhere else")
    } else {
        Ok(hex)
    }

In a few pages, we’ll cover guards, which offer another way to solve this problem.

Tuple and Struct Patterns

Tuple patterns match tuples. They’re useful any time you want to get multiple pieces of data involved in a single match:

fn describe_point(x: i32, y: i32) -> &'static str {
    use std::cmp::Ordering::*;
    match (x.cmp(&0), y.cmp(&0)) {
        (Equal, Equal) => "at the origin",
        (_, Equal) => "on the x axis",
        (Equal, _) => "on the y axis",
        (Greater, Greater) => "in the first quadrant",
        (Less, Greater) => "in the second quadrant",
        _ => "somewhere else"
    }
}

Struct patterns use curly braces, just like struct expressions. They contain a subpattern for each field:

match balloon.location {
    Point { x: 0, y: height } =>
        println!("straight up {} meters", height),
    Point { x: x, y: y } =>
        println!("at ({}m, {}m)", x, y)
}

In this example, if the first arm matches, then balloon.location.y is stored in the new local variable height.

Suppose balloon.location is Point { x: 30, y: 40 }. As always, Rust checks each component of each pattern in turn Figure 10-6.

Figure 10-6. Pattern matching with structs

The second arm matches, so the output would be “at (30m, 40m)”.

Patterns like Point { x: x, y: y } are common when matching structs, and the redundant names are visual clutter, so Rust has a shorthand for this: Point {x, y}. The meaning is the same. This pattern still stores a point’s x field in a new local x and its y field in a new local y.

Even with the shorthand, it is cumbersome to match a large struct when we only care about a few fields:

match get_account(id) {
    ...
    Some(Account {
            name, language,  // <--- the 2 things we care about
            id: _, status: _, address: _, birthday: _, eye_color: _,
            pet: _, security_question: _, hashed_innermost_secret: _,
            is_adamantium_preferred_customer: _ }) =>
        language.show_custom_greeting(name)
}

To avoid this, use .. to tell Rust you don’t care about any of the other fields:

Some(Account { name, language, .. }) =>
    language.show_custom_greeting(name)

Reference Patterns

Rust patterns support two features for working with references. ref patterns borrow parts of a matched value. & patterns match references. We’ll cover ref patterns first.

Matching on a noncopyable value moves the value. Continuing with the account example, this code would be invalid:

match account {
    Account { name, language, .. } => {
        ui.greet(&name, &language);
        ui.show_settings(&account);  // error: use of moved value `account`
    }
}

Here, the fields account.name and account.language are moved into local variables name and language. The rest of account is dropped. That’s why we can’t call methods on account afterward.

If name and language were both copyable values, Rust would copy the fields instead of moving them, and this code would be fine. But suppose these are Strings. What can we do?

We need a kind of pattern that borrows matched values instead of moving them. The ref keyword does just that:

match account {
    Account { ref name, ref language, .. } => {
        ui.greet(name, language);
        ui.show_settings(&account);  // ok
    }
}

Now the local variables name and language are references to the corresponding fields in account. Since account is only being borrowed, not consumed, it’s OK to continue calling methods on it.

You can use ref mut to borrow mut references:

match line_result {
    Err(ref err) => log_error(err),  // `err` is &Error (shared ref)
    Ok(ref mut line) => {            // `line` is &mut String (mut ref)
        trim_comments(line);         // modify the String in place
        handle(line);
    }
}

The pattern Ok(ref mut line) matches any success result and borrows a mut reference to the success value stored inside it.

The opposite kind of reference pattern is the & pattern. A pattern starting with & matches a reference.

match sphere.center() {
    &Point3d { x, y, z } => ...
}

In this example, suppose sphere.center() returns a reference to a private field of sphere, a common pattern in Rust. The value returned is the address of a Point3d. If the center is at the origin, then sphere.center() returns &Point3d { x: 0.0, y: 0.0, z: 0.0 }.

So pattern matching proceeds as shown in Figure 10-7.

Figure 10-7. Pattern matching with references

This is a bit tricky because Rust is following a pointer here, an action we usually associate with the * operator, not the & operator. The thing to remember is that patterns and expressions are natural opposites. The expression (x, y) makes two values into a new tuple, but the pattern (x, y) does the opposite: it matches a tuple and breaks out the two values. It’s the same with &. In an expression, & creates a reference. In a pattern, & matches a reference.

Matching a reference follows all the rules we’ve come to expect. Lifetimes are enforced. You can’t get mut access via a shared reference. And you can’t move a value out of a reference, even a mut reference. When we match &Point3d { x, y, z }, the variables x, y, and z receive copies of the coordinates, leaving the original Point3d value intact. It works because those fields are copyable. If we try the same thing on a struct with noncopyable fields, we’ll get an error:

match friend.borrow_car() {
    Some(&Car { engine, .. }) =>  // error: can't move out of borrow
        ...
    None => {}
}

Scrapping a borrowed car for parts is not nice, and Rust won’t stand for it. You can use a ref pattern to borrow a reference to a part. You just don’t own it.

    Some(&Car { ref engine, .. }) =>  // ok, engine is a reference

Let’s look at one more example of an & pattern. Suppose we have an iterator chars over the characters in a string, and it has a method chars.peek() that returns an Option<&char>: a reference to the next character, if any. (Peekable iterators do in fact return an Option<&ItemType>, as we’ll see in Chapter 15.)

A program can use an & pattern to get the pointed-to character:

match chars.peek() {
    Some(&c) => println!("coming up: {:?}", c),
    None => println!("end of chars")
}

Matching Multiple Possibilities

The vertical bar (|) can be used to combine several patterns in a single match arm:

let at_end =
    match chars.peek() {
        Some(&'\r') | Some(&'\n') | None => true,
        _ => false
    };

In an expression, | is the bitwise OR operator, but here it works more like the | symbol in a regular expression. at_end is set to true if chars.peek() matches any of the three patterns.

Use ... to match a whole range of values. Range patterns include the begin and end values, so '0' ... '9' matches all the ASCII digits:

match next_char {
    '0' ... '9' =>
        self.read_number(),
    'a' ... 'z' | 'A' ... 'Z' =>
        self.read_word(),
    ' ' | '\t' | '\n' =>
        self.skip_whitespace(),
    _ =>
        self.handle_punctuation()
}

Ranges in patterns are inclusive, so that both '0' and '9' match the pattern '0' ... '9'. By contrast, range expressions (written with two dots, as in for n in 0..100) are half-open, or exclusive (covering 0 but not 100). The reason for the inconsistency is simply that exclusive ranges are more useful for loops and slicing, but inclusive ranges are more useful in pattern matching.

Pattern Guards

Use the if keyword to add a guard to a match arm. The match succeeds only if the guard evaluates to true:

match robot.last_known_location() {
    Some(point) if self.distance_to(point) < 10 =>
        short_distance_strategy(point),
    Some(point) =>
        long_distance_strategy(point),
    None =>
        searching_strategy()
}

If a pattern moves any values, you can’t put a guard on it. The guard might evaluate to false, and then Rust would go on to the next pattern. But it can’t do that if you’ve moved bits out of the value to be matched. Therefore, the preceding code works only if point is copyable. If it’s not, we’ll get an error:

error[E0008]: cannot bind by-move into a pattern guard
  --> enums_move_into_guard.rs:19:18
   |
19 |             Some(point) if self.distance_to(point) < 10 =>
   |                  ^^^^^ moves value into pattern guard

The workaround, then, would be to change the pattern to borrow point instead of moving it: Some(ref point).

@ patterns

Finally, x @ pattern matches exactly like the given pattern, but on success, instead of creating variables for parts of the matched value, it creates a single variable x and moves or copies the whole value into it. For example, say you have this code:

match self.get_selection() {
    Shape::Rect(top_left, bottom_right) =>
        optimized_paint(&Shape::Rect(top_left, bottom_right)),
    other_shape =>
        paint_outline(other_shape.get_outline()),
}

Note that the first case unpacks a Shape::Rect value, only to rebuild an identical Shape::Rect value on the next line. This can be rewritten to use an @ pattern:

    rect @ Shape::Rect(..) =>
        optimized_paint(&rect),

@ patterns are also useful with ranges:

match chars.next() {
    Some(digit @ '0' ... '9') => read_number(digit, chars),
    ...
}

Where Patterns Are Allowed

Although patterns are most prominent in match expressions, they are also allowed in several other places, typically in place of an identifier. The meaning is always the same: instead of just storing a value in a single variable, Rust uses pattern matching to take the value apart.

This means patterns can be used to...

// ...unpack a struct into three new local variables
let Track { album, track_number, title, .. } = song;

// ...unpack a function argument that's a tuple
fn distance_to((x, y): (f64, f64)) -> f64 { ... }

// ...iterate over keys and values of a HashMap
for (id, document) in &cache_map {
    println!("Document #{}: {}", id, document.title);
}

// ...automatically dereference an argument to a closure
// (handy because sometimes other code passes you a reference
// when you'd rather have a copy)
let sum = numbers.fold(0, |a, &num| a + num);

Each of these saves two or three lines of boilerplate code. The same concept exists in some other languages: in JavaScript, it’s called destructuring; in Python, unpacking.

Note that in all four examples, we use patterns that are guaranteed to match. The pattern Point3d { x, y, z } matches every possible value of the Point3d struct type; (x, y) matches any (f64, f64) pair; and so on. Patterns that always match are special in Rust. They’re called irrefutable patterns, and they’re the only patterns allowed in the four places shown here (after let, in function arguments, after for, and in closure arguments).

A refutable pattern is one that might not match, like Ok(x), which doesn’t match an error result, or '0' ... '9', which doesn’t match the character 'Q'. Refutable patterns can be used in match arms, because match is designed for them: if one pattern fails to match, it’s clear what happens next. The four examples above are places in Rust programs where a pattern can be handy, but the language doesn’t allow for match failure.

Refutable patterns are also allowed in if let and while let expressions, which can be used to...

// ...handle just one enum variant specially
if let RoughTime::InTheFuture(_, _) = user.date_of_birth() {
    user.set_time_traveler(true);
}

// ...run some code only if a table lookup succeeds
if let Some(document) = cache_map.get(&id) {
    return send_cached_response(document);
}

// ...repeatedly try something until it succeeds
while let Err(err) = present_cheesy_anti_robot_task() {
    log_robot_attempt(err);
    // let the user try again (it might still be a human)
}

// ...manually loop over an iterator
while let Some(_) = lines.peek() {
    read_paragraph(&mut lines);
}

For details about these expressions, see “if let” and “Loops”.

Populating a Binary Tree

Earlier we promised to show how to implement a method, BinaryTree::add(), that adds a node to a BinaryTree of this type:

enum BinaryTree<T> {
    Empty,
    NonEmpty(Box<TreeNode<T>>)
}

struct TreeNode<T> {
    element: T,
    left: BinaryTree<T>,
    right: BinaryTree<T>
}

You now know enough about patterns to write this method. An explanation of binary search trees is beyond the scope of this book, but for readers already familiar with the topic, it’s worth seeing how it plays out in Rust.

 1  impl<T: Ord> BinaryTree<T> {
 2      fn add(&mut self, value: T) {
 3          match *self {
 4              BinaryTree::Empty =>
 5                  *self = BinaryTree::NonEmpty(Box::new(TreeNode {
 6                      element: value,
 7                      left: BinaryTree::Empty,
 8                      right: BinaryTree::Empty
 9                  })),
10              BinaryTree::NonEmpty(ref mut node) =>
11                  if value <= node.element {
12                      node.left.add(value);
13                  } else {
14                      node.right.add(value);
15                  }
16          }
17      }
18  }

Line 1 tells Rust that we’re defining a method on BinaryTrees of ordered types. This is exactly the same syntax we use to define methods on generic structs, explained in “Defining Methods with impl”.

If the existing tree *self is empty, that’s the easy case. Lines 5–9 run, changing the Empty tree to a NonEmpty one. The call to Box::new() here allocates a new TreeNode in the heap. When we’re done, the tree contains one element. Its left and right subtrees are both Empty.

If *self is not empty, we match the pattern on line 10:

BinaryTree::NonEmpty(ref mut node) =>

This pattern borrows a mutable reference to the Box<TreeNode<T>>, so we can access and modify data in that tree node. That reference is named node, and it’s in scope from line 11 to line 15. Since there’s already an element in this node, the code must recursively call .add() to add the new element to either the left or the right subtree.

The new method can be used like this:

let mut tree = BinaryTree::Empty;
tree.add("Mercury");
tree.add("Venus");
...

The Big Picture

Rust’s enums may be new to systems programming, but they are not a new idea. Traveling under various academic-sounding names, like algebraic data types, they’ve been used in functional programming languages for more than 40 years. It’s unclear why so few other languages in the C tradition have ever had them. Perhaps it is simply that for a programming language designer, combining variants, references, mutability, and memory safety is extremely challenging. Functional programming languages dispense with mutability. C unions, by contrast, have variants, pointers, and mutability—but are so spectacularly unsafe that even in C, they’re a last resort. Rust’s borrow checker is the magic that makes it possible to combine all four without compromise.

Programming is data processing. Getting data into the right shape can be the difference between a small, fast, elegant program and a slow, gigantic tangle of duct tape and virtual method calls.

This is the problem space enums address. They are a design tool for getting data into the right shape. For cases when a value may be one thing, or another thing, or perhaps nothing at all, enums are better than class hierarchies on every axis: faster, safer, less code, easier to document.

The limiting factor is flexibility. End users of an enum can’t extend it to add new variants. Variants can be added only by changing the enum declaration. And when that happens, existing code breaks. Every match expression that individually matches each variant of the enum must be revisited—it needs a new arm to handle the new variant. In some cases, trading flexibility for simplicity is just good sense. After all, the structure of JSON is not expected to change. And in some cases, revisiting all uses of an enum when it changes is exactly what we want. For example, when an enum is used in a compiler to represent the various operators of a programming language, adding a new operator should involve touching all code that handles operators.

But sometimes more flexibility is needed. For those situations, Rust has traits, the topic of our next chapter.

Chapter 11. Traits and Generics

[A] computer scientist tends to be able to deal with nonuniform structures—case 1, case 2, case 3—while a mathematician will tend to want one unifying axiom that governs an entire system.

Donald Knuth

One of the great discoveries in programming is that it’s possible to write code that operates on values of many different types, even types that haven’t been invented yet. Here are two examples:

Vec<T> is generic: you can create a vector of any type of value, including types defined in your program that the authors of Vec never anticipated.

Many things have .write() methods, including Files and TcpStreams. Your code can take a writer by reference, any writer, and send data to it. Your code doesn’t have to care what type of writer it is. Later, if someone adds a new type of writer, your code will already support it.

Of course, this capability is hardly new with Rust. It’s called polymorphism, and it was the hot new programming language technology of the 1970s. By now it’s effectively universal. Rust supports polymorphism with two related features: traits and generics. These concepts will be familiar to many programmers, but Rust takes a fresh approach inspired by Haskell’s typeclasses.

Traits are Rust’s take on interfaces or abstract base classes. At first, they look just like interfaces in Java or C#. The trait for writing bytes is called std::io::Write, and its definition in the standard library starts out like this:

trait Write {
    fn write(&mut self, buf: &[u8]) -> Result<usize>;
    fn flush(&mut self) -> Result<()>;

    fn write_all(&mut self, buf: &[u8]) -> Result<()> { ... }
    ...
}

This trait offers several methods; we’ve shown only the first three.

The standard types File and TcpStream both implement std::io::Write. So does Vec<u8>. All three types provide methods named .write(), .flush(), and so on. Code that uses a writer without caring about its type looks like this:

use std::io::Write;

fn say_hello(out: &mut Write) -> std::io::Result<()> {
    out.write_all(b"hello world\n")?;
    out.flush()
}

The type of out is &mut Write, meaning “a mutable reference to any value that implements the Write trait.”

use std::fs::File;
let mut local_file = File::create("hello.txt")?;
say_hello(&mut local_file)?;  // works

let mut bytes = vec![];
say_hello(&mut bytes)?;  // also works
assert_eq!(bytes, b"hello world\n");

This chapter begins by showing how traits are used, how they work, and how to define your own. But there is more to traits than we’ve hinted at so far. We’ll use them to add extension methods to existing types, even built-in types like str and bool. We’ll explain why adding a trait to a type costs no extra memory and how to use traits without virtual method call overhead. We’ll see that built-in traits are the hook into the language that Rust provides for operator overloading and other features. And we’ll cover the Self type, associated methods, and associated types, three features Rust lifted from Haskell that elegantly solve problems that other languages address with workarounds and hacks.

Generics are the other flavor of polymorphism in Rust. Like a C++ template, a generic function or type can be used with values of many different types.

/// Given two values, pick whichever one is less.
fn min<T: Ord>(value1: T, value2: T) -> T {
    if value1 <= value2 {
        value1
    } else {
        value2
    }
}

The <T: Ord> in this function means that min can be used with arguments of any type T that implements the Ord trait—that is, any ordered type. The compiler generates custom machine code for each type T that you actually use.

Generics and traits are closely related. Rust makes us declare the T: Ord requirement (called a bound) up front, before using the <= operator to compare two values of type T. So we’ll also talk about how &mut Write and <T: Write> are similar, how they’re different, and how to choose between these two ways of using traits.

Using Traits

A trait is a feature that any given type may or may not support. Most often, a trait represents a capability: something a type can do.

A value that implements std::io::Write can write out bytes.

A value that implements std::iter::Iterator can produce a sequence of values.

A value that implements std::clone::Clone can make clones of itself in memory.

A value that implements std::fmt::Debug can be printed using println!() with the {:?} format specifier.

These traits are all part of Rust’s standard library, and many standard types implement them.

std::fs::File implements the Write trait; it writes bytes to a local file. std::net::TcpStream writes to a network connection. Vec<u8> also implements Write. Each .write() call on a vector of bytes appends some data to the end.

Range<i32> (the type of 0..10) implements the Iterator trait, as do some iterator types associated with slices, hash tables, and so on.

Most standard library types implement Clone. The exceptions are mainly types like TcpStream that represent more than just data in memory.

Likewise, most standard library types support Debug.

There is one unusual rule about trait methods: the trait itself must be in scope. Otherwise, all its methods are hidden.

let mut buf: Vec<u8> = vec![];
buf.write_all(b"hello")?;  // error: no method named `write_all`

In this case, the compiler prints a friendly error message that suggests adding use std::io::Write; and indeed that fixes the problem:

use std::io::Write;

let mut buf: Vec<u8> = vec![];
buf.write_all(b"hello")?;  // ok

Rust has this rule because, as we’ll see later in this chapter, you can use traits to add new methods to any type—even standard library types like u32 and str. Third-party crates can do the same thing. Clearly, this could lead to naming conflicts! But since Rust makes you import the traits you plan to use, crates are free to take advantage of this superpower, and conflicts are rare in practice.

The reason Clone and Iterator methods work without any special imports is that they’re always in scope by default: they’re part of the standard prelude, names that Rust automatically imports into every module. In fact, the prelude is mostly a carefully chosen selection of traits. We’ll cover many of them in Chapter 13.

C++ and C# programmers will already have noticed that trait methods are like virtual methods. Still, calls like the one shown above are fast, as fast as any other method call. Simply put, there’s no polymorphism here. It’s obvious that buf is a vector, not a file or a network connection. The compiler can emit a simple call to Vec<u8>::write(). It can even inline the method. (C++ and C# will often do the same, although the possibility of subclassing sometimes precludes this.) Only calls through &mut Write incur the overhead of a virtual method call.

Trait Objects

There are two ways of using traits to write polymorphic code in Rust: trait objects and generics. We’ll present trait objects first and turn to generics in the next section.

Rust doesn’t permit variables of type Write:

use std::io::Write;

let mut buf: Vec<u8> = vec![];
let writer: Write = buf;  // error: `Write` does not have a constant size

A variable’s size has to be known at compile time, and types that implement Write can be any size.

This may be surprising if you’re coming from C# or Java, but the reason is simple. In Java, a variable of type OutputStream (the Java standard interface analogous to std::io::Write) is a reference to any object that implements OutputStream. The fact that it’s a reference goes without saying. It’s the same with interfaces in C# and most other languages.

What we want in Rust is the same thing, but in Rust, references are explicit:

let mut buf: Vec<u8> = vec![];
let writer: &mut Write = &mut buf;  // ok

A reference to a trait type, like writer, is called a trait object. Like any other reference, a trait object points to some value, it has a lifetime, and it can be either mut or shared.

What makes a trait object different is that Rust usually doesn’t know the type of the referent at compile time. So a trait object includes a little extra information about the referent’s type. This is strictly for Rust’s own use behind the scenes: when you call writer.write(data), Rust needs the type information to dynamically call the right write method depending on the type of *writer. You can’t query the type information directly, and Rust does not support downcasting from the trait object &mut Write back to a concrete type like Vec<u8>.

Trait Object Layout

In memory, a trait object is a fat pointer consisting of a pointer to the value, plus a pointer to a table representing that value’s type. Each trait object therefore takes up two machine words, as shown in Figure 11-1.

Figure 11-1. Trait objects in memory

C++ has this kind of run-time type information as well. It’s called a virtual table, or vtable. In Rust, as in C++, the vtable is generated once, at compile time, and shared by all objects of the same type. Everything shown in dark gray in Figure 11-1, including the vtable, is a private implementation detail of Rust. Again, these aren’t fields and data structures that you can access directly. Instead, the language automatically uses the vtable when you call a method of a trait object, to determine which implementation to call.

Seasoned C++ programmers will notice that Rust and C++ use memory a bit differently. In C++, the vtable pointer, or vptr, is stored as part of the struct. Rust uses fat pointers instead. The struct itself contains nothing but its fields. This way, a struct can implement dozens of traits without containing dozens of vptrs. Even types like i32, which aren’t big enough to accommodate a vptr, can implement traits.

Rust automatically converts ordinary references into trait objects when needed. This is why we’re able to pass &mut local_file to say_hello in this example:

let mut local_file = File::create("hello.txt")?;
say_hello(&mut local_file)?;

The type of &mut local_file is &mut File, and the type of the argument to say_hello is &mut Write. Since a File is a kind of writer, Rust allows this, automatically converting the plain reference to a trait object.

Likewise, Rust will happily convert a Box<File> to a Box<Write>, a value that owns a writer in the heap:

let w: Box<Write> = Box::new(local_file);

Box<Write>, like &mut Write, is a fat pointer: it contains the address of the writer itself and the address of the vtable. The same goes for other pointer types, like Rc<Write>.

This kind of conversion is the only way to create a trait object. What the computer is actually doing here is very simple. At the point where the conversion happens, Rust knows the referent’s true type (in this case, File), so it just adds the address of the appropriate vtable, turning the regular pointer into a fat pointer.

Generic Functions

At the beginning of this chapter, we showed a say_hello() function that took a trait object as an argument. Let’s rewrite that function as a generic function:

fn say_hello<W: Write>(out: &mut W) -> std::io::Result<()> {
    out.write_all(b"hello world\n")?;
    out.flush()
}

Only the type signature has changed:

fn say_hello(out: &mut Write)         // plain function

fn say_hello<W: Write>(out: &mut W)   // generic function

The phrase <W: Write> is what makes the function generic. This is a type parameter. It means that throughout the body of this function, W stands for some type that implements the Write trait. Type parameters are usually single uppercase letters, by convention.

Which type W stands for depends on how the generic function is used:

say_hello(&mut local_file)?;  // calls say_hello::<File>
say_hello(&mut bytes)?;       // calls say_hello::<Vec<u8>>

When you pass &mut local_file to the generic say_hello() function, you’re calling say_hello::<File>(). Rust generates machine code for this function that calls File::write_all() and File::flush(). When you pass &mut bytes, you’re calling say_hello::<Vec<u8>>(). Rust generates separate machine code for this version of the function, calling the corresponding Vec<u8> methods. In both cases, Rust infers the type W from the type of the argument. You can always spell out the type parameters:

say_hello::<File>(&mut local_file)?;

but it’s seldom necessary, because Rust can usually deduce the type parameters by looking at the arguments. Here, the say_hello generic function expects a &mut W argument, and we’re passing it a &mut File, so Rust infers that W = File.

If the generic function you’re calling doesn’t have any arguments that provide useful clues, you may have to spell it out:

// calling a generic method collect<C>() that takes no arguments
let v1 = (0 .. 1000).collect();  // error: can't infer type
let v2 = (0 .. 1000).collect::<Vec<i32>>(); // ok

Sometimes we need multiple abilities from a type parameter. For example, if we want to print out the top 10 most common values in a vector, we’ll need for those values to be printable:

use std::fmt::Debug;

fn top_ten<T: Debug>(values: &Vec<T>) { ... }

But this isn’t good enough. How are we planning to determine which values are the most common? The usual way is to use the values as keys in a hash table. That means the values need to support the Hash and Eq operations. The bounds on T must include these as well as Debug. The syntax for this uses the + sign:

fn top_ten<T: Debug + Hash + Eq>(values: &Vec<T>) { ... }

Some types implement Debug, some implement Hash, some support Eq; and a few, like u32 and String, implement all three, as shown in Figure 11-2.

Figure 11-2. Traits as sets of types

It’s also possible for a type parameter to have no bounds at all, but you can’t do much with a value if you haven’t specified any bounds for it. You can move it. You can put it into a box or vector. That’s about it.

Generic functions can have multiple type parameters:

/// Run a query on a large, partitioned data set.
/// See <http://research.google.com/archive/mapreduce.html>.
fn run_query<M: Mapper + Serialize, R: Reducer + Serialize>(
    data: &DataSet, map: M, reduce: R) -> Results
{ ... }

As this example shows, the bounds can get to be so long that they are hard on the eyes. Rust provides an alternative syntax using the keyword where:

fn run_query<M, R>(data: &DataSet, map: M, reduce: R) -> Results
    where M: Mapper + Serialize,
          R: Reducer + Serialize
{ ... }

The type parameters M and R are still declared up front, but the bounds are moved to separate lines. This kind of where clause is also allowed on generic structs, enums, type aliases, and methods—anywhere bounds are permitted.

Of course, an alternative to where clauses is to keep it simple: find a way to write the program without using generics quite so intensively.

“Receiving References as Parameters” introduced the syntax for lifetime parameters. A generic function can have both lifetime parameters and type parameters. Lifetime parameters come first.

/// Return a reference to the point in `candidates` that's
/// closest to the `target` point.
fn nearest<'t, 'c, P>(target: &'t P, candidates: &'c [P]) -> &'c P
    where P: MeasureDistance
{
    ...
}

This function takes two arguments, target and candidates. Both are references, and we give them distinct lifetimes 't and 'c (as discussed in “Distinct Lifetime Parameters”). Furthermore, the function works with any type P that implements the MeasureDistance trait, so we might use it on Point2d values in one program and Point3d values in another.

Lifetimes never have any impact on machine code. Two calls to nearest() using the same type P, but different lifetimes, will call the same compiled function. Only differing types cause Rust to compile multiple copies of a generic function.

Of course, functions are not the only kind of generic code in Rust.

We’ve already covered generic types in “Generic Structs” and “Generic Enums”.

An individual method can be generic, even if the type it’s defined on is not generic:

impl PancakeStack {
    fn push<T: Topping>(&mut self, goop: T) -> PancakeResult<()> {
        ...
    }
}

Type aliases can be generic, too:

type PancakeResult<T> = Result<T, PancakeError>;

We’ll cover generic traits later in this chapter.

All the features introduced in this section—bounds, where clauses, lifetime parameters, and so forth—can be used on all generic items, not just functions.

Which to Use

The choice of whether to use trait objects or generic code is subtle. Since both features are based on traits, they have a lot in common.

Trait objects are the right choice whenever you need a collection of values of mixed types, all together. It is technically possible to make generic salad:

trait Vegetable {
    ...
}

struct Salad<V: Vegetable> {
    veggies: Vec<V>
}

but this is a rather severe design. Each such salad consists entirely of a single type of vegetable. Not everyone is cut out for this sort of thing. One of your authors once paid $14 for a Salad<IcebergLettuce> and has never quite gotten over the experience.

How can we build a better salad? Since Vegetable values can be all different sizes, we can’t ask Rust for a Vec<Vegetable>:

struct Salad {
    veggies: Vec<Vegetable>  // error: `Vegetable` does not have
                             //        a constant size
}

Trait objects are the solution:

struct Salad {
    veggies: Vec<Box<Vegetable>>
}

Each Box<Vegetable> can own any type of vegetable, but the box itself has a constant size—two pointers—suitable for storing in a vector. Apart from the unfortunate mixed metaphor of having boxes in one’s food, this is precisely what’s called for, and it would work out just as well for shapes in a drawing app, monsters in a game, pluggable routing algorithms in a network router, and so on.

Another possible reason to use trait objects is to reduce the total amount of compiled code. Rust may have to compile a generic function many times, once for each type it’s used with. This could make the binary large, a phenomenon called code bloat in C++ circles. These days, memory is plentiful, and most of us have the luxury of ignoring code size; but constrained environments do exist.

Outside of situations involving salad or microcontrollers, generics have two important advantages over trait objects, with the result that in Rust, generics are the more common choice.

The first advantage is speed. Each time the Rust compiler generates machine code for a generic function, it knows which types it’s working with, so it knows at that time which write method to call. There’s no need for dynamic dispatch.

The generic min() function shown in the introduction is just as fast as if we had written separate functions min_u8, min_i64, min_string, and so on. The compiler can inline it, like any other function, so in a release build, a call to min::<i32> is likely just two or three instructions. A call with constant arguments, like min(5, 3), will be even faster: Rust can evaluate it at compile time, so that there’s no runtime cost at all.

Or consider this generic function call:

let mut sink = std::io::sink();
say_hello(&mut sink)?;

std::io::sink() returns a writer of type Sink that quietly discards all bytes written to it.

When Rust generates machine code for this, it could emit code that calls Sink::write_all, checks for errors, then calls Sink::flush. That’s what the body of the generic function says to do.

Or, Rust could look at those methods and realize the following:

Sink::write_all() does nothing.

Sink::flush() does nothing.

Neither method ever returns an error.

In short, Rust has all the information it needs to optimize away this function entirely.

Compare that to the behavior with trait objects. Rust never knows what type of value a trait object points to until run time. So even if you pass a Sink, the overhead of calling virtual methods and checking for errors still applies.

The second advantage of generics is that not every trait can support trait objects. Traits support several features, such as static methods, that work only with generics: they rule out trait objects entirely. We’ll point out these features as we come to them.

Defining and Implementing Traits

Defining a trait is simple. Give it a name and list the type signatures of the trait methods. If we’re writing a game, we might have a trait like this:

/// A trait for characters, items, and scenery -
/// anything in the game world that's visible on screen.
trait Visible {
    /// Render this object on the given canvas.
    fn draw(&self, canvas: &mut Canvas);

    /// Return true if clicking at (x, y) should
    /// select this object.
    fn hit_test(&self, x: i32, y: i32) -> bool;
}

To implement a trait, use the syntax impl TraitName for Type:

impl Visible for Broom {
    fn draw(&self, canvas: &mut Canvas) {
        for y in self.y - self.height - 1 .. self.y {
            canvas.write_at(self.x, y, '|');
        }
        canvas.write_at(self.x, self.y, 'M');
    }

    fn hit_test(&self, x: i32, y: i32) -> bool {
        self.x == x
        && self.y - self.height - 1 <= y
        && y <= self.y
    }
}

Note that this impl contains an implementation for each method of the Visible trait, and nothing else. Everything defined in a trait impl must actually be a feature of the trait; if we wanted to add a helper method in support of Broom::draw(), we would have to define it in a separate impl block:

impl Broom {
    /// Helper function used by Broom::draw() below.
    fn broomstick_range(&self) -> Range<i32> {
        self.y - self.height - 1 .. self.y
    }
}

impl Visible for Broom {
    fn draw(&self, canvas: &mut Canvas) {
        for y in self.broomstick_range() {
            ...
        }
        ...
    }
    ...
}

Default Methods

The Sink writer type we discussed earlier can be implemented in a few lines of code. First, we define the type:

/// A Writer that ignores whatever data you write to it.
pub struct Sink;

Sink is an empty struct, since we don’t need to store any data in it. Next, we provide an implementation of the Write trait for Sink:

use std::io::{Write, Result};

impl Write for Sink {
    fn write(&mut self, buf: &[u8]) -> Result<usize> {
        // Claim to have successfully written the whole buffer.
        Ok(buf.len())
    }

    fn flush(&mut self) -> Result<()> {
        Ok(())
    }
}

So far, this is very much like the Visible trait. But we have also seen that the Write trait has a write_all method:

out.write_all(b"hello world\n")?;

Why does Rust let us impl Write for Sink without defining this method? The answer is that the standard library’s definition of the Write trait contains a default implementation for write_all:

trait Write {
    fn write(&mut self, buf: &[u8]) -> Result<usize>;
    fn flush(&mut self) -> Result<()>;

    fn write_all(&mut self, buf: &[u8]) -> Result<()> {
        let mut bytes_written = 0;
        while bytes_written < buf.len() {
            bytes_written += self.write(&buf[bytes_written..])?;
        }
        Ok(())
    }

    ...
}

The write and flush methods are the basic methods that every writer must implement. A writer may also implement write_all, but if not, the default implementation shown above will be used.

Your own traits can include default implementations using the same syntax.

The most dramatic use of default methods in the standard library is the Iterator trait, which has one required method (.next()) and dozens of default methods. Chapter 15 explains why.

Traits and Other People’s Types

Rust lets you implement any trait on any type, as long as either the trait or the type is introduced in the current crate.

This means that any time you want to add a method to any type, you can use a trait to do it:

trait IsEmoji {
    fn is_emoji(&self) -> bool;
}

/// Implement IsEmoji for the built-in character type.
impl IsEmoji for char {
    fn is_emoji(&self) -> bool {
        ...
    }
}

assert_eq!('$'.is_emoji(), false);

Like any other trait method, this new is_emoji method is only visible when IsEmoji is in scope.

The sole purpose of this particular trait is to add a method to an existing type, char. This is called an extension trait. Of course, you can add this trait to types, too, by writing impl IsEmoji for str { ... } and so forth.

You can even use a generic impl block to add an extension trait to a whole family of types at once. The following extension trait adds a method to all Rust writers:

use std::io::{self, Write};

/// Trait for values to which you can send HTML.
trait WriteHtml {
    fn write_html(&mut self, &HtmlDocument) -> io::Result<()>;
}

/// You can write HTML to any std::io writer.
impl<W: Write> WriteHtml for W {
    fn write_html(&mut self, html: &HtmlDocument) -> io::Result<()> {
        ...
    }
}

The line impl<W: Write> WriteHtml for W means “for every type W that implements Write, here’s an implementation of WriteHtml for W.”

The serde library offers a nice example of how useful it can be to implement user-defined traits on standard types. serde is a serialization library. That is, you can use it to write Rust data structures to disk and reload them later. The library defines a trait, Serialize, that’s implemented for every data type the library supports. So in the serde source code, there is code implementing Serialize for bool, i8, i16, i32, array and tuple types, and so on, through all the standard data structures like Vec and HashMap.

The upshot of all this is that serde adds a .serialize() method to all these types. It can be used like this:

use serde::Serialize;
use serde_json;

pub fn save_configuration(config: &HashMap<String, String>)
    -> std::io::Result<()>
{
    // Create a JSON serializer to write the data to a file.
    let writer = File::create(config_filename())?;
    let mut serializer = serde_json::Serializer::new(writer);

    // The serde `.serialize()` method does the rest.
    config.serialize(&mut serializer)?;

    Ok(())
}

We said earlier that when you implement a trait, either the trait or the type must be new in the current crate. This is called the coherence rule. It helps Rust ensure that trait implementations are unique. Your code can’t impl Write for u8, because both Write and u8 are defined in the standard library. If Rust let crates do that, there could be multiple implementations of Write for u8, in different crates, and Rust would have no reasonable way to decide which implementation to use for a given method call.

(C++ has a similar uniqueness restriction: the One Definition Rule. In typical C++ fashion, it isn’t enforced by the compiler, except in the simplest cases, and you get undefined behavior if you break it.)

Self in Traits

A trait can use the keyword Self as a type. The standard Clone trait, for example, looks like this (slightly simplified):

pub trait Clone {
    fn clone(&self) -> Self;
    ...
}

Using Self as the return type here means that the type of x.clone() is the same as the type of x, whatever that might be. If x is a String, then the type of x.clone() is String—not Clone or any other cloneable type.

Likewise, if we define this trait:

pub trait Spliceable {
    fn splice(&self, other: &Self) -> Self;
}

with two implementations:

impl Spliceable for CherryTree {
    fn splice(&self, other: &Self) -> Self {
        ...
    }
}

impl Spliceable for Mammoth {
    fn splice(&self, other: &Self) -> Self {
        ...
    }
}

then inside the first impl, Self is simply an alias for CherryTree, and in the second, it’s an alias for Mammoth. This means that we can splice together two cherry trees or two mammoths, not that we can create a mammoth-cherry hybrid. The type of self and the type of other must match.

A trait that uses the Self type is incompatible with trait objects:

// error: the trait `Spliceable` cannot be made into an object
fn splice_anything(left: &Spliceable, right: &Spliceable) {
    let combo = left.splice(right);
    ...
}

The reason is something we’ll see again and again as we dig into the advanced features of traits. Rust rejects this code because it has no way to type-check the call left.splice(right). The whole point of trait objects is that the type isn’t known until runtime. Rust has no way to know at compile time if left and right will be the same type, as required.

Trait objects are really intended for the simplest kinds of traits, the kinds that could be implemented using interfaces in Java or abstract base classes in C++. The more advanced features of traits are useful, but they can’t coexist with trait objects because with trait objects, you lose the type information Rust needs to type-check your program.

Now, had we wanted genetically improbable splicing, we could have designed a trait-object-friendly trait:

pub trait MegaSpliceable {
    fn splice(&self, other: &MegaSpliceable) -> Box<MegaSpliceable>;
}

This trait is compatible with trait objects. There’s no problem type-checking calls to this .splice() method because the type of the argument other is not required to match the type of self, as long as both types are MegaSpliceable.

Subtraits

We can declare that a trait is an extension of another trait:

/// Someone in the game world, either the player or some other
/// pixie, gargoyle, squirrel, ogre, etc.
trait Creature: Visible {
    fn position(&self) -> (i32, i32);
    fn facing(&self) -> Direction;
    ...
}

The phrase trait Creature: Visible means that all creatures are visible. Every type that implements Creature must also implement the Visible trait:

impl Visible for Broom {
    ...
}

impl Creature for Broom {
    ...
}

We can implement the two traits in either order, but it’s an error to implement Creature for a type without also implementing Visible.

Subtraits are like subinterfaces in Java or C#. They’re a way to describe a trait that extends an existing trait with a few more methods. In this example, all your code that works with Creatures can also use the methods from the Visible trait.

Static Methods

In most object-oriented languages, interfaces can’t include static methods or constructors. However, Rust traits can include static methods and constructors, here is how:

trait StringSet {
    /// Return a new empty set.
    fn new() -> Self;

    /// Return a set that contains all the strings in `strings`.
    fn from_slice(strings: &[&str]) -> Self;

    /// Find out if this set contains a particular `value`.
    fn contains(&self, string: &str) -> bool;

    /// Add a string to this set.
    fn add(&mut self, string: &str);
}

Every type that implements the StringSet trait must implement these four associated functions. The first two, new() and from_slice(), don’t take a self argument. They serve as constructors.

In nongeneric code, these functions can be called using :: syntax, just like any other static method:

// Create sets of two hypothetical types that impl StringSet:
let set1 = SortedStringSet::new();
let set2 = HashedStringSet::new();

In generic code, it’s the same, except the type is often a type variable, as in the call to S::new() shown here:

/// Return the set of words in `document` that aren't in `wordlist`.
fn unknown_words<S: StringSet>(document: &Vec<String>, wordlist: &S) -> S {
    let mut unknowns = S::new();
    for word in document {
        if !wordlist.contains(word) {
            unknowns.add(word);
        }
    }
    unknowns
}

Like Java and C# interfaces, trait objects don’t support static methods. If you want to use &StringSet trait objects, you must change the trait, adding the bound where Self: Sized to each static method:

trait StringSet {
    fn new() -> Self
        where Self: Sized;

    fn from_slice(strings: &[&str]) -> Self
        where Self: Sized;

    fn contains(&self, string: &str) -> bool;

    fn add(&mut self, string: &str);
}

This bound tells Rust that trait objects are excused from supporting this method. StringSet trait objects are then allowed; they still don’t support the two static methods, but you can create them and use them to call .contains() and .add(). The same trick works for any other method that is incompatible with trait objects. (We will forgo the rather tedious technical explanation of why this works, but the Sized trait is covered in Chapter 13.)

Fully Qualified Method Calls

A method is just a special kind of function. These two calls are equivalent:

"hello".to_string()

str::to_string("hello")

The second form looks exactly like a static method call. This works even though the to_string method takes a self argument. Simply pass self as the function’s first argument.

Since to_string is a method of the standard ToString trait, there are two more forms you can use:

ToString::to_string("hello")

<str as ToString>::to_string("hello")

All four of these method calls do exactly the same thing. Most often, you’ll just write value.method(). The other forms are qualified method calls. They specify the type or trait that a method is associated with. The last form, with the angle brackets, specifies both: a fully qualified method call.

When you write "hello".to_string(), using the . operator, you don’t say exactly which to_string method you’re calling. Rust has a method lookup algorithm that figures this out, depending on the types, deref coercions, and so on. With fully qualified calls, you can say exactly which method you mean, and that can help in a few odd cases:

When two methods have the same name. The classic hokey example is the Outlaw with two .draw() methods from two different traits, one for drawing it on the screen and one for interacting with the law:

outlaw.draw();  // error: draw on screen or draw pistol?

Visible::draw(&outlaw);  // ok: draw on screen
HasPistol::draw(&outlaw);  // ok: corral

Normally you’re better off just renaming one of the methods, but sometimes you can’t.

When the type of the self argument can’t be inferred:

let zero = 0;  // type unspecified; could be `i8`, `u8`, ...

zero.abs();  // error: method `abs` not found
i64::abs(zero);  // ok

When using the function itself as a function value:

let words: Vec<String> =
    line.split_whitespace()  // iterator produces &str values
        .map(<str as ToString>::to_string)  // ok
        .collect();

Here the fully qualified <str as ToString>::to_string is just a way to name the specific function we want to pass to .map().

When calling trait methods in macros. We’ll explain in Chapter 20.

Fully qualified syntax also works for static methods. In the previous section, we wrote S::new() to create a new set in a generic function. We could also have written StringSet::new() or <S as StringSet>::new().

Traits That Define Relationships Between Types

So far, every trait we’ve looked at stands alone: a trait is a set of methods that types can implement. Traits can also be used in situations where there are multiple types that have to work together. They can describe relationships between types.

The std::iter::Iterator trait relates each iterator type with the type of value it produces.

The std::ops::Mul trait relates types that can be multiplied. In the expression a * b, the values a and b can be either the same type, or different types.

The rand crate includes both a trait for random number generators (rand::Rng) and a trait for types that can be randomly generated (rand::Rand). The traits themselves define exactly how these types work together.

You won’t need to create traits like these every day, but you’ll come across them throughout the standard library and in third-party crates. In this section, we’ll show how each of these examples is implemented, picking up relevant Rust language features as we need them. The key skill here is the ability to read traits and method signatures and figure out what they say about the types involved.

Associated Types (or How Iterators Work)

We’ll start with iterators. By now every object-oriented language has some sort of built-in support for iterators, objects that represent the traversal of some sequence of values.

Rust has a standard Iterator trait, defined like this:

pub trait Iterator {
    type Item;

    fn next(&mut self) -> Option<Self::Item>;
    ...
}

The first feature of this trait, type Item;, is an associated type. Each type that implements Iterator must specify what type of item it produces.

The second feature, the next() method, uses the associated type in its return value. next() returns an Option<Self::Item>: either Some(item), the next value in the sequence, or None when there are no more values to visit. The type is written as Self::Item, not just plain Item, because Item is a feature of each type of iterator, not a standalone type. As always, self and the Self type show up explicitly in the code everywhere their fields, methods, and so on are used.

Here’s what it looks like to implement Iterator for a type:

// (code from the std::env standard library module)
impl Iterator for Args {
    type Item = String;

    fn next(&mut self) -> Option<String> {
        ...
    }
    ...
}

std::env::Args is the type of iterator returned by the standard library function std::env::args() that we used in Chapter 2 to access command-line arguments. It produces String values, so the impl declares type Item = String;.

Generic code can use associated types:

/// Loop over an iterator, storing the values in a new vector.
fn collect_into_vector<I: Iterator>(iter: I) -> Vec<I::Item> {
    let mut results = Vec::new();
    for value in iter {
        results.push(value);
    }
    results
}

Inside the body of this function, Rust infers the type of value for us, which is nice; but we must spell out the return type of collect_into_vector, and the Item associated type is the only way to do that. (Vec<I> would be simply wrong: we would be claiming to return a vector of iterators!)

The preceding example is not code that you would write out yourself, because after reading Chapter 15, you’ll know that iterators already have a standard method that does this: iter.collect(). So let’s look at one more example before moving on.

/// Print out all the values produced by an iterator
fn dump<I>(iter: I)
    where I: Iterator
{
    for (index, value) in iter.enumerate() {
        println!("{}: {:?}", index, value);   // error
    }
}

This almost works. There is just one problem: value might not be a printable type.

error[E0277]: the trait bound `<I as std::iter::Iterator>::Item:
              std::fmt::Debug` is not satisfied
  --> traits_dump.rs:10:37
   |
10 | println!("{}: {:?}", index, value);   // error
   |                             ^^^^^ the trait `std::fmt::Debug`
   |                                   is not implemented for
   |                                   `<I as std::iter::Iterator>::Item`
   |
   = help: consider adding a
           `where <I as std::iter::Iterator>::Item: std::fmt::Debug` bound
   = note: required by `std::fmt::Debug::fmt`

The error message is slightly obfuscated by Rust’s use of the syntax <I as std::iter::Iterator>::Item, which is a long, maximally explicit way of saying I::Item. This is valid Rust syntax, but you’ll rarely actually need to write a type out that way.

The gist of the error message is that to make this generic function compile, we must ensure that I::Item implements the Debug trait, the trait for formatting values with {:?}. We can do this by placing a bound on I::Item:

use std::fmt::Debug;

fn dump<I>(iter: I)
    where I: Iterator, I::Item: Debug
{
    ...
}

Or, we could write, “I must be an iterator over String values”:

fn dump<I>(iter: I)
    where I: Iterator<Item=String>
{
    ...
}

Iterator<Item=String> is itself a trait. If you think of Iterator as the set of all iterator types, then Iterator<Item=String> is a subset of Iterator: the set of iterator types that produce Strings. This syntax can be used anywhere the name of a trait can be used, including trait object types:

fn dump(iter: &mut Iterator<Item=String>) {
    for (index, s) in iter.enumerate() {
        println!("{}: {:?}", index, s);
    }
}

Traits with associated types, like Iterator, are compatible with trait methods, but only if all the associated types are spelled out, as shown here. Otherwise, the type of s could be anything, and again, Rust would have no way to type-check this code.

We’ve shown a lot of examples involving iterators. It’s hard not to; they’re by far the most prominent use of associated types. But associated types are generally useful whenever a trait needs to cover more than just methods.

In a thread pool library, a Task trait, representing a unit of work, could have an associated Output type.

A Pattern trait, representing a way of searching a string, could have an associated Match type, representing all the information gathered by matching the pattern to the string.

trait Pattern {
    type Match;

    fn search(&self, string: &str) -> Option<Self::Match>;
}

/// You can search a string for a particular character.
impl Pattern for char {
    /// A "match" is just the location where the
    /// character was found.
    type Match = usize;

    fn search(&self, string: &str) -> Option<usize> {
        ...
    }
}

If you’re familiar with regular expressions, it’s easy to see how impl Pattern for RegExp would have a more elaborate Match type, probably a struct that would include the start and length of the match, the locations where parenthesized groups matched, and so on.

A library for working with relational databases might have a Database​Connection trait with associated types representing transactions, cursors, prepared statements, and so on.

Associated types are perfect for cases where each implementation has one specific related type: each type of Task produces a particular type of Output; each type of Pattern looks for a particular type of Match. However, as we’ll see, some relationships among types are not like this.

Generic Traits (or How Operator Overloading Works)

Multiplication in Rust uses this trait:

/// std::ops::Mul, the trait for types that support `*`.
pub trait Mul<RHS> {
    /// The resulting type after applying the `*` operator
    type Output;

    /// The method for the `*` operator
    fn mul(self, rhs: RHS) -> Self::Output;
}

Mul is a generic trait. The type parameter, RHS, is short for right hand side.

The type parameter here means the same thing that it means on a struct or function: Mul is a generic trait, and its instances Mul<f64>, Mul<String>, Mul<Size>, etc. are all different traits, just as min::<i32> and min::<String> are different functions and Vec<i32> and Vec<String> are different types.

A single type—say, WindowSize—can implement both Mul<f64> and Mul<i32>, and many more. You would then be able to multiply a WindowSize by many other types. Each implementation would have its own associated Output type.

The trait shown above is missing one minor detail. The real Mul trait looks like this:

pub trait Mul<RHS=Self> {
    ...
}

The syntax RHS=Self means that RHS defaults to Self. If I write impl Mul for Complex, without specifying Mul’s type parameter, it means impl Mul<Complex> for Complex. In a bound, if I write where T: Mul, it means where T: Mul<T>.

In Rust, the expression lhs * rhs is shorthand for Mul::mul(lhs, rhs). So overloading the * operator in Rust is as simple as implementing the Mul trait. We’ll show examples in the next chapter.

Buddy Traits (or How rand::random() Works)

There’s one more way to use traits to express relationships between types. This way is perhaps the simplest of the bunch, since you don’t have to learn any new language features to understand it: what we’ll call buddy traits are simply traits that are designed to work together.

There’s a good example inside the rand crate, a popular crate for generating random numbers. The main feature of rand is the random() function, which returns a random value:

use rand::random;
let x = random();

If Rust can’t infer the type of the random value, which is often the case, you must specify it:

let x = random::<f64>();   // a number, 0.0 <= x < 1.0
let b = random::<bool>();  // true or false

For many programs, this one generic function is all you need. But the rand crate also offers several different, but interoperable, random number generators. All the random number generators in the library implement a common trait:

/// A random number generator.
pub trait Rng {
    fn next_u32(&mut self) -> u32;
    ...
}

An Rng is simply a value that can spit out integers on demand. The rand library provides a few different implementations, including XorShiftRng (a fast pseudorandom number generator) and OsRng (much slower, but truly unpredictable, for use in cryptography).

The buddy trait is called Rand:

/// A type that can be randomly generated using an `Rng`.
pub trait Rand: Sized {
    fn rand<R: Rng>(rng: &mut R) -> Self;
}

Types like f64 and bool implement this trait. Pass any random number generator to their ::rand() method, and it returns a random value:

let x = f64::rand(rng);
let b = bool::rand(rng);

In fact random() is nothing but a thin wrapper that passes a globally allocated Rng to this rand method. One way to implement it is like this:

pub fn random<T: Rand>() -> T {
    T::rand(&mut global_rng())
}

When you see traits that use other traits as bounds, the way Rand::rand() uses Rng, you know that those two traits are mix-and-match: any Rng can generate values of every Rand type. Since the methods involved are generic, Rust generates optimized machine code for each combination of Rng and Rand that your program actually uses.

The two traits also serve to separate concerns. Whether you’re implementing Rand for your Monster type or implementing a spectacularly fast but not-so-random Rng, you don’t have to do anything special for those two pieces of code to be able to work together, as shown in Figure 11-3.

Figure 11-3. Buddy traits illustrated. The Rng types listed on the left are real random number generators provided by the rand crate.

The standard library’s support for computing hash codes provides another example of buddy traits. Types that implement Hash are hashable, so they can be used as hash table keys. Types that implement Hasher are hashing algorithms. The two are linked in the same way as Rand and Rng: Hash has a generic method Hash::hash() that accepts any type of Hasher as an argument.

Another example is the serde library’s Serialize trait, which you saw in “Traits and Other People’s Types”. It has a buddy trait we didn’t talk about: the Serializer trait, which represents the output format. serde supports pluggable serialization formats. There are Serializer implementations for JSON, YAML, a binary format called CBOR, and so on. Thanks to the close relationship between the two traits, every format automatically supports every serializable type.

In the last three sections, we’ve shown three ways traits can describe relationships between types. All of these can also be seen as ways of avoiding virtual method overhead and downcasts, since they allow Rust to know more concrete types at compile time.

Reverse-Engineering Bounds

Writing generic code can be a real slog when there’s no single trait that does everything you need. Suppose we have written this nongeneric function to do some computation:

fn dot(v1: &[i64], v2: &[i64]) -> i64 {
    let mut total = 0;
    for i in 0 .. v1.len() {
        total = total + v1[i] * v2[i];
    }
    total
}

Now we want to use the same code with floating-point values. We might try something like this:

fn dot<N>(v1: &[N], v2: &[N]) -> N {
    let mut total: N = 0;
    for i in 0 .. v1.len() {
        total = total + v1[i] * v2[i];
    }
    total
}

No such luck: Rust complains about the use of + and * and the type of 0. We can require N to be a type that supports + and * using the Add and Mul traits. Our use of 0 needs to change, though, because 0 is always an integer in Rust; the corresponding floating-point value is 0.0. Fortunately, there is a standard Default trait for types that have default values. For numeric types, the default is always 0.

use std::ops::{Add, Mul};

fn dot<N: Add + Mul + Default>(v1: &[N], v2: &[N]) -> N {
    let mut total = N::default();
    for i in 0 .. v1.len() {
        total = total + v1[i] * v2[i];
    }
    total
}

This is closer, but still does not quite work:

error[E0308]: mismatched types
  --> traits_generic_dot_2.rs:11:25
   |
11 | total = total + v1[i] * v2[i];
   |                 ^^^^^^^^^^^^^ expected type parameter, found associated type
   |
   = note: expected type `N`
              found type `<N as std::ops::Mul>::Output`

Our new code assumes that that multiplying two values of type N produces another value of type N. This isn’t necessarily the case. You can overload the multiplication operator to return whatever type you want. We need to somehow tell Rust that this generic function only works with types that have the normal flavor of multiplication, where multiplying N * N returns an N. We do this by replacing Mul with Mul<Output=N>, and the same for Add:

fn dot<N: Add<Output=N> + Mul<Output=N> + Default>(v1: &[N], v2: &[N]) -> N
{
    ...
}

At this point, the bounds are starting to pile up, making the code hard to read. Let’s move the bounds into a where clause:

fn dot<N>(v1: &[N], v2: &[N]) -> N
    where N: Add<Output=N> + Mul<Output=N> + Default
{
    ...
}

Great. But Rust still complains about this line of code:

error[E0508]: cannot move out of type `[N]`, a non-copy array
 --> traits_generic_dot_3.rs:7:25
  |
7 |         total = total + v1[i] * v2[i];
  |                         ^^^^^ cannot move out of here

This one might be a real puzzle, even though by now we’re familiar with the terminology. Yes, it would be illegal to move the value v1[i] out of the slice. But numbers are copyable. So what’s the problem?

The answer is that Rust doesn’t know v1[i] is a number. In fact, it isn’t—the type N can be any type that satisfies the bounds we’ve given it. If we also want N to be a copyable type, we must say so:

where N: Add<Output=N> + Mul<Output=N> + Default + Copy

With this, the code compiles and runs. The final code looks like this:

use std::ops::{Add, Mul};

fn dot<N>(v1: &[N], v2: &[N]) -> N
    where N: Add<Output=N> + Mul<Output=N> + Default + Copy
{
    let mut total = N::default();
    for i in 0 .. v1.len() {
        total = total + v1[i] * v2[i];
    }
    total
}

#[test]
fn test_dot() {
    assert_eq!(dot(&[1, 2, 3, 4], &[1, 1, 1, 1]), 10);
    assert_eq!(dot(&[53.0, 7.0], &[1.0, 5.0]), 88.0);
}

This occasionally happens in Rust: there is a period of intense arguing with the compiler, at the end of which the code looks rather nice, as if it had been a breeze to write, and runs beautifully.

What we’ve been doing here is reverse-engineering the bounds on N, using the compiler to guide and check our work. The reason it was a bit of a pain is that there wasn’t a single Number trait in the standard library that included all the operators and methods we wanted to use. As it happens, there’s a popular open source crate called num that defines such a trait! Had we known, we could have added num to our Cargo.toml and written:

use num::Num;

fn dot<N: Num + Copy>(v1: &[N], v2: &[N]) -> N {
    let mut total = N::zero();
    for i in 0 .. v1.len() {
        total = total + v1[i] * v2[i];
    }
    total
}

Just as in object-oriented programming, the right interface makes everything nice, in generic programming, the right trait makes everything nice.

Still, why go to all this trouble? Why didn’t Rust’s designers make the generics more like C++ templates, where the constraints are left implicit in the code, à la “duck typing?”

One advantage of Rust’s approach is forward compatibility of generic code. You can change the implementation of a public generic function or method, and if you didn’t change the signature, you haven’t broken any of its users.

Another advantage of bounds is that when you do get a compiler error, at least the compiler can tell you where the trouble is. C++ compiler error messages involving templates can be much longer than Rust’s, pointing at many different lines of code, because the compiler has no way to tell who’s to blame for a problem: the template—or its caller, which might also be a template—or that template’s caller...

Perhaps the most important advantage of writing out the bounds explicitly is simply that they are there, in the code and in the documentation. You can look at the signature of a generic function in Rust and see exactly what kind of arguments it accepts. The same can’t be said for templates. The work that goes into fully documenting argument types in C++ libraries like Boost is even more arduous than what we went through here. The Boost developers don’t have a compiler that checks their work.

Conclusion

Traits are one of the main organizing features in Rust, and with good reason. There’s nothing better to design a program or library around than a good interface.

This chapter was a blizzard of syntax, rules, and explanations. Now that we’ve laid a foundation, we can start talking about the many ways traits and generics are used in Rust code. The fact is, we’ve only begun to scratch the surface. The next two chapters cover common traits provided by the standard library. Upcoming chapters cover closures, iterators, input/output, and concurrency. Traits and generics play a central role in all of these topics.

Chapter 12. Operator Overloading

There is a range of views among mathematicians and philosophers as to the exact scope and definition of mathematics. ...All have severe problems, none has widespread acceptance, and no reconciliation seems possible.

Wikipedia, “Mathematics”

In the Mandelbrot set plotter we showed in Chapter 2, we used the num crate’s Complex type to represent a number on the complex plane:

#[derive(Clone, Copy, Debug)]
struct Complex<T> {
    /// Real portion of the complex number
    re: T,

    /// Imaginary portion of the complex number
    im: T
}

We were able to add and multiply Complex numbers just like any built-in numeric type, using Rust’s + and * operators:

z = z * z + c;

You can make your own types support arithmetic and other operators, too, just by implementing a few built-in traits. This is called operator overloading, and the effect is much like operator overloading in C++, C#, Python, and Ruby.

The traits for operator overloading fall into a few categories depending on what part of the language they support, as shown in Table 12-1. The remaining sections of this chapter cover each category in turn.

Table 12-1. Summary of traits for operator overloading

Category
Trait
Operator

Unary operators
std::ops::Neg
-x

std::ops::Not
!x

Arithmetic operators
std::ops::Add
x + y

std::ops::Sub
x - y

std::ops::Mul
x * y

std::ops::Div
x / y

std::ops::Rem
x % y

Bitwise operators
std::ops::BitAnd
x & y

std::ops::BitOr
x | y

std::ops::BitXor
x ^ y

std::ops::Shl
x << y

std::ops::Shr
x >> y

Compound assignment
arithmetic operators
std::ops::AddAssign
x += y

std::ops::SubAssign
x -= y

std::ops::MulAssign
x *= y

std::ops::DivAssign
x /= y

std::ops::RemAssign
x %= y

Compound assignment
bitwise operators
std::ops::BitAndAssign
x &= y

std::ops::BitOrAssign
x |= y

std::ops::BitXorAssign
x ^= y

std::ops::ShlAssign
x <<= y

std::ops::ShrAssign
x >>= y

Comparison
std::cmp::PartialEq
x == y, x != y

std::cmp::PartialOrd
x < y,  x <= y,  x > y,  x >= y

Indexing
std::ops::Index
x[y],  &x[y]

std::ops::IndexMut
x[y] = z,  &mut x[y]

Arithmetic and Bitwise Operators

In Rust, the expression a + b is actually shorthand for a.add(b), a call to the add method of the standard library’s std::ops::Add trait. Rust’s standard numeric types all implement std::ops::Add. To make the expression a + b work for Complex values, the num crate implements this trait for Complex as well. Similar traits cover the other operators: a * b is shorthand for a.mul(b), a method from the std::ops::Mul trait, std::ops::Neg covers the prefix - negation operator, and so on.

If you want to try writing out z.add(c), you’ll need to bring the Add trait into scope, so that its method is visible. That done, you can treat all arithmetic as function calls:¹

use std::ops::Add;

assert_eq!(4.125f32.add(5.75), 9.875);
assert_eq!(10.add(20), 10 + 20);

Here’s the definition of std::ops::Add:

trait Add<RHS=Self> {
    type Output;
    fn add(self, rhs: RHS) -> Self::Output;
}

In other words, the trait Add<T> is the ability to add a T value to yourself. For example, if you want to be able to add i32 and u32 values to your type, your type must implement both Add<i32> and Add<u32>. The trait’s type parameter RHS defaults to Self, so if you’re implementing addition between two values of the same type, you can simply write Add for that case. The associated type Output describes the result of the addition.

For example, to be able to add Complex<i32> values together, Complex<i32> must implement Add<Complex<i32>>. Since we’re adding a type to itself, we just write Add:

use std::ops::Add;

impl Add for Complex<i32> {
    type Output = Complex<i32>;
    fn add(self, rhs: Self) -> Self {
        Complex { re: self.re + rhs.re, im: self.im + rhs.im }
    }
}

Of course, we shouldn’t have to implement Add separately for Complex<i32>, Complex<f32>, Complex<f64>, and so on. All the definitions would look exactly the same except for the types involved, so we should be able to write a single generic implementation that covers them all, as long as the type of the complex components themselves supports addition:

use std::ops::Add;

impl<T> Add for Complex<T>
    where T: Add<Output=T>
{
    type Output = Self;
    fn add(self, rhs: Self) -> Self {
        Complex { re: self.re + rhs.re, im: self.im + rhs.im }
    }
}

By writing where T: Add<Output=T>, we restrict T to types that can be added to themselves, yielding another T value. This is a reasonable restriction, but we could loosen things still further: the Add trait doesn’t require both operands of + to have the same type, nor does it constrain the result type. So a maximally generic implementation would let the left- and right-hand operands vary independently, and produce a Complex value of whatever component type that addition produces:

use std::ops::Add;

impl<L, R, O> Add<Complex<R>> for Complex<L>
    where L: Add<R, Output=O>
{
    type Output = Complex<O>;
    fn add(self, rhs: Complex<R>) -> Self::Output {
        Complex { re: self.re + rhs.re, im: self.im + rhs.im }
    }
}

In practice, however, Rust tends to avoid supporting mixed-type operations. Since our type parameter L must implement Add<R, Output=O>, it usually follows that L, R, and O are all going to be the same type: there simply aren’t that many types available for L that implement anything else. So in the end, this maximally generic version may not be much more useful than the prior, simpler generic definition.

Rust’s built-in traits for arithmetic and bitwise operators come in three groups: unary operators, binary operators, and compound assignment operators. Within each group, the traits and their methods all have the same form, so we’ll cover one example from each.

Unary Operators

Aside from the dereferencing operator *, which we’ll cover separately in “Deref and DerefMut”, Rust has two unary operators that you can customize, shown in Table 12-2.

Table 12-2. Built-in traits for unary operators

Trait name | Expression | Equivalent expression
std::ops::Neg | -x | x.neg()
std::ops::Not | !x | x.not()

All of Rust’s numeric types implement std::ops::Neg, for the unary negation operator -; the integer types and bool implement std::ops::Not, for the unary complement operator !. There are also implementations for references to those types.

Note that ! complements bool values, and performs a bitwise complement (that is, flips the bits) when applied to integers; it plays the role of both the ! and ~ operators from C and C++.

These traits’ definitions are simple:

trait Neg {
    type Output;
    fn neg(self) -> Self::Output;
}

trait Not {
    type Output;
    fn not(self) -> Self::Output;
}

Negating a complex number simply negates each of its components. Here’s how we might write a generic implementation of negation for Complex values:

use std::ops::Neg;

impl<T, O> Neg for Complex<T>
    where T: Neg<Output=O>
{
    type Output = Complex<O>;
    fn neg(self) -> Complex<O> {
        Complex { re: -self.re, im: -self.im }
    }
}

Binary Operators

Rust’s binary arithmetic and bitwise operators and their corresponding built-in traits appear in Table 12-3.

Table 12-3. Built-in traits for binary operators

Category | Trait name | Expression | Equivalent expression
Arithmetic operators | std::ops::Add | x + y | x.add(y)
std::ops::Sub | x - y | x.sub(y)
std::ops::Mul | x * y | x.mul(y)
std::ops::Div | x / y | x.div(y)
std::ops::Rem | x % y | x.rem(y)
Bitwise operators | std::ops::BitAnd | x & y | x.bitand(y)
std::ops::BitOr | x | y | x.bitor(y)
std::ops::BitXor | x ^ y | x.bitxor(y)
std::ops::Shl | x << y | x.shl(y)
std::ops::Shr | x >> y | x.shr(y)

All of Rust’s numeric types implement the arithmetic operators. Rust’s integer types and bool implement the bitwise operators. There are also implementations that accept references to those types as either or both operands.

All of the traits here have the same general form. The definition of std::ops::BitXor, for the ^ operator, looks like this:

trait BitXor<RHS=Self> {
    type Output;
    fn bitxor(self, rhs: RHS) -> Self::Output;
}

At the beginning of this chapter, we also showed std::ops::Add, another trait in this category, along with several sample implementations.

The Shl and Shr traits deviate slightly from this pattern: they do not default their RHS type parameter to Self, so you must always supply the righthand operand type explicitly. The right operand of a << or >> operator is a bit shift distance, which doesn’t have much relationship to the type of the value being shifted.

You can use the + operator to concatenate a String with a &str slice or another String. However, Rust does not permit the left operand of + to be a &str, to discourage building up long strings by repeatedly concatenating small pieces on the left. (This performs poorly, requiring time quadratic in the final length of the string.) Generally, the write! macro is better for building up strings piece by piece; we show how to do this in “Appending and Inserting Text”.

Compound Assignment Operators

A compound assignment expression is one like x += y or x &= y: it takes two operands, performs some operation on them like addition or a bitwise AND, and stores the result back in the left operand. In Rust, the value of a compound assignment expression is always (), never the value stored.

Many languages have operators like these, and usually define them as shorthand for expressions like x = x + y or x = x & y. However, Rust doesn’t take that approach. Instead, x += y is shorthand for the method call x.add_assign(y), where add_assign is the sole method of the std::ops::AddAssign trait:

trait AddAssign<RHS=Self> {
    fn add_assign(&mut self, RHS);
}

Table 12-4 shows all of Rust’s compound assignment operators, and the built-in traits that implement them.

Table 12-4. Built-in traits for compound assignment operators

Category | Trait name | Expression | Equivalent expression
Arithmetic operators | std::ops::AddAssign | x += y | x.add_assign(y)
std::ops::SubAssign | x -= y | x.sub_assign(y)
std::ops::MulAssign | x *= y | x.mul_assign(y)
std::ops::DivAssign | x /= y | x.div_assign(y)
std::ops::RemAssign | x %= y | x.rem_assign(y)
Bitwise operators | std::ops::BitAndAssign | x &= y | x.bitand_assign(y)
std::ops::BitOrAssign | x |= y | x.bitor_assign(y)
std::ops::BitXorAssign | x ^= y | x.bitxor_assign(y)
std::ops::ShlAssign | x <<= y | x.shl_assign(y)
std::ops::ShrAssign | x >>= y | x.shr_assign(y)

All of Rust’s numeric types implement the arithmetic compound assignment operators. Rust’s integer types and bool implement the bitwise compound assignment operators.

A generic implementation of AddAssign for our Complex type is straightforward:

use std::ops::AddAssign;

impl<T> AddAssign for Complex<T>
    where T: AddAssign<T>
{
    fn add_assign(&mut self, rhs: Complex<T>) {
        self.re += rhs.re;
        self.im += rhs.im;
    }
}

The built-in trait for a compound assignment operator is completely independent of the built-in trait for the corresponding binary operator. Implementing std::ops::Add does not automatically implement std::ops::AddAssign; if you want Rust to permit your type as the lefthand operand of a += operator, you must implement AddAssign yourself.

As with the binary Shl and Shr traits, the ShlAssign and ShrAssign traits deviate slightly from the pattern followed by the other compound assignment traits: they do not default their RHS type parameter to Self, so you must always supply the right-hand operand type explicitly.

Equality Tests

Rust’s equality operators, == and !=, are shorthand for calls to the std::cmp::PartialEq trait’s eq and ne methods:

assert_eq!(x == y, x.eq(&y));
assert_eq!(x != y, x.ne(&y));

Here’s the definition of std::cmp::PartialEq:

trait PartialEq<Rhs: ?Sized = Self> {
    fn eq(&self, other: &Rhs) -> bool;
    fn ne(&self, other: &Rhs) -> bool { !self.eq(other) }
}

Since the ne method has a default definition, you only need to define eq to implement the PartialEq trait, so here’s a complete implementation for Complex:

impl<T: PartialEq> PartialEq for Complex<T> {
    fn eq(&self, other: &Complex<T>) -> bool {
        self.re == other.re && self.im == other.im
    }
}

In other words, for any component type T that itself can be compared for equality, this implements comparison for Complex<T>. Assuming we’ve also implemented std::ops::Mul for Complex somewhere along the line, we can now write:

let x = Complex { re: 5, im: 2 };
let y = Complex { re: 2, im: 5 };
assert_eq!(x * y, Complex { re: 0, im: 29 });

Implementations of PartialEq are almost always of the form shown here: they compare each field of the left operand to the corresponding field of the right. These get tedious to write, and equality is a common operation to support, so if you ask, Rust will generate an implementation of PartialEq for you automatically. Simply add PartialEq to the type definition’s derive attribute like so:

#[derive(Clone, Copy, Debug, PartialEq)]
struct Complex<T> {
    ...
}

Rust’s automatically generated implementation is essentially identical to our hand-written code, comparing each field or element of the type in turn. Rust can derive PartialEq implementations for enum types as well. Naturally, each of the values the type holds (or might hold, in the case of an enum) must itself implement PartialEq.

Unlike the arithmetic and bitwise traits, which take their operands by value, PartialEq takes its operands by reference. This means that comparing non-Copy values like Strings, Vecs, or HashMaps doesn’t cause them to be moved, which would be troublesome:

let s = "d\x6fv\x65t\x61i\x6c".to_string();
let t = "\x64o\x76e\x74a\x69l".to_string();
assert!(s == t);  // s and t are only borrowed...

// ... so they still have their values here.
assert_eq!(format!("{} {}", s, t), "dovetail dovetail");

This leads us to the trait’s bound on the Rhs type parameter, which is of a kind we haven’t seen before:

where Rhs: ?Sized

This relaxes Rust’s usual requirement that type parameters must be sized types, letting us write traits like PartialEq<str> or PartialEq<[T]>. The eq and ne methods take parameters of type &Rhs, and comparing something with a &str or a &[T] is completely reasonable. Since str implements PartialEq<str>, the following assertions are equivalent:

assert!("ungula" != "ungulate");
assert!("ungula".ne("ungulate"));

Here, both Self and Rhs would be the unsized type str, making ne’s self and rhs parameters both &str values. We’ll discuss sized types, unsized types, and the Sized trait in detail in “Sized”.

Why is this trait called PartialEq? The traditional mathematical definition of an equivalence relation, of which equality is one instance, imposes three requirements. For any values x and y:

If x == y is true, then y == x must be true as well. In other words, swapping the two sides of an equality comparison doesn’t affect the result.

If x == y and y == z, then it must be the case that x == z. Given any chain of values, each equal to the next, each value in the chain is directly equal to every other. Equality is contagious.

It must always be true that x == x.

That last requirement might seem too obvious to be worth stating, but this is exactly where things go awry. Rust’s f32 and f64 are IEEE standard floating-point values. According to that standard, expressions like 0.0/0.0 and others with no appropriate value must produce special not-a-number values, usually referred to as NaN values. The standard further requires that a NaN value be treated as unequal to every other value—including itself. For example, the standard requires all the following behaviors:

assert!(f64::is_nan(0.0/0.0));
assert_eq!(0.0/0.0 == 0.0/0.0, false);
assert_eq!(0.0/0.0 != 0.0/0.0, true);

Furthermore, any ordered comparison with a NaN value must return false:

assert_eq!(0.0/0.0 < 0.0/0.0, false);
assert_eq!(0.0/0.0 > 0.0/0.0, false);
assert_eq!(0.0/0.0 <= 0.0/0.0, false);
assert_eq!(0.0/0.0 >= 0.0/0.0, false);

So while Rust’s == operator meets the first two requirements for equivalence relations, it clearly doesn’t meet the third when used on IEEE floating-point values. This is called a partial equivalence relation, so Rust uses the name PartialEq for the == operator’s built-in trait. If you write generic code with type parameters known only to be PartialEq, you may assume the first two requirements hold, but you should not assume that values always equal themselves.

That can be a bit counterintuitive, and may lead to bugs if you’re not vigilant. If you’d prefer your generic code to require a full equivalence relation, you can instead use the std::cmp::Eq trait as a bound, which represents a full equivalence relation: if a type implements Eq, then x == x must be true for every value x of that type. In practice, almost every type that implements PartialEq should implement Eq as well; f32 and f64 are the only types in the standard library that are PartialEq but not Eq.

The standard library defines Eq as an extension of PartialEq, adding no new methods:

trait Eq: PartialEq<Self> { }

If your type is PartialEq, and you would like it to be Eq as well, you must explicitly implement Eq, even though you need not actually define any new functions or types to do so. So implementing Eq for our Complex type is quick:

impl<T: Eq> Eq for Complex<T> { }

We could implement it even more succinctly by just including Eq in the derive attribute on the Complex type definition:

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
struct Complex<T> {
    ...
}

Derived implementations on a generic type may depend on the type parameters. With the derive attribute, Complex<i32> would implement Eq, because i32 does, but Complex<f32> would implement only PartialEq, since f32 doesn’t implement Eq.

When you implement std::cmp::PartialEq yourself, Rust can’t check that your definitions for the eq and ne methods actually behave as required for partial or full equivalence. They could do anything you like. Rust simply takes your word that you’ve implemented equality in a way that meets the expectations of the trait’s users.

Although the definition of PartialEq provides a default definition for ne, you can provide your own implementation if you like. However, you must ensure that ne and eq are exact inverses of each other. Users of the PartialEq trait will assume this is so.

Ordered Comparisons

Rust specifies the behavior of the ordered comparison operators <, >, <=, and >= all in terms of a single trait, std::cmp::PartialOrd:

trait PartialOrd<Rhs = Self>: PartialEq<Rhs> where Rhs: ?Sized {
    fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;

    fn lt(&self, other: &Rhs) -> bool { ... }
    fn le(&self, other: &Rhs) -> bool { ... }
    fn gt(&self, other: &Rhs) -> bool { ... }
    fn ge(&self, other: &Rhs) -> bool { ... }
}

Note that PartialOrd<Rhs> extends PartialEq<Rhs>: you can do ordered comparisons only on types that you can also compare for equality.

The only method of PartialOrd you must implement yourself is partial_cmp. When partial_cmp returns Some(o), then o indicates self’s relationship to other:

enum Ordering {
    Less,       // self < other
    Equal,      // self == other
    Greater,    // self > other
}

But if partial_cmp returns None, that means self and other are unordered with respect to each other: neither is greater than the other, nor are they equal. Among all of Rust’s primitive types, only comparisons between floating-point values ever return None: specifically, comparing a NaN (not-a-number) value with anything else returns None. We give some more background on NaN values in “Equality Tests”.

Like the other binary operators, to compare values of two types Left and Right, Left must implement PartialOrd<Right>. Expressions like x < y or x >= y are shorthand for calls to PartialOrd methods, as shown in Table 12-5.

Table 12-5. Ordered comparison operators and PartialOrd methods

Expression
Equivalent method call
Default definition

x < y
x.lt(y)
x.partial_cmp(&y) == Some(Less)

x > y
x.gt(y)
x.partial_cmp(&y) == Some(Greater)

x <= y
x.le(y)
match x.partial_cmp(&y) {
  Some(Less) | Some(Equal) => true,
  _ => false,
}

x >= y
x.ge(y)
match x.partial_cmp(&y) {
  Some(Greater) | Some(Equal) => true,
  _ => false,
}

As in prior examples, the equivalent method call code shown assumes that std::cmp::PartialOrd and std::cmp::Ordering are in scope.

If you know that values of two types are always ordered with respect to each other, then you can implement the stricter std::cmp::Ord trait:

trait Ord: Eq + PartialOrd<Self> {
    fn cmp(&self, other: &Self) -> Ordering;
}

The cmp method here simply returns an Ordering, instead of an Option<Ordering> like partial_cmp: cmp always declares its arguments equal, or indicates their relative order. Almost all types that implement PartialOrd should also implement Ord. In the standard library, f32 and f64 are the only exceptions to this rule.

Since there’s no natural ordering on complex numbers, we can’t use our Complex type from the previous sections to show a sample implementation of PartialOrd. Instead, suppose you’re working with the following type, representing the set of numbers falling within a given half-open interval:

#[derive(Debug, PartialEq)]
struct Interval<T> {
    lower: T, // inclusive
    upper: T  // exclusive
}

You’d like to make values of this type partially ordered: one interval is less than another if it falls entirely before the other, with no overlap. If two unequal intervals overlap, they’re unordered: some element of each side is less than some element of the other. And two equal intervals are simply equal. The following implementation of PartialOrd implements those rules:

use std::cmp::{Ordering, PartialOrd};

impl<T: PartialOrd> PartialOrd<Interval<T>> for Interval<T> {
    fn partial_cmp(&self, other: &Interval<T>) -> Option<Ordering> {
        if self == other { Some(Ordering::Equal) }
        else if self.lower >= other.upper { Some(Ordering::Greater) }
        else if self.upper <= other.lower { Some(Ordering::Less) }
        else { None }
    }
}

With that implementation in place, you can write the following:

assert!(Interval { lower: 10, upper: 20 } <  Interval { lower: 20, upper: 40 });
assert!(Interval { lower: 7,  upper: 8  } >= Interval { lower: 0,  upper: 1  });
assert!(Interval { lower: 7,  upper: 8  } <= Interval { lower: 7,  upper: 8  });

// Overlapping intervals aren't ordered with respect to each other.
let left  = Interval { lower: 10, upper: 30 };
let right = Interval { lower: 20, upper: 40 };
assert!(!(left < right));
assert!(!(left >= right));

Index and IndexMut

You can specify how an indexing expression like a[i] works on your type by implementing the std::ops::Index and std::ops::IndexMut traits. Arrays support the [] operator directly, but on any other type, the expression a[i] is normally shorthand for *a.index(i), where index is a method of the std::ops::Index trait. However, if the expression is being assigned to or borrowed mutably, it’s instead shorthand for *a.index_mut(i), a call to the method of the std::ops::IndexMut trait.

Here are the traits’ definitions:

trait Index<Idx> {
    type Output: ?Sized;
    fn index(&self, index: Idx) -> &Self::Output;
}

trait IndexMut<Idx>: Index<Idx> {
    fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
}

Note that these traits take the type of the index expression as a parameter. You can index a slice with a single usize, referring to a single element, because slices implement Index<usize>. But you can refer to a subslice with an expression like a[i..j] because they also implement Index<Range<usize>>. That expression is shorthand for:

*a.index(std::ops::Range { start: i, end: j })

Rust’s HashMap and BTreeMap collections let you use any hashable or ordered type as the index. The following code works because HashMap<&str, i32> implements Index<&str>:

use std::collections::HashMap;
let mut m = HashMap::new();
m.insert("十", 10);
m.insert("百", 100);
m.insert("千", 1000);
m.insert("万", 1_0000);
m.insert("億", 1_0000_0000);

assert_eq!(m["十"], 10);
assert_eq!(m["千"], 1000);

Those indexing expressions are equivalent to:

use std::ops::Index;
assert_eq!(*m.index("十"), 10);
assert_eq!(*m.index("千"), 1000);

The Index trait’s associated type Output specifies what type an indexing expression produces: for our HashMap, the Index implementation’s Output type is i32.

The IndexMut trait extends Index with an index_mut method that takes a mutable reference to self, and returns a mutable reference to an Output value. Rust automatically selects index_mut when the indexing expression occurs in a context where it’s necessary. For example, suppose we write the following:

let mut desserts = vec!["Howalon".to_string(),
                        "Soan papdi".to_string()];
desserts[0].push_str(" (fictional)");
desserts[1].push_str(" (real)");

Because the push_str method operates on &mut self, those last two lines are equivalent to:

use std::ops::IndexMut;
(*desserts.index_mut(0)).push_str(" (fictional)");
(*desserts.index_mut(1)).push_str(" (real)");

One limitation of IndexMut is that, by design, it must return a mutable reference to some value. This is why you can’t use an expression like m["十"] = 10; to insert a value into the HashMap m: the table would need to create an entry for "十" first, with some default value, and return a mutable reference to that. But not all types have cheap default values, and some may be expensive to drop; it would be a waste to create such a value only to be immediately dropped by the assignment. (There are plans to improve this in later versions of the language.)

The most common use of indexing is for collections. For example, suppose we are working with bitmapped images, like the ones we created in the Mandelbrot set plotter in Chapter 2. Recall that our program contained code like this:

pixels[row * bounds.0 + column] = ...;

It would be nicer to have an Image<u8> type that acts like a two-dimensional array, allowing us to access pixels without having to write out all the arithmetic:

image[row][column] = ...;

To do this, we’ll need to declare a struct:

struct Image<P> {
    width: usize,
    pixels: Vec<P>
}

impl<P: Default + Copy> Image<P> {
    /// Create a new image of the given size.
    fn new(width: usize, height: usize) -> Image<P> {
        Image {
            width,
            pixels: vec![P::default(); width * height]
        }
    }
}

And here are implementations of Index and IndexMut that would fit the bill:

impl<P> std::ops::Index<usize> for Image<P> {
    type Output = [P];
    fn index(&self, row: usize) -> &[P] {
        let start = row * self.width;
        &self.pixels[start .. start + self.width]
    }
}

impl<P> std::ops::IndexMut<usize> for Image<P> {
    fn index_mut(&mut self, row: usize) -> &mut [P] {
        let start = row * self.width;
        &mut self.pixels[start .. start + self.width]
    }
}

When you index into an Image, you get back a slice of pixels; indexing the slice gives you an individual pixel.

Note that when we write image[row][column], if row is out of bounds, our .index() method will try to index self.pixels out of range, triggering a panic. This is how Index and IndexMut implementations are supposed to behave: out-of-bounds access is detected and causes a panic, the same as when you index an array, slice, or vector out of bounds.

Other Operators

Not all operators can be overloaded in Rust. As of Rust 1.17, the error-checking ? operator works only with Result values. Similarly, the logical operators && and || are limited to Boolean values only. The .. operator always creates Range values, the & operator always borrows references, and the = operator always moves or copies values. None of them can be overloaded.

The dereferencing operator, *val, and the dot operator for accessing fields and calling methods, as in val.field and val.method(), can be overloaded using the Deref and DerefMut traits, which are covered in the next chapter. (We did not include them here because these traits do more than just overload a few operators.)

Rust does not support overloading the function call operator, f(x). Instead, when you need a callable value, you’ll typically just write a closure. We’ll explain how this works and cover the Fn, FnMut, and FnOnce special traits in Chapter 14.

¹ Lisp programmers rejoice! The expression <i32 as Add>::add is the + operator on i32, captured as a function value.

Chapter 13. Utility Traits

Science is nothing else than the search to discover unity in the wild variety of nature—or, more exactly, in the variety of our experience. Poetry, painting, the arts are the same search, in Coleridge’s phrase, for unity in variety.

Jacob Bronowski

Apart from operator overloading, which we covered in the previous chapter, several other built-in traits let you hook into parts of the Rust language and standard library:

You can use the Drop trait to clean up values when they go out of scope, like destructors in C++.

Smart pointer types, like Box<T> and Rc<T>, can implement the Deref trait to make the pointer reflect the methods of the wrapped value.

By implementing the From<T> and Into<T> traits, you can tell Rust how to convert a value from one type to another.

This chapter is a grab bag of useful traits from the Rust standard library. We’ll cover each of the traits shown in Table 13-1.

There are other important standard library traits as well. We’ll cover Iterator and IntoIterator in Chapter 15. The Hash trait, for computing hash codes, is covered in Chapter 16. And a pair of traits that mark thread-safe types, Send and Sync, are covered in Chapter 19.

Table 13-1. Summary of utility traits

Trait | Description
Drop | Destructors. Cleanup code that Rust runs automatically whenever a value is dropped.
Sized | Marker trait for types with a fixed size known at compile time, as opposed to types (such as slices) that are dynamically sized.
Clone | Types that support cloning values.
Copy | Marker trait for types that can be cloned simply by making a byte-for-byte copy of the memory containing the value.
Deref and DerefMut | Traits for smart pointer types.
Default | Types that have a sensible “default value.”
AsRef and AsMut | Conversion traits for borrowing one type of reference from another.
Borrow and BorrowMut | Conversion traits, like AsRef/AsMut, but additionally guaranteeing consistent hashing, ordering, and equality.
From and Into | Conversion traits for transforming one type of value into another.
ToOwned | Conversion trait for converting a reference to an owned value.

Drop

When a value’s owner goes away, we say that Rust drops the value. Dropping a value entails freeing whatever other values, heap storage, and system resources the value owns. Drops occur under a variety of circumstances: when a variable goes out of scope; when an expression’s value is discarded by the ; operator; when you truncate a vector, removing elements from its end; and so on.

For the most part, Rust handles dropping values for you automatically. For example, suppose you define the following type:

struct Appellation {
    name: String,
    nicknames: Vec<String>
}

An Appellation owns heap storage for the strings’ contents and the vector’s buffer of elements. Rust takes care of cleaning all that up whenever an Appellation is dropped, without any further coding necessary on your part. However, if you want, you can customize how Rust drops values of your type by implementing the std::ops::Drop trait:

trait Drop {
    fn drop(&mut self);
}

An implementation of Drop is analogous to a destructor in C++, or a finalizer in other languages. When a value is dropped, if it implements std::ops::Drop, Rust calls its drop method, before proceeding to drop whatever values its fields or elements own, as it normally would. This implicit invocation of drop is the only way to call that method; if you try to invoke it explicitly yourself, Rust flags that as an error.

Because Rust calls Drop::drop on a value before dropping its fields or elements, the value the method receives is always still fully initialized. An implementation of Drop for our Appellation type can make full use of its fields:

impl Drop for Appellation {
    fn drop(&mut self) {
        print!("Dropping {}", self.name);
        if !self.nicknames.is_empty() {
            print!(" (AKA {})", self.nicknames.join(", "));
        }
        println!("");
    }
}

Given that implementation, we can write the following:

{
    let mut a = Appellation { name: "Zeus".to_string(),
                              nicknames: vec!["cloud collector".to_string(),
                                              "king of the gods".to_string()] };

    println!("before assignment");
    a = Appellation { name: "Hera".to_string(), nicknames: vec![] };
    println!("at end of block");
}

When we assign the second Appellation to a, the first is dropped, and when we leave the scope of a, the second is dropped. This code prints the following:

before assignment
Dropping Zeus (AKA cloud collector, king of the gods)
at end of block
Dropping Hera

Since our std::ops::Drop implementation for Appellation does nothing but print a message, how, exactly, does its memory get cleaned up? The Vec type implements Drop, dropping each of its elements and then freeing the heap-allocated buffer they occupied. A String uses a Vec<u8> internally to hold its text, so String need not implement Drop itself; it lets its Vec take care of freeing the characters. The same principle extends to Appellation values: when one gets dropped, in the end it is Vec’s implementation of Drop that actually takes care of freeing each of the strings’ contents, and finally freeing the buffer holding the vector’s elements. As for the memory that holds the Appellation value itself, it too has some owner, perhaps a local variable or some data structure, which is responsible for freeing it.

If a variable’s value gets moved elsewhere, so that the variable is uninitialized when it goes out of scope, then Rust will not try to drop that variable: there is no value in it to drop.

This principle holds even when a variable may or may not have had its value moved away, depending on the flow of control. In cases like this, Rust keeps track of the variable’s state with an invisible flag indicating whether the variable’s value needs to be dropped or not:

let p;
{
    let q = Appellation { name: "Cardamine hirsuta".to_string(),
                          nicknames: vec!["shotweed".to_string(),
                                          "bittercress".to_string()] };
    if complicated_condition() {
        p = q;
    }
}
println!("Sproing! What was that?");

Depending on whether complicated_condition returns true or false, either p or q will end up owning the Appellation, with the other uninitialized. Where it lands determines whether it is dropped before or after the println!, since q goes out of scope before the println!, and p after. Although a value may be moved from place to place, Rust drops it only once.

You usually won’t need to implement std::ops::Drop unless you’re defining a type that owns resources Rust doesn’t already know about. For example, on Unix systems, Rust’s standard library uses the following type internally to represent an operating system file descriptor:

struct FileDesc {
    fd: c_int,
}

The fd field of a FileDesc is simply the number of the file descriptor that should be closed when the program is done with it; c_int is an alias for i32. The standard library implements Drop for FileDesc as follows:

impl Drop for FileDesc {
    fn drop(&mut self) {
        let _ = unsafe { libc::close(self.fd) };
    }
}

Here, libc::close is the Rust name for the C library’s close function. Rust code may call C functions only within unsafe blocks, so the library uses one here.

If a type implements Drop, it cannot implement the Copy trait. If a type is Copy, that means that simple byte-for-byte duplication is sufficient to produce an independent copy of the value. But it is typically a mistake to call the same drop method more than once on the same data.

The standard prelude includes a function to drop a value, drop, but its definition is anything but magical:

fn drop<T>(_x: T) { }

In other words, it receives its argument by value, taking ownership from the caller—and then does nothing with it. Rust drops the value of _x when it goes out of scope, as it would for any other variable.

Sized

A sized type is one whose values all have the same size in memory. Almost all types in Rust are sized: every u64 takes eight bytes, every (f32, f32, f32) tuple twelve. Even enums are sized: no matter which variant is actually present, an enum always occupies enough space to hold its largest variant. And although a Vec<T> owns a heap-allocated buffer whose size can vary, the Vec value itself is a pointer to the buffer, its capacity, and its length, so Vec<T> is a sized type.

However, Rust also has a few unsized types whose values are not all the same size. For example, the string slice type str (note, without an &) is unsized. The string literals "diminutive" and "big" are references to str slices that occupy ten and three bytes. Both are shown in Figure 13-1. Array slice types like [T] (again, without an &) are unsized, too: a shared reference like &[u8] can point to a [u8] slice of any size. Because the str and [T] types denote sets of values of varying sizes, they are unsized types.

Figure 13-1. References to unsized values

The other common kind of unsized type in Rust is the referent of a trait object. As we explained in “Trait Objects”, a trait object is a pointer to some value that implements a given trait. For example, the types &std::io::Write and Box<std::io::Write> are pointers to some value that implements the Write trait. The referent might be a file, or a network socket, or some type of your own for which you have implemented Write. Since the set of types that implement Write is open-ended, Write considered as a type is unsized: its values have various sizes.

Rust can’t store unsized values in variables or pass them as arguments. You can only deal with them through pointers like &str or Box<Write>, which themselves are sized. As shown in Figure 13-1, a pointer to an unsized value is always a fat pointer, two words wide: a pointer to a slice also carries the slice’s length, and a trait object also carries a pointer to a vtable of method implementations.

Trait objects and pointers to slices are nicely symmetrical. In both cases, the type lacks information necessary to use it: you can’t index a [u8] without knowing its length, nor can you invoke a method on a Box<Write> without knowing the implementation of Write appropriate to the specific value it refers to. And in both cases, the fat pointer fills in the information missing from the type, carrying a length or a vtable pointer. The omitted static information is replaced with dynamic information.

All sized types implement the std::marker::Sized trait, which has no methods or associated types. Rust implements it automatically for all types to which it applies; you can’t implement it yourself. The only use for Sized is as a bound for type variables: a bound like T: Sized requires T to be a type whose size is known at compile time. Traits of this sort are called marker traits, because the Rust language itself uses them to mark certain types as having characteristics of interest.

Since unsized types are so limited, most generic type variables should be restricted to Sized types. In fact, this is necessary so often that it is the implicit default in Rust: if you write struct S<T> { ... }, Rust understands you to mean struct S<T: Sized> { ... }. If you do not want to constrain T this way, you must explicitly opt out, writing struct S<T: ?Sized> { ... }. The ?Sized syntax is specific to this case, and means “not necessarily Sized.” For example, if you write struct S<T: ?Sized> { b: Box<T> }, then Rust will allow you to write S<str> and S<Write>, where the box becomes a fat pointer, as well as S<i32> and S<String>, where the box is an ordinary pointer.

Despite their restrictions, unsized types make Rust’s type system work more smoothly. Reading the standard library documentation, you will occasionally come across a ?Sized bound on a type variable; this almost always means that the given type is only pointed to, and allows the associated code to work with slices and trait objects as well as ordinary values. When a type variable has the ?Sized bound, people often say it is questionably sized: it might be Sized, or it might not.

Aside from slices and trait objects, there is one more kind of unsized type. A struct type’s last field (but only its last) may be unsized, and such a struct is itself unsized. For example, an Rc<T> reference-counted pointer is implemented internally as a pointer to the private type RcBox<T>, which stores the reference count alongside the T. Here’s a simplified definition of RcBox:

struct RcBox<T: ?Sized> {
    ref_count: usize,
    value: T,
}

The value field is the T to which Rc<T> is counting references; Rc<T> dereferences to a pointer to this field. The ref_count field holds the reference count.

You can use RcBox with sized types, like RcBox<String>; the result is a sized struct type. Or you can use it with unsized types, like RcBox<std::fmt::Display> (where Display is the trait for types that can be formatted by println! and similar macros); RcBox<Display> is an unsized struct type.

You can’t build an RcBox<Display> value directly. Instead, you must first create an ordinary, sized RcBox whose value type implements Display, like RcBox<String>. Rust then lets you convert a reference &RcBox<String> to a fat reference &RcBox<Display>:

let boxed_lunch: RcBox<String> = RcBox {
    ref_count: 1,
    value: "lunch".to_string()
};

use std::fmt::Display;
let boxed_displayable: &RcBox<Display> = &boxed_lunch;

This conversion happens implicitly when passing values to functions, so you can pass a &RcBox<String> to a function that expects an &RcBox<Display>:

fn display(boxed: &RcBox<Display>) {
    println!("For your enjoyment: {}", &boxed.value);
}

display(&boxed_lunch);

This produces the following output:

For your enjoyment: lunch

Clone

The std::clone::Clone trait is for types that can make copies of themselves. Clone is defined as follows:

trait Clone: Sized {
    fn clone(&self) -> Self;
    fn clone_from(&mut self, source: &Self) {
        *self = source.clone()
    }
}

The clone method should construct an independent copy of self and return it. Since this method’s return type is Self, and functions may not return unsized values, the Clone trait itself extends the Sized trait: this has the effect of bounding implementations’ Self types to be Sized.

Cloning a value usually entails allocating copies of anything it owns, as well, so a clone can be expensive, in both time and memory. For example, cloning a Vec<String> not only copies the vector, but also copies each of its String elements. This is why Rust doesn’t just clone values automatically, but instead requires you to make an explicit method call. The reference-counted pointer types like Rc<T> and Arc<T> are exceptions: cloning one of these simply increments the reference count and hands you a new pointer.

The clone_from method modifies self into a copy of source. The default definition of clone_from simply clones source, and then moves that into *self. This always works, but for some types, there is a faster way to get the same effect. For example, suppose s and t are Strings. The statement s = t.clone(); must clone t, drop the old value of s, and then move the cloned value into s; that’s one heap allocation, and one heap deallocation. But if the heap buffer belonging to the original s has enough capacity to hold t’s contents, no allocation or deallocation is necessary: you can simply copy t’s text into s’s buffer, and adjust the length. In generic code, you should use clone_from whenever possible, to permit this optimization when it is available.

If your Clone implementation simply applies clone to each field or element of your type, and then constructs a new value from those clones, and the default definition of clone_from is good enough, then Rust will implement that for you: simply put #[derive(Clone)] above your type definition.

Pretty much every type in the standard library that makes sense to copy implements Clone. Primitive types like bool and i32 do. Container types like String, Vec<T>, and HashMap do, too. Some types don’t make sense to copy, like std::sync::Mutex; those don’t implement Clone. Some types like std::fs::File can be copied, but the copy might fail if the operating system doesn’t have the necessary resources; these types don’t implement Clone, since clone must be infallible. Instead, std::fs::File provides a try_clone method, which returns a std::io::Result<File>, which can report a failure.

Copy

In Chapter 4, we explained that, for most types, assignment moves values, rather than copying them. Moving values makes it much simpler to track the resources they own. But in “Copy Types: The Exception to Moves”, we pointed out the exception: simple types that don’t own any resources can be Copy types, where assignment makes a copy of the source, rather than moving the value and leaving the source uninitialized.

At that time, we left it vague exactly what Copy was, but now we can tell you: a type is Copy if it implements the std::marker::Copy marker trait, which is defined as follows:

trait Copy: Clone { }

This is certainly easy to implement for your own types:

impl Copy for MyType { }

But because Copy is a marker trait with special meaning to the language, Rust permits a type to implement Copy only if a shallow byte-for-byte copy is all it needs. Types that own any other resources, like heap buffers or operating system handles, cannot implement Copy.

Any type that implements the Drop trait cannot be Copy. Rust presumes that if a type needs special clean-up code, it must also require special copying code, and thus can’t be Copy.

As with Clone, you can ask Rust to derive Copy for you, using #[derive(Copy)]. You will often see both derived at once, with #[derive(Copy, Clone)].

Think carefully before making a type Copy. Although doing so makes the type easier to use, it places heavy restrictions on its implementation. Implicit copies can also be expensive. We explain these factors in detail in “Copy Types: The Exception to Moves”.

Deref and DerefMut

You can specify how dereferencing operators like * and . behave on your types by implementing the std::ops::Deref and std::ops::DerefMut traits. Pointer types like Box<T> and Rc<T> implement these traits so that they can behave as Rust’s built-in pointer types do. For example, if you have a Box<Complex> value b, then *b refers to the Complex value that b points to, and b.re refers to its real component. If the context assigns or borrows a mutable reference to the referent, Rust uses the DerefMut (“dereference mutably”) trait; otherwise, read-only access is enough, and it uses Deref.

The traits are defined like this:

trait Deref {
    type Target: ?Sized;
    fn deref(&self) -> &Self::Target;
}

trait DerefMut: Deref {
    fn deref_mut(&mut self) -> &mut Self::Target;
}

The deref and deref_mut methods take a &Self reference and return a &Self::Target reference. Target should be something that Self contains, owns, or refers to: for Box<Complex> the Target type is Complex. Note that DerefMut extends Deref: if you can dereference something and modify it, certainly you should be able to borrow a shared reference to it as well. Since the methods return a reference with the same lifetime as &self, self remains borrowed for as long as the returned reference lives.

The Deref and DerefMut traits play another role as well. Since deref takes a &Self reference and returns a &Self::Target reference, Rust uses this to automatically convert references of the former type into the latter. In other words, if inserting a deref call would prevent a type mismatch, Rust inserts one for you. Implementing DerefMut enables the corresponding conversion for mutable references. These are called the deref coercions: one type is being “coerced” into behaving as another.

Although the deref coercions aren’t anything you couldn’t write out explicitly yourself, they’re convenient:

If you have some Rc<String> value r, and want to apply String::find to it, you can simply write r.find('?'), instead of (*r).find('?'): the method call implicitly borrows r, and &Rc<String> coerces to &String, because Rc<T> implements Deref<Target=T>.

You can use methods like split_at on String values, even though split_at is a method of the str slice type, because String implements Deref<Target=str>. There’s no need for String to reimplement all of str’s methods, since you can coerce a &str from a &String.

If you have a vector of bytes v, and you want to pass it to a function that expects a byte slice &[u8], you can simply pass &v as the argument, since Vec<T> implements Deref<Target=[T]>.

Rust will apply several deref coercions in succession if necessary. For example, using the coercions mentioned before, you can apply split_at directly to an Rc<String>, since &Rc<String> dereferences to &String, which dereferences to &str, which has the split_at method.

For example, suppose you have the following type:

struct Selector<T> {
    /// Elements available in this `Selector`.
    elements: Vec<T>,

    /// The index of the "current" element in `elements`. A `Selector`
    /// behaves like a pointer to the current element.
    current: usize
}

To make the Selector behave as the doc comment claims, you must implement Deref and DerefMut for the type:

use std::ops::{Deref, DerefMut};

impl<T> Deref for Selector<T> {
    type Target = T;
    fn deref(&self) -> &T {
        &self.elements[self.current]
    }
}

impl<T> DerefMut for Selector<T> {
    fn deref_mut(&mut self) -> &mut T {
        &mut self.elements[self.current]
    }
}

Given those implementations, you can use a Selector like this:

let mut s = Selector { elements: vec!['x', 'y', 'z'],
                       current: 2 };

// Because `Selector` implements `Deref`, we can use the `*` operator to
// refer to its current element.
assert_eq!(*s, 'z');

// Assert that 'z' is alphabetic, using a method of `char` directly on a
// `Selector`, via deref coercion.
assert!(s.is_alphabetic());

// Change the 'z' to a 'w', by assigning to the `Selector`'s referent.
*s = 'w';

assert_eq!(s.elements, ['x', 'y', 'w']);

The Deref and DerefMut traits are designed for implementing smart pointer types, like Box, Rc, and Arc, and types that serve as owning versions of something you would also frequently use by reference, the way Vec<T> and String serve as owning versions of [T] and str. You should not implement Deref and DerefMut for a type just to make the Target type’s methods appear on it automatically, the way a C++ base class’s methods are visible on a subclass. This will not always work as you expect, and can be confusing when it goes awry.

The deref coercions come with a caveat that can cause some confusion: Rust applies them to resolve type conflicts, but not to satisfy bounds on type variables. For example, the following code works fine:

let s = Selector { elements: vec!["good", "bad", "ugly"],
                   current: 2 };

fn show_it(thing: &str) { println!("{}", thing); }
show_it(&s);

In the call show_it(&s), Rust sees an argument of type &Selector<&str> and a parameter of type &str, finds the Deref<Target=str> implementation, and rewrites the call to show_it(s.deref()), just as needed.

However, if you change show_it into a generic function, Rust is suddenly no longer cooperative:

use std::fmt::Display;
fn show_it_generic<T: Display>(thing: T) { println!("{}", thing); }
show_it_generic(&s);

Rust complains:

error[E0277]: the trait bound `Selector<&str>: Display` is not satisfied
    |
542 |         show_it_generic(&s);
    |         ^^^^^^^^^^^^^^^ trait `Selector<&str>: Display` not satisfied
    |

This can be bewildering: How could making a function generic introduce an error? True, Selector<&str> does not implement Display itself, but it dereferences to &str, which certainly does.

Since you’re passing an argument of type &Selector<&str>, and the function’s parameter type is &T, the type variable T must be Selector<&str>. Then, Rust checks whether the bound T: Display is satisfied: since it does not apply deref coercions to satisfy bounds on type variables, this check fails.

To work around this problem, you can spell out the coercion using the as operator:

show_it_generic(&s as &str);

Default

Some types have a reasonably obvious default value: the default vector or string is empty, the default number is zero, the default Option is None, and so on. Types like this can implement the std::default::Default trait:

trait Default {
    fn default() -> Self;
}

The default method simply returns a fresh value of type Self. String’s implementation of Default is straightforward:

impl Default for String {
    fn default() -> String {
        String::new()
    }
}

All of Rust’s collection types—Vec, HashMap, BinaryHeap, and so on—implement Default, with default methods that return an empty collection. This is helpful when you need to build a collection of values, but want to let your caller decide exactly what sort of collection to build. For example, the Iterator trait’s partition method splits the values the iterator produces into two collections, using a closure to decide where each value goes:

use std::collections::HashSet;
let squares = [4, 9, 16, 25, 36, 49, 64];
let (powers_of_two, impure): (HashSet<i32>, HashSet<i32>)
    = squares.iter().partition(|&n| n & (n-1) == 0);

assert_eq!(powers_of_two.len(), 3);
assert_eq!(impure.len(), 4);

The closure |&n| n & (n-1) == 0 uses some bit-fiddling to recognize numbers that are powers of two, and partition uses that to produce two HashSets. But of course, partition isn’t specific to HashSets; you can use it to produce any sort of collection you like, as long as the collection type implements Default, to produce an empty collection to start with, and Extend<T>, to add a T to the collection. String implements Default and Extend<char>, so you can write:

let (upper, lower): (String, String)
    = "Great Teacher Onizuka".chars().partition(|&c| c.is_uppercase());
assert_eq!(upper, "GTO");
assert_eq!(lower, "reat eacher nizuka");

Another common use of Default is to produce default values for structs that represent a large collection of parameters, most of which you won’t usually need to change. For example, the glium crate provides Rust bindings for the powerful and complex OpenGL graphics library. The glium::DrawParameters struct includes 22 fields, each controlling a different detail of how OpenGL should render some bit of graphics. The glium draw function expects a DrawParameters struct as an argument. Since DrawParameters implements Default, you can create one to pass to draw, mentioning only those fields you want to change:

let params = glium::DrawParameters {
    line_width: Some(0.02),
    point_size: Some(0.02),
    .. Default::default()
};

target.draw(..., &params).unwrap();

This calls Default::default() to create a DrawParameters value initialized with the default values for all its fields, and then uses the .. syntax for structs to create a new one with the line_width and point_size fields changed, ready for you to pass it to target.draw.

If a type T implements Default, then the standard library implements Default automatically for Rc<T>, Arc<T>, Box<T>, Cell<T>, RefCell<T>, Cow<T>, Mutex<T>, and RwLock<T>. The default value for the type Rc<T>, for example, is an Rc pointing to the default value for type T.

If all the element types of a tuple type implement Default, then the tuple type does too, defaulting to a tuple holding each element’s default value.

Rust does not implicitly implement Default for struct types, but if all of a struct’s fields implement Default, you can implement Default for the struct automatically using #[derive(Default)].

The default value of any Option<T> is None.

AsRef and AsMut

When a type implements AsRef<T>, that means you can borrow a &T from it efficiently. AsMut is the analogue for mutable references. Their definitions are as follows:

trait AsRef<T: ?Sized> {
    fn as_ref(&self) -> &T;
}

trait AsMut<T: ?Sized> {
    fn as_mut(&mut self) -> &mut T;
}

So, for example, Vec<T> implements AsRef<[T]>, and String implements AsRef<str>. You can also borrow a String’s contents as an array of bytes, so String implements AsRef<[u8]> as well.

AsRef is typically used to make functions more flexible in the argument types they accept. For example, the std::fs::File::open function is declared like this:

fn open<P: AsRef<Path>>(path: P) -> Result<File>

What open really wants is a &Path, the type representing a filesystem path. But with this signature, open accepts anything it can borrow a &Path from—that is, anything that implements AsRef<Path>. Such types include String and str, the operating system interface string types OsString and OsStr, and of course PathBuf and Path; see the library documentation for the full list. This is what allows you to pass string literals to open:

let dot_emacs = std::fs::File::open("/home/jimb/.emacs")?;

All of the standard library’s filesystem access functions accept path arguments this way. For callers, the effect resembles that of an overloaded function in C++, although Rust takes a different approach toward establishing which argument types are acceptable.

But this can’t be the whole story. A string literal is a &str, but the type that implements AsRef<Path> is str, without an &. And as we explained in “Deref and DerefMut”, Rust doesn’t try deref coercions to satisfy type variable bounds, so they won’t help here either.

Fortunately, the standard library includes the blanket implementation:

impl<'a, T, U> AsRef<U> for &'a T
    where T: AsRef<U>,
          T: ?Sized, U: ?Sized
{
    fn as_ref(&self) -> &U {
        (*self).as_ref()
    }
}

In other words, for any types T and U, if T: AsRef<U>, then &T: AsRef<U> as well: simply follow the reference and proceed as before. In particular, since str: AsRef<Path>, then &str: AsRef<Path> as well. In a sense, this is a way to get a limited form of deref coercion in checking AsRef bounds on type variables.

You might assume that, if a type implements AsRef<T>, it should also implement AsMut<T>. However, there are cases where this isn’t appropriate. For example, we’ve mentioned that String implements AsRef<[u8]>; this makes sense, as each String certainly has a buffer of bytes that can be useful to access as binary data. However, String further guarantees that those bytes are a well-formed UTF-8 encoding of Unicode text; if String implemented AsMut<[u8]>, that would let callers change the String’s bytes to anything they wanted, and you could no longer trust a String to be well-formed UTF-8. It only makes sense for a type to implement AsMut<T> if modifying the given T cannot violate the type’s invariants.

Although AsRef and AsMut are pretty simple, providing standard, generic traits for reference conversion avoids the proliferation of more specific conversion traits. You should avoid defining your own AsFoo traits when you could just implement AsRef<Foo>.

Borrow and BorrowMut

The std::borrow::Borrow trait is similar to AsRef: if a type implements Borrow<T>, then its borrow method efficiently borrows a &T from it. But Borrow imposes more restrictions: a type should implement Borrow<T> only when a &T hashes and compares the same way as the value it’s borrowed from. (Rust doesn’t enforce this; it’s just the documented intent of the trait.) This makes Borrow valuable in dealing with keys in hash tables and trees, or when dealing with values that will be hashed or compared for some other reason.

This distinction matters when borrowing from Strings, for example: String implements AsRef<&str>, AsRef<[u8]>, and AsRef<Path>, but those three target types will generally have different hash values. Only the &str slice is guaranteed to hash like the equivalent String, so String implements only Borrow<str>.

Borrow’s definition is identical to that of AsRef; only the names have been changed:

trait Borrow<Borrowed: ?Sized> {
    fn borrow(&self) -> &Borrowed;
}

Borrow is designed to address a specific situation with generic hash tables and other associative collection types. For example, suppose you have a std::collections​::HashMap<String, i32>, mapping strings to numbers. This table’s keys are Strings; each entry owns one. What should the signature of the method that looks up an entry in this table be? Here’s a first attempt:

impl HashMap<K, V> where K: Eq + Hash
{
    fn get(&self, key: K) -> Option<&V> { ... }
}

This makes sense: to look up an entry, you must provide a key of the appropriate type for the table. But in this case, K is String; this signature would force you to pass a String by value to every call to get, which is clearly wasteful. You really just need a reference to the key:

impl HashMap<K, V> where K: Eq + Hash
{
    fn get(&self, key: &K) -> Option<&V> { ... }
}

This is slightly better, but now you have to pass the key as a &String, so if you wanted to look up a constant string, you’d have to write:

hashtable.get(&"twenty-two".to_string())

This is ridiculous: it allocates a String buffer on the heap and copies the text into it, just so it can borrow it as a &String, pass it to get, and then drop it.

It should be good enough to pass anything that can be hashed and compared with our key type; a &str should be perfectly adequate, for example. So here’s the final iteration, which is what you’ll find in the standard library:

impl HashMap<K, V> where K: Eq + Hash
{
    fn get<Q: ?Sized>(&self, key: &Q) -> Option<&V>
        where K: Borrow<Q>,
              Q: Eq + Hash
    { ... }
}

In other words, if you can borrow an entry’s key as a &Q, and the resulting reference hashes and compares just the way the key itself would, then clearly &Q ought to be an acceptable key type. Since String implements Borrow<str> and Borrow<String>, this final version of get allows you to pass either &String or &str as a key, as needed.

Vec<T> and [T: N] implement Borrow<[T]>. Every string-like type allows borrowing its corresponding slice type: String implements Borrow<str>, PathBuf implements Borrow<Path>, and so on. And all the standard library’s associative collection types use Borrow to decide which types can be passed to their lookup functions.

The standard library includes a blanket implementation so that every type T can be borrowed from itself: T: Borrow<T>. This ensures that &K is always an acceptable type for looking up entries in a HashMap<K, V>.

As a convenience, every &mut T type also implements Borrow<T>, returning a shared reference &T as usual. This allows you to pass mutable references to collection lookup functions without having to reborrow a shared reference, emulating Rust’s usual implicit coercion from mutable references to shared references.

The BorrowMut trait is the analogue of Borrow for mutable references:

trait BorrowMut<Borrowed: ?Sized>: Borrow<Borrowed> {
    fn borrow_mut(&mut self) -> &mut Borrowed;
}

The same expectations described for Borrow apply to BorrowMut as well.

From and Into

The std::convert::From and std::convert::Into traits represent conversions that consume a value of one type, and return a value of another. Whereas the AsRef and AsMut traits borrow a reference of one type from another, From and Into take ownership of their argument, transform it, and then return ownership of the result back to the caller.

Their definitions are nicely symmetrical:

trait Into<T>: Sized {
    fn into(self) -> T;
}

trait From<T>: Sized {
    fn from(T) -> Self;
}

The standard library automatically implements the trivial conversion from each type to itself: every type T implements From<T> and Into<T>.

Although the traits simply provide two ways to do the same thing, they lend themselves to different uses.

You generally use Into to make your functions more flexible in the arguments they accept. For example, if you write:

use std::net::Ipv4Addr;
fn ping<A>(address: A) -> std::io::Result<bool>
    where A: Into<Ipv4Addr>
{
    let ipv4_address = address.into();
    ...
}

then ping can accept not just an Ipv4Addr as an argument, but also a u32 or a [u8; 4] array, since those types both conveniently happen to implement Into<Ipv4Addr>. (It’s sometimes useful to treat an IPv4 address as a single 32-bit value, or an array of four bytes.) Because the only thing ping knows about address is that it implements Into<Ipv4Addr>, there’s no need to specify which type you want when you call into; there’s only one that could possibly work, so type inference fills it in for you.

As with AsRef in the previous section, the effect is much like that of overloading a function in C++. With the definition of ping from before, we can make any of these calls:

println!("{:?}", ping(Ipv4Addr::new(23, 21, 68, 141))); // pass an Ipv4Addr
println!("{:?}", ping([66, 146, 219, 98]));             // pass a [u8; 4]
println!("{:?}", ping(0xd076eb94_u32));                 // pass a u32

The From trait, however, plays a different role. The from method serves as a generic constructor for producing an instance of a type from some other single value. For example, rather than Ipv4Addr having two methods named from_array and from_u32, it simply implements From<[u8;4]> and From<u32>, allowing us to write:

let addr1 = Ipv4Addr::from([66, 146, 219, 98]);
let addr2 = Ipv4Addr::from(0xd076eb94_u32);

We can let type inference sort out which implementation applies.

Given an appropriate From implementation, the standard library automatically implements the corresponding Into trait. When you define your own type, if it has single-argument constructors, you should write them as implementations of From<T> for the appropriate types; you’ll get the corresponding Into implementations for free.

Because the from and into conversion methods take ownership of their arguments, a conversion can reuse the original value’s resources to construct the converted value. For example, suppose you write:

let text = "Beautiful Soup".to_string();
let bytes: Vec<u8> = text.into();

The implementation of Into<Vec<u8>> for String simply takes the String’s heap buffer and repurposes it, unchanged, as the returned vector’s element buffer. The conversion has no need to allocate or copy the text. This is another case where moves enable efficient implementations.

These conversions also provide a nice way to relax a value of a constrained type into something more flexible, without weakening the constrained type’s guarantees. For example, a String guarantees that its contents are always valid UTF-8; its mutating methods are carefully restricted to ensure that nothing you can do will ever introduce bad UTF-8. But this example efficiently “demotes” a String to a block of plain bytes that you can do anything you like with: perhaps you’re going to compress it, or combine it with other binary data that isn’t UTF-8. Because into takes its argument by value, text is no longer initialized after the conversion, meaning that we can freely access the former String’s buffer without being able to corrupt any extant String.

However, cheap conversions are not part of Into and From’s contract. Whereas AsRef and AsMut conversions are expected to be cheap, From and Into conversions may allocate, copy, or otherwise process the value’s contents. For example, String implements From<&str>, which copies the string slice into a new heap-allocated buffer for the String. And std::collections::BinaryHeap<T> implements From<Vec<T>>, which compares and reorders the elements according to its algorithm’s requirements.

Note that From and Into are restricted to conversions that never fail. The methods’ type signatures don’t provide any way to indicate that a given conversion didn’t work out. To provide fallible conversions into or out of your types, it’s best to have a function or method that returns a Result type.

Before From and Into were added to the standard library, Rust code was full of ad hoc conversion traits and construction methods, each specific to a single type. From and Into codify conventions that you can follow to make your types easier to use, since your users are already familiar with them.

ToOwned

Given a reference, the usual way to produce an owned copy of its referent is to call clone, assuming the type implements std::clone::Clone. But what if you want to clone a &str or a &[i32]? What you probably want is a String or a Vec<i32>, but Clone’s definition doesn’t permit that: by definition, cloning a &T must always return a value of type T, and str and [u8] are unsized; they aren’t even types that a function could return.

The std::borrow::ToOwned trait provides a slightly looser way to convert a reference to an owned value:

trait ToOwned {
    type Owned: Borrow<Self>;
    fn to_owned(&self) -> Self::Owned;
}

Unlike clone, which must return exactly Self, to_owned can return anything you could borrow a &Self from: the Owned type must implement Borrow<Self>. You can borrow a &[T] from a Vec<T>, so [T] can implement ToOwned<Owned=Vec<T>>, as long as T implements Clone, so that we can copy the slice’s elements into the vector. Similarly, str implements ToOwned<Owned=String>, Path implements ToOwned<Owned=PathBuf>, and so on.

Borrow and ToOwned at Work: The Humble Cow

Making good use of Rust involves thinking through questions of ownership, like whether a function should receive a parameter by reference or by value. Usually you can settle on one approach or the other, and the parameter’s type reflects your decision. But in some cases you cannot decide whether to borrow or own until the program is running; the std::borrow::Cow type (for “clone on write”) provides one way to do this.

Its definition is shown here:

enum Cow<'a, B: ?Sized + 'a>
    where B: ToOwned
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

A Cow<B> either borrows a shared reference to a B, or owns a value from which we could borrow such a reference. Since Cow implements Deref, you can call methods on it as if it were a shared reference to a B: if it’s Owned, it borrows a shared reference to the owned value; and if it’s Borrowed, it just hands out the reference it’s holding.

You can also get a mutable reference to a Cow’s value by calling its to_mut method, which returns a &mut B. If the Cow happens to be Cow::Borrowed, to_mut simply calls the reference’s to_owned method to get its own copy of the referent, changes the Cow into a Cow::Owned, and borrows a mutable reference to the newly owned value. This is the “clone on write” behavior the type’s name refers to.

Similarly, Cow has an into_owned method that promotes the reference to an owned value if necessary, and then returns it, moving ownership to the caller and consuming the Cow in the process.

One common use for Cow is to return either a statically allocated string constant or a computed string. For example, suppose you need to convert an error enum to a message. Most of the variants can be handled with fixed strings, but some of them have additional data that should be included in the message. You can return a Cow<'static, str>:

use std::path::PathBuf;
use std::borrow::Cow;
fn describe(error: &Error) -> Cow<'static, str> {
    match *error {
        Error::OutOfMemory => "out of memory".into(),
        Error::StackOverflow => "stack overflow".into(),
        Error::MachineOnFire => "machine on fire".into(),
        Error::Unfathomable => "machine bewildered".into(),
        Error::FileNotFound(ref path) => {
            format!("file not found: {}", path.display()).into()
        }
    }
}

This code uses Cow’s implementation of Into to construct the values. Most arms of this match statement return a Cow::Borrowed referring to a statically allocated string. But when we get a FileNotFound variant, we use format! to construct a message incorporating the given filename. This arm of the match statement produces a Cow::Owned value.

Callers of describe that don’t need to change the value can simply treat the Cow as a &str:

println!("Disaster has struck: {}", describe(&error));

Callers who do need an owned value can readily produce one:

let mut log: Vec<String> = Vec::new();
...
log.push(describe(&error).into_owned());

Using Cow helps describe and its callers put off allocation until the moment it becomes necessary.

Chapter 14. Closures

Save the environment! Create a closure today!

Cormac Flanagan

Sorting a vector of integers is easy.

integers.sort();

It is, therefore, a sad fact that when we want some data sorted, it’s hardly ever a vector of integers. We typically have records of some kind, and the built-in sort method typically does not work:

struct City {
    name: String,
    population: i64,
    country: String,
    ...
}

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort();  // error: how do you want them sorted?
}

Rust complains that City does not implement std::cmp::Ord. We need to specify the sort order, like this:

/// Helper function for sorting cities by population.
fn city_population_descending(city: &City) -> i64 {
    -city.population
}

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort_by_key(city_population_descending);  // ok
}

The helper function, city_population_descending, takes a City record and extracts the key, the field by which we want to sort our data. (It returns a negative number because sort arranges numbers in increasing order, and we want decreasing order: the most populous city first.) The sort_by_key method takes this key-function as a parameter.

This works fine, but it’s more concise to write the helper function as a closure, an anonymous function expression:

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort_by_key(|city| -city.population);
}

The closure here is |city| -city.population. It takes an argument city and returns -city.population. Rust infers the argument type and return type from how the closure is used.

Other examples of standard library features that accept closures include:

Iterator methods such as map and filter, for working with sequential data. We’ll cover these methods in Chapter 15.

Threading APIs like thread::spawn, which starts a new system thread. Concurrency is all about moving work to other threads, and closures conveniently represent units of work. We’ll cover these features in Chapter 19.

Some methods that conditionally need to compute a default value, like the or_insert_with method of HashMap entries. This method either gets or creates an entry in a HashMap, and it’s used when the default value is expensive to compute. The default value is passed in as a closure that is called only if a new entry must be created.

Of course, anonymous functions are everywhere these days, even in languages like Java, C#, Python, and C++ that didn’t originally have them. From now on we’ll assume you’ve seen anonymous functions before and focus on what makes Rust’s closures a little different. In this chapter, you’ll learn how to use closures with standard library methods, how a closure can “capture” variables in its scope, how to write your own functions and methods that take closures as arguments, and how to store closures for later use as callbacks. We’ll also explain how Rust closures work and why they’re faster than you might expect.

Capturing Variables

A closure can use data that belongs to an enclosing function. For example:

/// Sort by any of several different statistics.
fn sort_by_statistic(cities: &mut Vec<City>, stat: Statistic) {
    cities.sort_by_key(|city| -city.get_statistic(stat));
}

The closure here uses stat, which is owned by the enclosing function, sort_by_statistic. We say that the closure “captures” stat. This is one of the classic features of closures, so naturally, Rust supports it; but in Rust, this feature comes with a string attached.

In most languages with closures, garbage collection plays an important role. For example, consider this JavaScript code:

// Start an animation that rearranges the rows in a table of cities.
function startSortingAnimation(cities, stat) {
    // Helper function that we'll use to sort the table.
    // Note that this function refers to stat.
    function keyfn(city) {
        return city.get_statistic(stat);
    }

    if (pendingSort)
        pendingSort.cancel();

    // Now kick off an animation, passing keyfn to it.
    // The sorting algorithm will call keyfn later.
    pendingSort = new SortingAnimation(cities, keyfn);
}

The closure keyfn is stored in the new SortingAnimation object. It’s meant to be called after startSortingAnimation returns. Now, normally when a function returns, all its variables and arguments go out of scope and are discarded. But here, the JavaScript engine must keep stat around somehow, since the closure uses it. Most JavaScript engines do this by allocating stat in the heap and letting the garbage collector reclaim it later.

Rust doesn’t have garbage collection. How will this work? To answer this question, we’ll look at two examples.

Closures That Borrow

First, let’s repeat the opening example of this section:

fn sort_by_statistic(cities: &mut Vec<City>, stat: Statistic) {
    cities.sort_by_key(|city| -city.get_statistic(stat));
}

In this case, when Rust creates the closure, it automatically borrows a reference to stat. It stands to reason: the closure refers to stat, so it must have a reference to it.

The rest is simple. The closure is subject to the rules about borrowing and lifetimes that we described in Chapter 5. In particular, since the closure contains a reference to stat, Rust won’t let it outlive stat. Since the closure is only used during sorting, this example is fine.

In short, Rust ensures safety by using lifetimes instead of garbage collection. Rust’s way is faster: even a fast GC allocation will be slower than storing stat on the stack, as Rust does in this case.

Closures That Steal

The second example is trickier:

use std::thread;

fn start_sorting_thread(mut cities: Vec<City>, stat: Statistic)
    -> thread::JoinHandle<Vec<City>>
{
    let key_fn = |city: &City| -> i64 { -city.get_statistic(stat) };

    thread::spawn(|| {
        cities.sort_by_key(key_fn);
        cities
    })
}

This is a bit more like what our JavaScript example was doing: thread::spawn takes a closure and calls it in a new system thread. Note that || is the closure’s empty argument list.

The new thread runs in parallel with the caller. When the closure returns, the new thread exits. (The closure’s return value is sent back to the calling thread as a JoinHandle value. We’ll cover that in Chapter 19.)

Again, the closure key_fn contains a reference to stat. But this time, Rust can’t guarantee that the reference is used safely. Rust therefore rejects this program:

error[E0373]: closure may outlive the current function, but it borrows `stat`,
              which is owned by the current function
  --> closures_sort_thread.rs:33:18
   |
33 | let key_fn = |city: &City| -> i64 { -city.get_statistic(stat) };
   |              ^^^^^^^^^^^^^^^^^^^^                       ^^^^
   |              |                                      `stat` is borrowed here
   |              may outlive borrowed value `stat`

In fact, there are two problems here, because cities is shared unsafely as well. Quite simply, the new thread created by thread::spawn can’t be expected to finish its work before cities and stat are destroyed at the end of the function.

The solution to both problems is the same: tell Rust to move cities and stat into the closures that use them instead of borrowing references to them.

fn start_sorting_thread(mut cities: Vec<City>, stat: Statistic)
    -> thread::JoinHandle<Vec<City>>
{
    let key_fn = move |city: &City| -> i64 { -city.get_statistic(stat) };

    thread::spawn(move || {
        cities.sort_by_key(key_fn);
        cities
    })
}

The only thing we’ve changed is to add the move keyword before each of the two closures. The move keyword tells Rust that a closure doesn’t borrow the variables it uses: it steals them.

The first closure, key_fn, takes ownership of stat. Then the second closure takes ownership of both cities and key_fn.

Rust thus offers two ways for closures to get data from enclosing scopes: moves and borrowing. Really there is nothing more to say than that; closures follow the same rules about moves and borrowing that we already covered in Chapters 4 and 5. A few cases in point:

Just as everywhere else in the language, if a closure would move a value of a copyable type, like i32, it copies the value instead. So if Statistic happened to be a copyable type, we could keep using stat even after creating a move closure that uses it.

Values of noncopyable types, like Vec<City>, really are moved: the code above transfers cities to the new thread, by way of the move closure. Rust would not let us access cities by name after creating the closure.

As it happens, this code doesn’t need to use cities after the point where the closure moves it. If we did, though, the workaround would be easy: we could tell Rust to clone cities and store the copy in a different variable. The closure would only steal one of the copies—whichever one it refers to.

We get something important by accepting Rust’s strict rules: thread safety. It is precisely because the vector is moved, rather than being shared across threads, that we know the old thread won’t free the vector while the new thread is modifying it.

Function and Closure Types

Throughout this chapter, we’ve seen functions and closures used as values. Naturally, this means that they have types. For example:

fn city_population_descending(city: &City) -> i64 {
    -city.population
}

This function takes one argument (a &City) and returns an i64. It has the type fn(&City) -> i64.

You can do all the same things with functions that you do with other values. You can store them in variables. You can use all the usual Rust syntax to compute function values:

let my_key_fn: fn(&City) -> i64 =
    if user.prefs.by_population {
        city_population_descending
    } else {
        city_monster_attack_risk_descending
    };

cities.sort_by_key(my_key_fn);

Structs may have function-typed fields. Generic types like Vec can store scads of functions, as long as they all share the same fn type. And function values are tiny: a fn value is the memory address of the function’s machine code, just like a function pointer in C++.

A function can take another function as an argument. For example:

/// Given a list of cities and a test function,
/// return how many cities pass the test.
fn count_selected_cities(cities: &Vec<City>,
                         test_fn: fn(&City) -> bool) -> usize
{
    let mut count = 0;
    for city in cities {
        if test_fn(city) {
            count += 1;
        }
    }
    count
}

/// An example of a test function. Note that the type of
/// this function is `fn(&City) -> bool`, the same as
/// the `test_fn` argument to `count_selected_cities`.
fn has_monster_attacks(city: &City) -> bool {
    city.monster_attack_risk > 0.0
}

// How many cities are at risk for monster attack?
let n = count_selected_cities(&my_cities, has_monster_attacks);

If you’re familiar with function pointers in C/C++, you’ll see that Rust’s function values are exactly the same thing.

After all this, it may come as a surprise that closures do not have the same type as functions:

let limit = preferences.acceptable_monster_risk();
let n = count_selected_cities(
    &my_cities,
    |city| city.monster_attack_risk > limit);  // error: type mismatch

The second argument causes a type error. To support closures, we must change the type signature of this function. It needs to look like this:

fn count_selected_cities<F>(cities: &Vec<City>, test_fn: F) -> usize
    where F: Fn(&City) -> bool
{
    let mut count = 0;
    for city in cities {
        if test_fn(city) {
            count += 1;
        }
    }
    count
}

We have changed only the type signature of count_selected_cities, not the body. The new version is generic. It takes a test_fn of any type F as long as F implements the special trait Fn(&City) -> bool. This trait is automatically implemented by all functions and closures that take a single &City as an argument and return a Boolean value.

fn(&City) -> bool    // fn type (functions only)
Fn(&City) -> bool    // Fn trait (both functions and closures)

This special syntax is built into the language. The -> and return type are optional; if omitted, the return type is ().

The new version of count_selected_cities accepts either a function or a closure:

count_selected_cities(
    &my_cities,
    has_monster_attacks);  // ok

count_selected_cities(
    &my_cities,
    |city| city.monster_attack_risk > limit);  // also ok

Why didn’t our first attempt work? Well, a closure is callable, but it’s not a fn. The closure |city| city.monster_attack_risk > limit has its own type that’s not a fn type.

In fact, every closure you write has its own type, because a closure may contain data: values either borrowed or stolen from enclosing scopes. This could be any number of variables, in any combination of types. So every closure has an ad hoc type created by the compiler, large enough to hold that data. No two closures have exactly the same type. But every closure implements a Fn trait; the closure in our example implements Fn(&City) -> i64.

Since every closure has its own type, code that works with closures usually needs to be generic, like count_selected_cities. It’s a little clunky to spell out the generic types each time, but to see the advantages of this design, just read on.

Closure Performance

Rust’s closures are designed to be fast: faster than function pointers, fast enough that you can use them even in red-hot, performance-sensitive code. If you’re familiar with C++ lambdas, you’ll find that Rust closures are just as fast and compact, but safer.

In most languages, closures are allocated in the heap, dynamically dispatched, and garbage collected. So creating them, calling them, and collecting them each cost a tiny bit of extra CPU time. Worse, closures tend to rule out inlining, a key technique compilers use to eliminate function call overhead and enable a raft of other optimizations. All told, closures are slow enough in these languages that it can be worth manually removing them from tight inner loops.

Rust closures have none of these performance drawbacks. They’re not garbage collected. Like everything else in Rust, they aren’t allocated on the heap unless you put them in a Box, Vec, or other container. And since each closure has a distinct type, whenever the Rust compiler knows the type of the closure you’re calling, it can inline the code for that particular closure. This makes it OK to use closures in tight loops, and Rust programs often do so, enthusiastically, as you’ll see in Chapter 15.

Figure 14-1 shows how Rust closures are laid out in memory. At the top of the figure, we show a couple of local variables that our closures will refer to: a string food and a simple enum weather, whose numeric value happens to be 27.

Figure 14-1. Layout of closures in memory

Closure (a) uses both variables. Apparently we’re looking for cities that have both tacos and tornadoes. In memory, this closure looks like a small struct containing references to the variables it uses.

Note that it doesn’t contain a pointer to its code! That’s not necessary: as long as Rust knows the closure’s type, it knows which code to run when you call it.

Closure (b) is exactly the same, except it’s a move closure, so it contains values instead of references.

Closure (c) doesn’t use any variables from its environment. The struct is empty, so this closure does not take up any memory at all.

As the figure shows, these closures don’t take up much space. But even those few bytes are not always needed in practice. Often, the compiler can inline all calls to a closure, and then even the small structs shown in this figure are optimized away.

In “Callbacks”, we’ll show how to allocate closures in the heap and call them dynamically, using trait objects. That is a bit slower, but it is still as fast as any other trait object method.

Closures and Safety

The next few pages complete our explanation of how closures interact with Rust’s safety system. As we said earlier in this chapter, most of the story is simply that when a closure is created, it either moves or borrows the captured variables. But some of the consequences are not exactly obvious. In particular, we’ll be talking about what happens when a closure drops or modifies a captured value.

Closures That Kill

We have seen closures that borrow values and closures that steal them; it was only a matter of time before they went all the way bad.

Of course, kill is not really the right terminology. In Rust, we drop values. The most straightforward way to do it is to call drop():

let my_str = "hello".to_string();
let f = || drop(my_str);

When f is called, my_str is dropped.

So what happens if we call it twice?

f();
f();

Let’s think it through. The first time we call f, it drops my_str, which means the memory where the string is stored is freed, returned to the system. The second time we call f, the same thing happens. It’s a double free, a classic mistake in C++ programming that triggers undefined behavior.

Dropping a String twice would be an equally bad idea in Rust. Fortunately, Rust can’t be fooled so easily:

f();  // ok
f();  // error: use of moved value

Rust knows this closure can’t be called twice.

A closure that can only be called once may seem like a rather extraordinary thing. But we’ve been talking throughout this book about ownership and lifetimes. The idea of values being used up (that is, moved) is one of the core concepts in Rust. It works the same with closures as with everything else.

FnOnce

Let’s try once more to trick Rust into dropping a String twice. This time, we’ll use this generic function:

fn call_twice<F>(closure: F) where F: Fn() {
    closure();
    closure();
}

This generic function may be passed any closure that implements the trait Fn(): that is, closures that take no arguments and return (). (As with functions, the return type can be omitted if it’s (); Fn() is shorthand for Fn() -> ().)

Now what happens if we pass our unsafe closure to this generic function?

let my_str = "hello".to_string();
let f = || drop(my_str);
call_twice(f);

Again, the closure will drop my_str when it’s called. Calling it twice would be a double free. But again, Rust is not fooled:

error[E0525]: expected a closure that implements the `Fn` trait, but
              this closure only implements `FnOnce`
  --> closures_twice.rs:12:13
   |
12 |     let f = || drop(my_str);
   |             ^^^^^^^^^^^^^^^
   |
note: the requirement to implement `Fn` derives from here
  --> closures_twice.rs:13:5
   |
13 |     call_twice(f);
   |     ^^^^^^^^^^

This error message tells us more about how Rust handles “closures that kill.” They could have been banned from the language entirely, but cleanup closures are useful sometimes. So instead, Rust restricts their use. Closures that drop values, like f, are not allowed to have Fn. They are, quite literally, no Fn at all. They implement a less powerful trait, FnOnce, the trait of closures that can be called once.

The first time you call a FnOnce closure, the closure itself is used up. It’s as though the two traits, Fn and FnOnce, were defined like this:

// Pseudocode for `Fn` and `FnOnce` traits with no arguments.
trait Fn() -> R {
    fn call(&self) -> R;
}

trait FnOnce() -> R {
    fn call_once(self) -> R;
}

Just as an arithmetic expression like a + b is shorthand for a method call, Add::add(a, b), Rust treats closure() as shorthand for one of the two trait methods shown above. For a Fn closure, closure() expands to closure.call(). This method takes self by reference, so the closure is not moved. But if the closure is only safe to call once, then closure() expands to closure.call_once(). That method takes self by value, so the closure is used up.

Of course we’ve been deliberately stirring up trouble here by using drop(). In practice, you’ll mostly get into this situation by accident. It doesn’t happen often, but once in a great while you’ll write some closure code that unintentionally uses up a value:

let dict = produce_glossary();
let debug_dump_dict = || {
    for (key, value) in dict {  // oops!
        println!("{:?} - {:?}", key, value);
    }
};

Then, when you call debug_dump_dict() more than once, you’ll get an error message like this:

error[E0382]: use of moved value: `debug_dump_dict`
  --> closures_debug_dump_dict.rs:18:5
   |
17 |     debug_dump_dict();
   |     --------------- value moved here
18 |     debug_dump_dict();
   |     ^^^^^^^^^^^^^^^ value used here after move
   |
   = help: closure was moved because it only implements `FnOnce`

To debug this, we have to figure out why the closure is a FnOnce. Which value is being used up here? The only one we’re referring to at all is dict. Ah, there’s the bug: we’re using up dict by iterating over it directly. We should be looping over &dict rather than plain dict, to access the values by reference:

let debug_dump_dict = || {
    for (key, value) in &dict {  // does not use up dict
        println!("{:?} - {:?}", key, value);
    }
};

This fixes the error; the function is now a Fn and can be called any number of times.

FnMut

There is one more kind of closure, the kind that contains mutable data or mut references.

Rust considers non-mut values safe to share across threads. But it wouldn’t be safe to share non-mut closures that contain mut data: calling such a closure from multiple threads could lead to all sorts of race conditions as multiple threads try to read and write the same data at the same time.

Therefore, Rust has one more category of closure, FnMut, the category of closures that write. FnMut closures are called by mut reference, as if they were defined like this:

// Pseudocode for `Fn`, `FnMut`, and `FnOnce` traits.
trait Fn() -> R {
    fn call(&self) -> R;
}

trait FnMut() -> R {
    fn call_mut(&mut self) -> R;
}

trait FnOnce() -> R {
    fn call_once(self) -> R;
}

Any closure that requires mut access to a value, but doesn’t drop any values, is a FnMut closure. For example:

let mut i = 0;
let incr = || {
    i += 1;  // incr borrows a mut reference to i
    println!("Ding! i is now: {}", i);
};
call_twice(incr);

The way we wrote call_twice, it requires a Fn. Since incr is a FnMut and not a Fn, this code fails to compile. There’s an easy fix, though. To understand the fix, let’s take a step back and summarize what you’ve learned about the three categories of Rust closures.

Fn is the family of closures and functions that you can call multiple times without restriction. This highest category also includes all fn functions.

FnMut is the family of closures that can be called multiple times if the closure itself is declared mut.

FnOnce is the family of closures that can be called once, if the caller owns the closure.

Every Fn meets the requirements for FnMut, and every FnMut meets the requirements for FnOnce. As shown in Figure 14-2, they’re not three separate categories.

Figure 14-2. Venn diagram of the three closure categories

Instead, Fn() is a subtrait of FnMut(), which is a subtrait of FnOnce(). This makes Fn the most exclusive and most powerful category. FnMut and FnOnce are broader categories that include closures with usage restrictions.

Now that we’ve organized what we know, it’s clear that to accept the widest possible swath of closures, our call_twice function really ought to accept all FnMut closures, like this:

fn call_twice<F>(mut closure: F) where F: FnMut() {
    closure();
    closure();
}

The bound on the first line was F: Fn(), and now it’s F: FnMut(). With this change, we still accept all Fn closures, and we additionally can use call_twice on closures that mutate data:

let mut i = 0;
call_twice(|| i += 1);  // ok!
assert_eq!(i, 2);

Callbacks

A lot of libraries use callbacks as part of their API: functions provided by the user, for the library to call later. In fact, you’ve seen some APIs like that already in this book. Back in Chapter 2, we used the Iron framework to write a simple web server. It looked like this:

fn main() {
    let mut router = Router::new();

    router.get("/", get_form, "root");
    router.post("/gcd", post_gcd, "gcd");

    println!("Serving on http://localhost:3000...");
    Iron::new(router).http("localhost:3000").unwrap();
}

The purpose of the router is to route incoming requests from the Internet to the bit of Rust code that handles that particular kind of request. In this example, get_form and post_gcd were the names of some functions that we declared elsewhere in the program, using the fn keyword. But we could have passed closures instead, like this:

let mut router = Router::new();

router.get("/", |_: &mut Request| {
    Ok(get_form_response())
}, "root");
router.post("/gcd", |request: &mut Request| {
    let numbers = get_numbers(request)?;
    Ok(get_gcd_response(numbers))
}, "gcd");

This is because Iron was written to accept any thread-safe Fn as an argument.

How can we do that in our own programs? Let’s try writing our own very simple router from scratch, without using any code from Iron. We can begin by declaring a few types to represent HTTP requests and responses:

struct Request {
    method: String,
    url: String,
    headers: HashMap<String, String>,
    body: Vec<u8>
}

struct Response {
    code: u32,
    headers: HashMap<String, String>,
    body: Vec<u8>
}

Now the job of a router is simply to store a table that maps URLs to callbacks, so that the right callback can be called on demand. (For simplicity’s sake, we’ll only allow users to create routes that match a single exact URL.)

struct BasicRouter<C> where C: Fn(&Request) -> Response {
    routes: HashMap<String, C>
}

impl<C> BasicRouter<C> where C: Fn(&Request) -> Response {
    /// Create an empty router.
    fn new() -> BasicRouter<C> {
        BasicRouter { routes: HashMap::new() }
    }

    /// Add a route to the router.
    fn add_route(&mut self, url: &str, callback: C) {
        self.routes.insert(url.to_string(), callback);
    }
}

Unfortunately, we’ve made a mistake. Did you notice it?

This router works fine as long as we only add one route to it:

let mut router = BasicRouter::new();
router.add_route("/", |_| get_form_response());

This much compiles and runs. Unfortunately, if we add another route:

router.add_route("/gcd", |req| get_gcd_response(req));

then we get errors:

error[E0308]: mismatched types
  --> closures_bad_router.rs:41:30
   |
41 |     router.add_route("/gcd", |req| get_gcd_response(req));
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                              expected closure, found a different closure
   |
   = note: expected type `[closure@closures_bad_router.rs:40:27: 40:50]`
              found type `[closure@closures_bad_router.rs:41:30: 41:57]`
note: no two closures, even if identical, have the same type
help: consider boxing your closure and/or using it as a trait object

Our mistake was in how we defined the BasicRouter type:

struct BasicRouter<C> where C: Fn(&Request) -> Response {
    routes: HashMap<String, C>
}

We unwittingly declared that each BasicRouter has a single callback type C, and all the callbacks in the HashMap are of that type. Back in “Which to Use”, we showed a Salad type that had the same problem.

struct Salad<V: Vegetable> {
    veggies: Vec<V>
}

The solution here is the same as for the salad: since we want to support a variety of types, we need to use boxes and trait objects.

type BoxedCallback = Box<Fn(&Request) -> Response>;

struct BasicRouter {
    routes: HashMap<String, BoxedCallback>
}

Each box can contain a different type of closure, so a single HashMap can contain all sorts of callbacks. Note that the type parameter C is gone.

This requires a few adjustments to the methods:

impl BasicRouter {
    // Create an empty router.
    fn new() -> BasicRouter {
        BasicRouter { routes: HashMap::new() }
    }

    // Add a route to the router.
    fn add_route<C>(&mut self, url: &str, callback: C)
        where C: Fn(&Request) -> Response + 'static
    {
        self.routes.insert(url.to_string(), Box::new(callback));
    }
}

(Note the two bounds on C in the type signature for add_route: a particular Fn trait, and the 'static lifetime. Rust makes us add this 'static bound. Without it, the call to Box::new(callback) would be an error, because it’s not safe to store a closure if it contains borrowed references to variables that are about to go out of scope.)

Finally, our simple router is ready to handle incoming requests:

impl BasicRouter {
    fn handle_request(&self, request: &Request) -> Response {
        match self.routes.get(&request.url) {
            None => not_found_response(),
            Some(callback) => callback(request)
        }
    }
}

Using Closures Effectively

As we’ve seen, Rust’s closures are different from closures in most other languages. The biggest difference is that in languages with GC, you can use local variables in a closure without having to think about lifetimes or ownership. Without GC, things are different. Some design patterns that are commonplace in Java, C#, and JavaScript won’t work in Rust without changes.

For example, take the Model-View-Controller design pattern (MVC for short), illustrated in Figure 14-3. For every element of a user interface, an MVC framework creates three objects: a model representing that UI element’s state, a view that’s responsible for its appearance, and a controller that handles user interaction. Countless variations on MVC have been implemented over the years, but the general idea is that three objects divvy up the UI responsibilities somehow.

Here’s the problem. Typically, each object has a reference to one or both of the others, directly or through a callback, as shown in Figure 14-3. Whenever anything happens to one of the objects, it notifies the others, so everything updates promptly. The question of which object “owns” the others never comes up.

Figure 14-3. The Model-View-Controller design pattern

You can’t implement this pattern in Rust without making some changes. Ownership must be made explicit, and reference cycles must be eliminated. The model and the controller can’t have direct references to each other.

Rust’s radical wager is that good alternative designs exist. Sometimes you can fix a problem with closure ownership and lifetimes by having each closure receive the references it needs as arguments. Sometimes you can assign each thing in the system a number and pass around the numbers instead of references. Or you can implement one of the many variations on MVC where the objects don’t all have references to each other. Or model your toolkit after a non-MVC system with unidirectional data flow, like Facebook’s Flux architecture, shown in Figure 14-4.

Figure 14-4. The Flux architecture, an alternative to MVC

In short, if you try to use Rust closures to make a “sea of objects,” you’re going to have a hard time. But there are alternatives. In this case, it seems software engineering as a discipline is already gravitating to the alternatives anyway, because they’re simpler.

In the next chapter, we turn to a topic where closures really shine. We’ll be writing a kind of code that takes full advantage of the concision, speed, and efficiency of Rust closures and that’s fun to write, easy to read, and eminently practical. Up next: Rust iterators.

Chapter 15. Iterators

It was the end of a very long day.

Phil

An iterator is a value that produces a sequence of values, typically for a loop to operate on. Rust’s standard library provides iterators that traverse vectors, strings, hash tables, and other collections, but also iterators to produce lines of text from an input stream, connections arriving at a network server, values received from other threads over a communications channel, and so on. And of course, you can implement iterators for your own purposes. Rust’s for loop provides a natural syntax for using iterators, but iterators themselves also provide a rich set of methods for mapping, filtering, joining, collecting, and so on.

Rust’s iterators are flexible, expressive, and efficient. Consider the following function, which returns the sum of the first n positive integers (often called the nth triangle number):

fn triangle(n: i32) -> i32 {
    let mut sum = 0;
    for i in 1..n+1 {
        sum += i;
    }
    sum
}

The expression 1..n+1 is a Range<i32> value. A Range<i32> is an iterator that produces the integers from its start value (inclusive) to its end value (exclusive), so you can use it as the operand of the for loop to sum the values from 1 to n.

But iterators also have a fold method, which you can use in the equivalent definition:

fn triangle(n: i32) -> i32 {
    (1..n+1).fold(0, |sum, item| sum + item)
}

Starting with 0 as the running total, fold takes each value that 1..n+1 produces and applies the closure |sum, item| sum + item to the running total and the value. The closure’s return value is taken as the new running total. The last value it returns is what fold itself returns—in this case, the total of the entire sequence. This may look strange if you’re used to for and while loops, but once you’ve gotten used to it, fold is a legible and concise alternative.

This is pretty standard fare for functional programming languages, which put a premium on expressiveness. But Rust’s iterators were carefully designed to ensure that the compiler can translate them into excellent machine code as well. In a release build of the second definition shown before, Rust knows the definition of fold, and inlines it into triangle. Next, the closure |sum, item| sum + item is inlined into that. Finally, Rust examines the combined code and recognizes that there’s a simpler way to sum the numbers from one to n: the sum is always equal to n * (n+1) / 2. Rust translates the entire body of triangle, loop, closure, and all, into a single multiplication instruction and a few other bits of arithmetic.

This example happens to involve simple arithmetic, but iterators also perform well when put to heavier use. They’re another example of Rust providing flexible abstractions that impose little or no overhead in typical use.

The rest of this chapter falls into five parts:

First we’ll explain the Iterator and IntoIterator traits, which are the foundation of Rust’s iterators.

Then we’ll go over the three stages of a typical iterator pipeline: creating an iterator from some sort of value source; adapting one sort of iterator into another by selecting or processing values as they go by; and then consuming the values the iterator produces.

Finally, we’ll show how to implement iterators for your own types.

There are a lot of methods, so it’s fine to skim a section once you’ve got the general idea. But iterators are very common in idiomatic Rust, and being familiar with the tools that come with them is essential to mastering the language.

The Iterator and IntoIterator Traits

An iterator is any value that implements the std::iter::Iterator trait:

trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
    ... // many default methods
}

Item is the type of value the iterator produces. The next method either returns Some(v), where v is the iterator’s next value, or returns None to indicate the end of the sequence. Here we’ve omitted Iterator’s many default methods; we’ll cover them individually throughout the rest of this chapter.

If there’s a natural way to iterate over some type, it can implement std::iter::IntoIterator, whose into_iter method takes a value and returns an iterator over it:

trait IntoIterator where Self::IntoIter::Item == Self::Item {
    type Item;
    type IntoIter: Iterator;
    fn into_iter(self) -> Self::IntoIter;
}

IntoIter is the type of the iterator value itself, and Item is the type of value it produces. We call any type that implements IntoIterator an iterable, because it’s something you could iterate over if you asked.

Rust’s for loop brings all these parts together nicely. To iterate over a vector’s elements, you can write:

println!("There's:");
let v = vec!["antimony", "arsenic", "aluminum", "selenium"];

for element in &v {
    println!("{}", element);
}

Under the hood, every for loop is just shorthand for calls to IntoIterator and Iterator methods:

let mut iterator = (&v).into_iter();
while let Some(element) = iterator.next() {
    println!("{}", element);
}

The for loop uses IntoIterator::into_iter to convert its operand &v into an iterator, and then calls Iterator::next repeatedly. Each time that returns Some(element), the for loop executes its body; and if it returns None, the loop finishes.

Although a for loop always calls into_iter on its operand, you can also pass iterators to for loops directly; this occurs when you loop over a Range, for example. All iterators automatically implement IntoIterator, with an into_iter method that simply returns the iterator.

If you call an iterator’s next method again after it has returned None, the Iterator trait doesn’t specify what it should do. Most iterators will just return None again, but not all. (If this causes problems, the fuse adaptor covered in “fuse” can help.)

Here’s some terminology for iterators:

As we’ve said, an iterator is any type that implements Iterator.

An iterable is any type that implements IntoIterator: you can get an iterator over it by calling its into_iter method. The vector reference &v is the iterable in this case.

An iterator produces values.

The values an iterator produces are items. Here, the items are "antimony", "arsenic", and so on.

The code that receives the items an iterator produces is the consumer. In this example, the for loop consumes the iterator’s items.

Creating Iterators

The Rust standard library documentation explains in detail what sort of iterators each type provides, but the library follows some general conventions to help you get oriented and find what you need.

iter and iter_mut Methods

Most collection types provide iter and iter_mut methods that return the natural iterators over the type, producing a shared or mutable reference to each item. Slices like &[T] and &str have iter and iter_mut methods too. These methods are the most common way to get an iterator, if you’re not going to let a for loop take care of it for you:

let v = vec![4, 20, 12, 8, 6];
let mut iterator = v.iter();
assert_eq!(iterator.next(), Some(&4));
assert_eq!(iterator.next(), Some(&20));
assert_eq!(iterator.next(), Some(&12));
assert_eq!(iterator.next(), Some(&8));
assert_eq!(iterator.next(), Some(&6));
assert_eq!(iterator.next(), None);

This iterator’s item type is &i32: each call to next produces a reference to the next element, until we reach the end of the vector.

Each type is free to implement iter and iter_mut in whatever way makes the most sense for its purpose. The iter method on std::path::Path returns an iterator that produces one path component at a time:

use std::ffi::OsStr;
use std::path::Path;

let path = Path::new("C:/Users/JimB/Downloads/Fedora.iso");
let mut iterator = path.iter();
assert_eq!(iterator.next(), Some(OsStr::new("C:")));
assert_eq!(iterator.next(), Some(OsStr::new("Users")));
assert_eq!(iterator.next(), Some(OsStr::new("JimB")));
...

This iterator’s item type is &std::ffi::OsStr, a borrowed slice of a string of the sort accepted by operating system calls.

IntoIterator Implementations

When a type implements IntoIterator, you can call its into_iter method yourself, just as a for loop would:

// You should usually use HashSet, but its iteration order is
// nondeterministic, so BTreeSet works better in examples.
use std::collections::BTreeSet;
let mut favorites = BTreeSet::new();
favorites.insert("Lucy in the Sky With Diamonds".to_string());
favorites.insert("Liebesträume No. 3".to_string());

let mut it = favorites.into_iter();
assert_eq!(it.next(), Some("Liebesträume No. 3".to_string()));
assert_eq!(it.next(), Some("Lucy in the Sky With Diamonds".to_string()));
assert_eq!(it.next(), None);

Most collections actually provide several implementations of IntoIterator, for shared references, mutable references, and moves:

Given a shared reference to the collection, into_iter returns an iterator that produces shared references to its items. For example, in the preceding code, (&favorites).into_iter() would return an iterator whose Item type is &String.

Given a mutable reference to the collection, into_iter returns an iterator that produces mutable references to the items. For example, if vector is some Vec<String>, the call (&mut vector).into_iter() returns an iterator whose Item type is &mut String.

When passed the collection by value, into_iter returns an iterator that takes ownership of the collection and returns items by value; the items’ ownership moves from the collection to the consumer, and the original collection is consumed in the process. For example, the call favorites.into_iter() in the preceding code returns an iterator that produces each string by value; the consumer receives ownership of each string. When the iterator is dropped, any elements remaining in the BTreeSet are dropped too, and the set’s now-empty husk is disposed of.

Since a for loop applies IntoIterator::into_iter to its operand, these three implementations are what create the following idioms for iterating over shared or mutable references to a collection, or consuming the collection and taking ownership of its elements:

for element in &collection { ... }
for element in &mut collection { ... }
for element in collection { ... }

Each of these simply results in a call to one of the IntoIterator implementations listed here.

Not every type provides all three implementations. For example, HashSet, BTreeSet and BinaryHeap don’t implement IntoIterator on mutable references, since modifying their elements would probably violate the type’s invariants: the modified value might have a different hash value, or be ordered differently with respect to its neighbors, so modifying it would leave it incorrectly placed. Other types do support mutation, but only partially. For example, HashMap and BTreeMap produce mutable reference to their entries’ values, but only shared references to their keys, for similar reasons to those given earlier.

The general principle is that iteration should be efficient and predictable, so rather than providing implementations that are expensive or could exhibit surprising behavior (for example, rehashing modified HashSet entries, and potentially revisiting them later in the iteration), Rust omits them entirely.

Slices implement two of the three IntoIterator variants; since they don’t own their elements, there is no “by value” case. Instead, into_iter for &[T] and &mut [T] returns an iterator that produces shared and mutable references to the elements. If you imagine the underlying slice type [T] as a collection of some sort, this fits neatly into the overall pattern.

You may have noticed that the first two IntoIterator variants, for shared and mutable references, are equivalent to calling iter or iter_mut on the referent. Why does Rust provide both?

IntoIterator is what makes for loops work, so that’s obviously necessary. But when you’re not using a for loop, favorites.iter() is clearer than (&favorites).into_iter(). Iteration by shared reference is something you’ll need frequently, so iter and iter_mut are still valuable for their ergonomics.

IntoIterator can also be useful in generic code: you can use a bound like T: IntoIterator to restrict the type variable T to types that can be iterated over. Or, you can write T: IntoIterator<Item=U> to further require the iteration to produce a particular type U. For example, this function dumps values from any iterable whose items are printable with the "{:?}" format:

use std::fmt::Debug;

fn dump<T, U>(t: T)
    where T: IntoIterator<Item=U>,
          U: Debug
{
    for u in t {
        println!("{:?}", u);
    }
}

You can’t write this generic function using iter and iter_mut, since they’re not methods of any trait: most iterable types just happen to have methods by those names.

drain Methods

Many collection types provide a drain method that takes a mutable reference to the collection and returns an iterator that passes ownership of each element to the consumer. However, unlike the into_iter() method, which takes the collection by value and consumes it, drain merely borrows a mutable reference to the collection, and when the iterator is dropped, it removes any remaining elements from the collection, and leaves it empty.

On types that can be indexed by a range, like Strings, vectors, and VecDeques, the drain method takes a range of elements to remove, rather than draining the entire sequence:

use std::iter::FromIterator;

let mut outer = "Earth".to_string();
let inner = String::from_iter(outer.drain(1..4));

assert_eq!(outer, "Eh");
assert_eq!(inner, "art");

If you do need to drain the entire sequence, use the full range, .., as the argument.

Other Iterator Sources

The previous sections are mostly concerned with collection types like vectors and HashMap, but there are many other types in the standard library that support iteration. Table 15-1 summarizes the more interesting ones, but there are many more. We cover some of these methods in more detail in the chapters dedicated to the specific types (namely, Chapters 16, 17, and 18).

Table 15-1. Other iterators in the standard library

Type or trait
Expression
Notes

std::ops::Range
1..10
Endpoints must be an integer type to be iterable. Range includes start value, and excludes end value.

std::ops::RangeFrom
1..
Unbounded iteration. Start must be an integer. May panic or overflow if the value reaches the limit of the type.

Option<T>
Some(10).iter()
Behaves like a vector whose length is either 0 (None) or 1 (Some(v)).

Result<T, E>
Ok("blah").iter()
Similar to Option, producing Ok values.

Vec<T>, &[T]
v.windows(16)
Produces every contiguous slice of the given length, from left to right. The windows overlap.

v.chunks(16)
Produces nonoverlapping, contiguous slices of the given length, from left to right.

v.chunks_mut(1024)
Like chunks, but slices are mutable.

v.split(|byte| byte & 1 != 0)
Produces slices separated by elements that match the given predicate.

v.split_mut(...)
As above, but produces mutable slices.

v.rsplit(...)
Like split, but produces slices from right to left.

v.splitn(n, ...)
Like split, but produces at most n slices.

String, &str
s.bytes()
Produces the bytes of the UTF-8 form.

s.chars()
Produces the chars the UTF-8 represents.

s.split_whitespace()
Splits string by whitespace, and produces slices of nonspace characters.

s.lines()
Produces slices of the lines of the string.

s.split('/')
Splits string on a given pattern, producing the slices between matches. Patterns can be many things: characters, strings, closures.

s.matches(char::is_numeric)
Produces slices matching the given pattern.

std::collections::HashMap,
std::collections::BTreeMap
map.keys(),
map.values()
Produces shared references to keys or values of the map.

map.values_mut()
Produces mutable references to entries’ values.

std::collections::HashSet,
std::collections::BTreeSet
set1.union(set2)
Produces shared references to elements of union of set1 and set2.

set1.intersection(set2)
Produces shared references to elements of intersection of set1 and set2.

std::sync::mpsc::Receiver
recv.iter()
Produces values sent from another thread on the corresponding Sender.

std::io::Read
stream.bytes()
Produces bytes from an I/O stream.

stream.chars()
Parses stream as UTF-8 and produces chars.

std::io::BufRead
bufstream.lines()
Parses stream as UTF-8, produces lines as Strings.

bufstream.split(0)
Splits stream on given byte, produces inter-byte Vec<u8> buffers.

std::fs::ReadDir
std::fs::read_dir(path)
Produces directory entries.

std::net::TcpListener
listener.incoming()
Produces incoming network connections.

Free functions
std::iter::empty()
Returns None immediately.

std::iter::once(5)
Produces the given value, and then ends.

std::iter::repeat("#9")
Produces the given value forever.

Iterator Adapters

Once you have an iterator in hand, the Iterator trait provides a broad selection of adapter methods, or simply adapters, that consume one iterator and build a new one with useful behaviors. To see how adapters work, we’ll show how to use two of the most popular ones.

map and filter

The Iterator trait’s map adapter lets you transform an iterator by applying a closure to its items. The filter adapter lets you filter out items from an iterator, using a closure to decide which to keep and which to drop.

For example, suppose you’re iterating over lines of text, and want to omit leading and trailing whitespace from each line. The standard library’s str::trim method drops leading and trailing whitespace from a single &str, returning a new, trimmed &str that borrows from the original. You can use the map adapter to apply str::trim to each line from the iterator:

let text = "  ponies  \n   giraffes\niguanas  \nsquid".to_string();
let v: Vec<&str> = text.lines()
    .map(str::trim)
    .collect();
assert_eq!(v, ["ponies", "giraffes", "iguanas", "squid"]);

The text.lines() call returns an iterator that produces the string’s lines. Calling map on that iterator returns a second iterator that applies str::trim to each line, and produces the results as its items. Finally, collect gathers those items into a vector.

The iterator map returns is, of course, itself a candidate for further adaptation. If you want to exclude iguanas from the result, you can write the following:

let text = "  ponies  \n   giraffes\niguanas  \nsquid".to_string();
let v: Vec<&str> = text.lines()
    .map(str::trim)
    .filter(|s| *s != "iguanas")
    .collect();
assert_eq!(v, ["ponies", "giraffes", "squid"]);

Here, filter returns a third iterator that produces only those items from the map iterator for which the closure |s| *s != "iguanas" returns true. A chain of iterator adapters is like a pipeline in the Unix shell: each adapter has a single purpose, and it’s clear how the sequence is being transformed as one reads from left to right.

These adapters’ signatures are as follows:

fn map<B, F>(self, f: F) -> some Iterator<Item=B>
    where Self: Sized, F: FnMut(Self::Item) -> B;

fn filter<P>(self, predicate: P) -> some Iterator<Item=Self::Item>
    where Self: Sized, P: FnMut(&Self::Item) -> bool;

The some Iterator<...> notation we’re using for the return types is not valid Rust.¹ The real return types are opaque struct types, which aren’t informative; what matters in practice is that these methods return iterators with the given Item type.

Since most adapters take self by value, they require Self to be Sized (which all common iterators are).

A map iterator passes each item to its closure by value, and in turn, passes along ownership of the closure’s result to its consumer. A filter iterator passes each item to its closure by shared reference, retaining ownership in case the item is selected to be passed on to its consumer. This is why the example must dereference s to compare it with "iguanas": the filter iterator’s item type is &str, so the type of the closure’s argument s is &&str.

There are two important points to notice about iterator adapters.

First, simply calling an adapter on an iterator doesn’t consume any items; it just returns a new iterator, ready to produce its own items by drawing from the first iterator as needed. In a chain of adapters, the only way to make any work actually get done is to call next on the final iterator.

So in our earlier example, the method call text.lines() itself doesn’t actually parse any lines from the string; it just returns an iterator that would parse lines if asked. Similarly, map and filter just return new iterators that would map or filter if asked. No work takes place until collect starts calling next on the filter iterator.

This point is especially important if you use adapters that have side effects. For example, this code prints nothing at all:

["earth", "water", "air", "fire"]
    .iter().map(|elt| println!("{}", elt));

The iter call returns an iterator over the array’s elements, and the map call returns a second iterator that applies the closure to each value the first produces. But there is nothing here that ever actually demands a value from the whole chain, so no next method ever runs. In fact, Rust will warn you about this:

warning: unused result which must be used:
iterator adaptors are lazy and do nothing unless consumed
    |
387 | /         ["earth", "water", "air", "fire"]
388 | |             .iter().map(|elt| println!("{}", elt));
    | |___________________________________________________^
    |
= note: #[warn(unused_must_use)] on by default

The term “lazy” in the error message is not a disparaging term; it’s just jargon for any mechanism that puts off a computation until its value is needed. It is Rust’s convention that iterators should do the minimum work necessary to satisfy each call to next; In the example, there are no such calls at all, so no work takes place.

The second important point is that iterator adapters are a zero-overhead abstraction. Since map, filter, and their companions are generic, applying them to an iterator specializes their code for the specific iterator type involved. This means that Rust has enough information to inline each iterator’s next method into its consumer, and then translate the entire arrangement into machine code as a unit. So the lines/map/filter chain of iterators we showed before is as efficient as the code you would probably write by hand:

for line in text.lines() {
    let line = line.trim();
    if line != "iguanas" {
        v.push(line);
    }
}

The rest of this section covers the various adapters available on the Iterator trait.

filter_map and flat_map

The map adapter is fine in situations where each incoming item produces one outgoing item. But what if you want to delete certain items from the iteration instead of processing them, or replace single items with zero or more items? The filter_map and flat_map adapters grant you this flexibility.

The filter_map adapter is similar to map except that it lets its closure either transform the item into a new item (as map does) or drop the item from the iteration. Thus, it’s a bit like a combination of filter and map. Its signature is as follows:

fn filter_map<B, F>(self, f: F) -> some Iterator<Item=B>
    where Self: Sized, F: FnMut(Self::Item) -> Option<B>;

This is the same as map’s signature, except that here the closure returns Option<B>, not simply B. When the closure returns None, the item is dropped from the iteration; when it returns Some(b), then b is the next item the filter_map iterator produces.

For example, suppose you want to scan a string for whitespace-separated words that can be parsed as numbers, and process the numbers, dropping the other words. You can write:

use std::str::FromStr;

let text = "1\nfrond .25  289\n3.1415 estuary\n";
for number in text.split_whitespace()
                  .filter_map(|w| f64::from_str(w).ok()) {
    println!("{:4.2}", number.sqrt());
}

This prints the following:

1.00
0.50
17.00
1.77

The closure given to filter_map tries to parse each whitespace-separated slice using f64::from_str. That returns a Result<f64, ParseFloatError>, which .ok() turns into an Option<f64>: a parse error becomes None, whereas a successful parse result becomes Some(v). The filter_map iterator drops all the None values, and produces the value v for each Some(v).

But what’s the point in fusing map and filter into a single operation like this, instead of just using those adapters directly? The filter_map adapter shows its value in situations like the one just shown, when the best way to decide whether to include the item in the iteration is to actually try to process it. You can do the same thing with only filter and map, but it’s a bit ungainly:

text.split_whitespace()
    .map(|w| f64::from_str(w))
    .filter(|r| r.is_ok())
    .map(|r| r.unwrap())

You can think of the flat_map adapter as continuing in the same vein as map and filter_map, except that now the closure can return not just one item (as with map) or zero or one items (as with filter_map), but a sequence of any number of items. The flat_map iterator produces the concatenation of the sequences the closure returns.

The signature of flat_map is shown here:

fn flat_map<U, F>(self, f: F) -> some Iterator<Item=U::Item>
    where F: FnMut(Self::Item) -> U, U: IntoIterator;

The closure passed to flat_map must return an iterable, but any sort of iterable will do.²

For example, suppose we have a table mapping countries to their major cities. Given a list of countries, how can we iterate over their major cities?

use std::collections::HashMap;

let mut major_cities = HashMap::new();
major_cities.insert("Japan", vec!["Tokyo", "Kyoto"]);
major_cities.insert("The United States", vec!["Portland", "Nashville"]);
major_cities.insert("Brazil", vec!["São Paulo", "Brasília"]);
major_cities.insert("Kenya", vec!["Nairobi", "Mombasa"]);
major_cities.insert("The Netherlands", vec!["Amsterdam", "Utrecht"]);

let countries = ["Japan", "Brazil", "Kenya"];

for &city in countries.iter().flat_map(|country| &major_cities[country]) {
    println!("{}", city);
}

This prints the following:

Tokyo
Kyoto
São Paulo
Brasília
Nairobi
Mombasa

One way to look at this would be to say that, for each country, we retrieve the vector of its cities, concatenate all the vectors together into a single sequence, and print that.

But remember that iterators are lazy: it’s only the for loop’s calls to the flat_map iterator’s next method that cause work to be done. The full concatenated sequence is never constructed in memory. Instead, what we have here is a little state machine that draws from the city iterator, one item at a time, until it’s exhausted, and only then produces a new city iterator for the next country. The effect is that of a nested loop, but packaged up for use as an iterator.

scan

The scan adapter resembles map, except that the closure is given a mutable value it can consult, and has the option of terminating the iteration early. It takes an initial state value, and then a closure that accepts a mutable reference to the state, and the next item from the underlying iterator. The closure must return an Option, which the scan iterator takes as its next item.

For example, here’s an iterator chain that squares another iterator’s items, and terminates the iteration once their sum exceeds 10:

let iter = (0..10)
    .scan(0, |sum, item| {
        *sum += item;
        if *sum > 10 {
            None
        } else {
            Some(item * item)
        }
    });

assert_eq!(iter.collect::<Vec<i32>>(), vec![0, 1, 4, 9, 16]);

The closure’s sum argument is a mutable reference to a value private to the iterator and initialized to scan’s first argument—in this case, 0. The closure updates *sum, checks whether it has exceeded the limit, and returns the iterator’s next result.

take and take_while

The Iterator trait’s take and take_while adapters let you end an iteration after a certain number of items, or when a closure decides to cut things off. Their signatures are as follows:

fn take(self, n: usize) -> some Iterator<Item=Self::Item>
    where Self: Sized;

fn take_while<P>(self, predicate: P) -> some Iterator<Item=Self::Item>
    where Self: Sized, P: FnMut(&Self::Item) -> bool;

Both take ownership of an iterator and return a new iterator that passes along items from the first one, possibly ending the sequence earlier. The take iterator returns None after producing at most n items. The take_while iterator applies predicate to each item, and returns None in place of the first item for which predicate returns false, and on every subsequent call to next.

For example, given an email message with a blank line separating the headers from the message body, you can use take_while to iterate over only the headers:

let message = "To: jimb\r\n\
               From: superego <editor@oreilly.com>\r\n\
               \r\n\
               Did you get any writing done today?\r\n\
               When will you stop wasting time plotting fractals?\r\n";
for header in message.lines().take_while(|l| !l.is_empty()) {
    println!("{}" , header);
}

Recall from “String Literals” that when a line in a string ends with a backslash, Rust doesn’t include the indentation of the next line in the string, so none of the lines in the string have any leading whitespace. This means that the third line of message is blank. The take_while adapter terminates the iteration as soon as it sees that blank line, so this code prints only the two lines:

To: jimb
From: superego <editor@oreilly.com>

skip and skip_while

The Iterator trait’s skip and skip_while methods are the complement of take and take_while: they drop a certain number of items from the beginning of an iteration, or drop items until a closure finds one acceptable, and then pass the remaining items through unchanged. Their signatures are as follows:

fn skip(self, n: usize) -> some Iterator<Item=Self::Item>
    where Self: Sized;

fn skip_while<P>(self, predicate: P) -> some Iterator<Item=Self::Item>
    where Self: Sized, P: FnMut(&Self::Item) -> bool;

One common use for the skip adapter is to skip the command name when iterating over a program’s command-line arguments. In Chapter 2, our greatest common denominator calculator used the following code to loop over its command-line arguments:

for arg in std::env::args().skip(1) {
    ...
}

The std::env::args function returns an iterator that produces the program’s arguments as Strings, the first item being the name of the program itself. That’s not a string we want to process in this loop. Calling skip(1) on that iterator returns a new iterator that drops the program name the first time it’s called, and then produces all the subsequent arguments.

The skip_while adapter uses a closure to decide how many items to drop from the beginning of the sequence. You can iterate over the body lines of the message from the previous section like this:

for body in message.lines()
    .skip_while(|l| !l.is_empty())
    .skip(1) {
    println!("{}" , body);
}

This uses skip_while to skip nonblank lines, but that iterator does produce the blank line itself—after all, the closure returned false for that line. So we use the skip method as well to drop that, giving us an iterator whose first item will be the message body’s first line. Taken together with the declaration of message from the previous section, this code prints:

Did you get any writing done today?
When will you stop wasting time plotting fractals?

peekable

A peekable iterator lets you peek at the next item that will be produced without actually consuming it. You can turn almost any iterator into a peekable iterator by calling the Iterator trait’s peekable method:

fn peekable(self) -> std::iter::Peekable<Self>
    where Self: Sized;

Here, Peekable<Self> is a struct that implements Iterator<Item=Self::Item>, and Self is the type of the underlying iterator.

A Peekable iterator has an additional method peek that returns an Option<&Item>: None if the underlying iterator is done, and otherwise Some(r), where r is a shared reference to the next item. (Note that, if the iterator’s item type is already a reference to something, this ends up being a reference to a reference.)

Calling peek tries to draw the next item from the underlying iterator, and if there is one, caches it until the next call to next. All the other Iterator methods on Peekable know about this cache: for example, iter.last() on a peekable iterator iter knows to check the cache after exhausting the underlying iterator.

Peekable iterators are essential when you can’t decide how many items to consume from an iterator until you’ve gone too far. For example, if you’re parsing numbers from a stream of characters, you can’t decide where the number ends until you’ve seen the first non-number character following it:

use std::iter::Peekable;

fn parse_number<I>(tokens: &mut Peekable<I>) -> u32
    where I: Iterator<Item=char>
{
    let mut n = 0;
    loop {
        match tokens.peek() {
            Some(r) if r.is_digit(10) => {
                n = n * 10 + r.to_digit(10).unwrap();
            }
            _ => return n
        }
        tokens.next();
    }
}

let mut chars = "226153980,1766319049".chars().peekable();
assert_eq!(parse_number(&mut chars), 226153980);
// Look, `parse_number` didn't consume the comma! So we will.
assert_eq!(chars.next(), Some(','));
assert_eq!(parse_number(&mut chars), 1766319049);
assert_eq!(chars.next(), None);

The parse_number function uses peek to check the next character, and consumes it only if it is a digit. If it isn’t a digit or the iterator is exhausted (that is, if peek returns None), we return the number we’ve parsed and leave the next character in the iterator, ready to be consumed.

fuse

Once an Iterator has returned None, the trait doesn’t specify how it ought to behave if you call its next method again. Most iterators just return None again, but not all. If your code counts on that behavior, you may be in for a surprise.

The fuse adapter takes any iterator and turns into one that will definitely continue to return None once it has done so the first time:

struct Flaky(bool);

impl Iterator for Flaky {
    type Item = &'static str;
    fn next(&mut self) -> Option<Self::Item> {
        if self.0 {
            self.0 = false;
            Some("totally the last item")
        } else {
            self.0 = true; // D'oh!
            None
        }
    }
}

let mut flaky = Flaky(true);
assert_eq!(flaky.next(), Some("totally the last item"));
assert_eq!(flaky.next(), None);
assert_eq!(flaky.next(), Some("totally the last item"));

let mut not_flaky = Flaky(true).fuse();
assert_eq!(not_flaky.next(), Some("totally the last item"));
assert_eq!(not_flaky.next(), None);
assert_eq!(not_flaky.next(), None);

The fuse adapter is probably most useful in generic code that needs to work with iterators of uncertain origin. Rather than hoping that every iterator you’ll have to deal with will be well-behaved, you can use fuse to make sure.

Reversible Iterators and rev

Some iterators are able to draw items from both ends of the sequence. You can reverse such iterators by using the rev adapter. For example, an iterator over a vector could just as easily draw items from the end of the vector as from the start. Such iterators can implement the std::iter::DoubleEndedIterator trait, which extends Iterator:

trait DoubleEndedIterator: Iterator {
    fn next_back(&mut self) -> Option<Self::Item>;
}

You can think of a double-ended iterator as having two fingers marking the current front and back of the sequence. Drawing items from either end advances that finger toward the other; when the two meet, the iteration is done:

use std::iter::DoubleEndedIterator;

let bee_parts = ["head", "thorax", "abdomen"];

let mut iter = bee_parts.iter();
assert_eq!(iter.next(),      Some(&"head"));
assert_eq!(iter.next_back(), Some(&"abdomen"));
assert_eq!(iter.next(),      Some(&"thorax"));

assert_eq!(iter.next_back(), None);
assert_eq!(iter.next(),      None);

The structure of an iterator over a slice makes this behavior easy to implement: it is literally a pair of pointers to the start and end of the range of elements we haven’t yet produced; next and next_back simply draw an item from the one or the other. Iterators for ordered collections like BTreeSet and BTreeMap are double-ended too: their next_back method draws the greatest elements or entries first. In general, the standard library provides double-ended iteration whenever it’s practical.

But not all iterators can do this so easily: an iterator producing values from other threads arriving at a channel’s Receiver has no way to anticipate what the last value received might be. In general, you’ll need to check the standard library’s documentation to see which iterators implement DoubleEndedIterator and which don’t.

If an iterator is double-ended, you can reverse it with the rev adapter:

fn rev(self) -> some Iterator<Item=Self>
    where Self: Sized + DoubleEndedIterator;

The returned iterator is also double-ended: its next and next_back methods are simply exchanged:

let meals = ["breakfast", "lunch", "dinner"];

let mut iter = meals.iter().rev();
assert_eq!(iter.next(), Some(&"dinner"));
assert_eq!(iter.next(), Some(&"lunch"));
assert_eq!(iter.next(), Some(&"breakfast"));
assert_eq!(iter.next(), None);

Most iterator adapters, if applied to a reversible iterator, return another reversible iterator. For example, map and filter preserve reversibility.

inspect

The inspect adapter is handy for debugging pipelines of iterator adapters, but it isn’t used much in production code. It simply applies a closure to a shared reference to each item, and then passes the item through. The closure can’t affect the items, but it can do things like print them or make assertions about them.

This example shows a case in which converting a string to uppercase changes its length:

let upper_case: String = "große".chars()
    .inspect(|c| println!("before: {:?}", c))
    .flat_map(|c| c.to_uppercase())
    .inspect(|c| println!(" after:     {:?}", c))
    .collect();
assert_eq!(upper_case, "GROSSE");

The uppercase equivalent of the lowercase German letter “ß” is “SS”, which is why char::to_uppercase returns an iterator over characters, not a single replacement character. The preceding code uses flat_map to concatenate all the sequences that to_uppercase returns into a single String, printing the following as it does so:

before: 'g'
 after:     'G'
before: 'r'
 after:     'R'
before: 'o'
 after:     'O'
before: 'ß'
 after:     'S'
 after:     'S'
before: 'e'
 after:     'E'

chain

The chain adapter appends one iterator to another. More precisely, i1.chain(i2) returns an iterator that draws items from i1 until it’s exhausted, and then draws items from i2.

The chain adapter’s signature is as follows:

fn chain<U>(self, other: U) -> some Iterator<Item=Self::Item>
    where Self: Sized, U: IntoIterator<Item=Self::Item>;

In other words, you can chain an iterator together with any iterable that produces the same item type.

For example:

let v: Vec<i32> = (1..4).chain(vec![20, 30, 40]).collect();
assert_eq!(v, [1, 2, 3, 20, 30, 40]);

A chain iterator is reversible, if both of its underlying iterators are:

let v: Vec<i32> = (1..4).chain(vec![20, 30, 40]).rev().collect();
assert_eq!(v, [40, 30, 20, 3, 2, 1]);

A chain iterator keeps track of whether each of the two underlying iterators has returned None, and directs next and next_back calls to one or the other as appropriate.

enumerate

The Iterator trait’s enumerate adapter attaches a running index to the sequence, taking an iterator that produces items A, B, C, ... and returning an iterator that produces pairs (0, A), (1, B), (2, C), .... It looks trivial at first glance, but it’s used surprisingly often.

Consumers can use that index to distinguish one item from another, and establish the context in which to process each one. For example, the Mandelbrot set plotter in Chapter 2 splits the image into eight horizontal bands and assigns each one to a different thread. That code uses enumerate to tell each thread which portion of the image its band corresponds to.

Starting with a rectangular buffer of pixels:

let mut pixels = vec![0; columns * rows];

It uses chunks_mut to split the image into horizontal bands, one per thread:

let threads = 8;
let band_rows = rows / threads + 1;
...
let bands: Vec<&mut [u8]> = pixels.chunks_mut(band_rows * columns).collect();

And then it iterates over the bands, starting a thread for each one:

for (i, band) in bands.into_iter().enumerate() {
    let top = band_rows * i;
    // start a thread to render rows `top..top + band_rows`
}

Each iteration gets a pair (i, band), where band is the &mut [u8] slice of the pixel buffer the thread should draw into, and i is the index of that band in the overall image, courtesy of the enumerate adapter. Given the boundaries of the plot and the size of the bands, this is enough information for the thread to determine which portion of the image it has been assigned, and thus what to draw into band.

zip

The zip adapter combines two iterators into a single iterator that produces pairs holding one value from each iterator, like a zipper joining its two sides into a single seam. The zipped iterator ends when either of the two underlying iterators ends.

For example, you can get the same effect as the enumerate adapter by zipping the half-open range 0.. with the other iterator:

let v: Vec<_> = (0..).zip("ABCD".chars()).collect();
assert_eq!(v, vec![(0, 'A'), (1, 'B'), (2, 'C'), (3, 'D')]);

In this sense, you can think of zip as a generalization of enumerate: whereas enumerate attaches indices to the sequence, zip attaches any arbitrary iterator’s items. We suggested before that enumerate can help provide context for processing items; zip is a more flexible way to do the same.

The argument to zip doesn’t need to be an iterator itself; it can be any iterable:

use std::iter::repeat;

let endings = vec!["once", "twice", "chicken soup with rice"];
let rhyme: Vec<_> = repeat("going")
    .zip(endings)
    .collect();
assert_eq!(rhyme, vec![("going", "once"),
                       ("going", "twice"),
                       ("going", "chicken soup with rice")]);

by_ref

Throughout this section, we’ve been attaching adapters to iterators. Once you’ve done so, can you ever take the adapter off again? Usually, no: adapters take ownership of the underlying iterator, and provide no method to give it back.

An iterator’s by_ref method borrows a mutable reference to the iterator, so that you can apply adaptors to the reference. When you’re done consuming items from these adaptors, you drop them, the borrow ends, and you regain access to your original iterator.

For example, earlier in the chapter we showed how to use take_while and skip_while to process the header lines and body of a mail message. But what if you want to do both, using the same underlying iterator? Using by_ref, we can use take_while to handle the headers, and when that’s done, get the underlying iterator back, which take_while has left exactly in position to handle the message body:

let message = "To: jimb\r\n\
               From: id\r\n\
               \r\n\
               Oooooh, donuts!!\r\n";

let mut lines = message.lines();

println!("Headers:");
for header in lines.by_ref().take_while(|l| !l.is_empty()) {
    println!("{}" , header);
}

println!("\nBody:");
for body in lines {
    println!("{}" , body);
}

The call lines.by_ref() borrows a mutable reference to the iterator, and it is this reference that the take_while iterator takes ownership of. That iterator goes out of scope at the end of the first for loop, meaning that the borrow has ended, so you can use lines again in the second for loop. This prints the following:

Headers:
To: jimb
From: id

Body:
Oooooh, donuts!!

The by_ref adapter’s definition is trivial: it returns a mutable reference to the iterator. Then, the standard library includes this strange little implementation:

impl<'a, I: Iterator + ?Sized> Iterator for &'a mut I {
    type Item = I::Item;
    fn next(&mut self) -> Option<I::Item> {
        (**self).next()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
}

In other words, if I is some iterator type, then &mut I is an iterator too, whose next and size_hint methods defer to its referent. When you call an adapter on a mutable reference to an iterator, the adapter takes ownership of the reference, not the iterator itself. That’s just a borrow that ends when the adapter goes out of scope.

cloned

The cloned adapter takes an iterator that produces references, and returns an iterator that produces values cloned from those references. Naturally, the referent type must implement Clone. For example:

let a = ['1', '2', '3', '∞'];

assert_eq!(a.iter().next(),          Some(&'1'));
assert_eq!(a.iter().cloned().next(), Some('1'));

cycle

The cycle adapter returns an iterator that endlessly repeats the sequence produced by the underlying iterator. The underlying iterator must implement std::clone::Clone, so that cycle can save its initial state and reuse it each time the cycle starts again.

For example:

let dirs = ["North", "East", "South", "West"];
let mut spin = dirs.iter().cycle();
assert_eq!(spin.next(), Some(&"North"));
assert_eq!(spin.next(), Some(&"East"));
assert_eq!(spin.next(), Some(&"South"));
assert_eq!(spin.next(), Some(&"West"));
assert_eq!(spin.next(), Some(&"North"));
assert_eq!(spin.next(), Some(&"East"));

Or, for a really gratuitous use of iterators:

use std::iter::{once, repeat};

let fizzes = repeat("").take(2).chain(once("fizz")).cycle();
let buzzes = repeat("").take(4).chain(once("buzz")).cycle();
let fizzes_buzzes = fizzes.zip(buzzes);

let fizz_buzz = (1..100).zip(fizzes_buzzes)
    .map(|tuple|
         match tuple {
             (i, ("", "")) => i.to_string(),
             (_, (fizz, buzz)) => format!("{}{}", fizz, buzz)
         });

for line in fizz_buzz {
    println!("{}", line);
}

This plays a children’s word game, now sometimes used as a job interview question for coders, in which the players take turns counting, replacing any number divisible by three with the word “fizz”, and any number divisible by five with “buzz”. Numbers divisible by both become “fizzbuzz”.

Consuming Iterators

So far we’ve covered creating iterators, and adapting them into new iterators; here we finish off the process by showing ways to consume them.

Of course, you can consume an iterator with a for loop, or call next explicitly, but there are many common tasks that you shouldn’t have to write out again and again. The Iterator trait provides a broad selection of methods to cover many of these.

Simple Accumulation: count, sum, product

The count method draws items from an iterator until it returns None, and tells you how many it got. Here’s a short program that counts the number of lines on its standard input:

use std::io::prelude::*;

fn main() {
    let stdin = std::io::stdin();
    println!("{}", stdin.lock().lines().count());
}

The sum and product methods compute the sum or product of the iterator’s items, which must be integers or floating-point numbers:

fn triangle(n: u64) -> u64 {
    (1..n+1).sum()
}
assert_eq!(triangle(20), 210);

fn factorial(n: u64) -> u64 {
    (1..n+1).product()
}
assert_eq!(factorial(20), 2432902008176640000);

(You can extend sum and product to work with other types by implementing the std::iter::Sum and std::iter::Product traits, which we won’t describe in this book.)

max, min

The min and max methods on Iterator return the least or greatest item the iterator produces. The iterator’s item type must implement std::cmp::Ord, so that items can be compared with one another. For example:

assert_eq!([-2, 0, 1, 0, -2, -5].iter().max(), Some(&1));
assert_eq!([-2, 0, 1, 0, -2, -5].iter().min(), Some(&-5));

These methods return an Option<Self::Item>, so that they can return None if the iterator produces no items.

As explained in “Equality Tests”, Rust’s floating-point types f32 and f64 implement only std::cmp::PartialOrd, not std::cmp::Ord, so you can’t use the min and max methods to compute the least or greatest of a sequence of floating-point numbers. This is not a popular aspect of Rust’s design, but it is deliberate: it’s not clear what such functions should do with IEEE NaN values. Simply ignoring them would risk masking more serious problems in the code.

If you know how you would like to handle NaN values, you can use the max_by and min_by iterator methods instead, which let you supply your own comparison function.

max_by, min_by

The max_by and min_by methods return the maximum or minimum item the iterator produces, as determined by a comparison function you provide:

use std::cmp::{PartialOrd, Ordering};

// Compare two f64 values. Panic if given a NaN.
fn cmp(lhs: &&f64, rhs: &&f64) -> Ordering {
    lhs.partial_cmp(rhs).unwrap()
}

let numbers = [1.0, 4.0, 2.0];
assert_eq!(numbers.iter().max_by(cmp), Some(&4.0));
assert_eq!(numbers.iter().min_by(cmp), Some(&1.0));

let numbers = [1.0, 4.0, std::f64::NAN, 2.0];
assert_eq!(numbers.iter().max_by(cmp), Some(&4.0)); // panics

(The double references in cmp’s parameters arise because numbers.iter() produces references to the elements, and then max_by and min_by pass the closure references to the iterator’s items.)

max_by_key, min_by_key

The max_by_key and min_by_key methods on Iterator let you select the maximum or minimum item as determined by a closure applied to each item. The closure can select some field of the item, or perform a computation on the items. Since you’re often interested in data associated with some minimum or maximum, not just the extremum itself, these functions are often more useful than min and max. Their signatures are as follows:

fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
    where Self: Sized, F: FnMut(&Self::Item) -> B;

fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
    where Self: Sized, F: FnMut(&Self::Item) -> B;

That is, given a closure that takes an item and returns any ordered type B, return the item for which the closure returned the maximum or minimum B, or None if no items were produced.

For example, if you need to scan a hash table of cities to find the cities with the largest and smallest populations, you could write:

use std::collections::HashMap;

let mut populations = HashMap::new();
populations.insert("Portland",  583_776);
populations.insert("Fossil",        449);
populations.insert("Greenhorn",       2);
populations.insert("Boring",      7_762);
populations.insert("The Dalles", 15_340);

assert_eq!(populations.iter().max_by_key(|&(_name, pop)| pop),
           Some((&"Portland", &583_776)));
assert_eq!(populations.iter().min_by_key(|&(_name, pop)| pop),
           Some((&"Greenhorn", &2)));

The closure |&(_name, pop)| pop gets applied to each item the iterator produces, and returns the value to use for comparison—in this case, the city’s population. The value returned is the entire item, not just the value the closure returns. (Naturally, if you were making queries like this often, you’d probably want to arrange for a more efficient way to find the entries than making a linear search through the table.)

Comparing Item Sequences

You can use the < and == operators to compare strings, vectors, and slices, assuming their individual elements can be compared. Although iterators do not support Rust’s comparison operators, they do provide methods like eq and lt that do the same job, drawing pairs of items from the iterators and comparing them until a decision can be reached. For example:

let packed =  "Helen of Troy";
let spaced =  "Helen   of    Troy";
let obscure = "Helen of Sandusky"; // nice person, just not famous

assert!(packed != spaced);
assert!(packed.split_whitespace().eq(spaced.split_whitespace()));

// This is true because ' ' < 'o'.
assert!(spaced < obscure);

// This is true because 'Troy' > 'Sandusky'.
assert!(spaced.split_whitespace().gt(obscure.split_whitespace()));

The calls to split_whitespace return iterators over the whitespace-separated words of the string. Using the eq and gt methods on these iterators performs a word-by-word comparison, instead of a character-by-character comparison. These are all possible because &str implements PartialOrd and PartialEq.

Iterators provide the eq and ne methods for equality comparisons, and lt, le, gt, and ge methods for ordered comparisons. The cmp and partial_cmp methods behave like the corresponding methods of the Ord and PartialOrd traits.

any and all

The any and all methods apply a closure to each item the iterator produces, and return true if the closure returns true for any item, or for all the items:

let id = "Iterator";

assert!( id.chars().any(char::is_uppercase));
assert!(!id.chars().all(char::is_uppercase));

These methods consume only as many items as they need to determine the answer. For example, if the closure ever returns true for a given item, then any returns true immediately, without drawing any more items from the iterator.

position, rposition, and ExactSizeIterator

The position method applies a closure to each item from the iterator and returns the index of the first item for which the closure returns true. More precisely, it returns an Option of the index: if the closure returns true for no item, position returns None. It stops drawing items as soon as the closure returns true. For example:

let text = "Xerxes";
assert_eq!(text.chars().position(|c| c == 'e'), Some(1));
assert_eq!(text.chars().position(|c| c == 'z'), None);

The rposition method is the same, except that it searches from the right. For example:

let bytes = b"Xerxes";
assert_eq!(bytes.iter().rposition(|&c| c == b'e'), Some(4));
assert_eq!(bytes.iter().rposition(|&c| c == b'X'), Some(0));

The rposition method requires a reversible iterator, so that it can draw items from the right end of the sequence. It also requires an exact-size iterator, so that it can assign indices the same way position would, starting with 0 at the left. An exact-size iterator is one that implements the std::iter::ExactSizeIterator trait:

pub trait ExactSizeIterator: Iterator {
    fn len(&self) -> usize { ... }
    fn is_empty(&self) -> bool { ... }
}

The len method returns the number of items remaining, and the is_empty method returns true if iteration is complete.

Naturally, not every iterator knows how many items it will produce in advance; in the preceding examples, the chars iterator on &str does not (UTF-8 is a variable-width encoding), so you can’t use rposition on strings. But an iterator over an array of bytes certainly knows the array’s length, so it can implement ExactSizeIterator.

fold

The fold method is a very general tool for accumulating some sort of result over the entire sequence of items an iterator produces. Given an initial value, which we’ll call the accumulator, and a closure, fold repeatedly applies the closure to the current accumulator and the next item from the iterator. The value the closure returns is taken as the new accumulator, to be passed to the closure with the next item. The final accumulator value is what fold itself returns. If the sequence is empty, fold simply returns the initial accumulator.

Many of the other methods for consuming an iterator’s values can be written as uses of fold:

let a = [5, 6, 7, 8, 9, 10];

assert_eq!(a.iter().fold(0, |n, _| n+1), 6);        // count
assert_eq!(a.iter().fold(0, |n, i| n+i), 45);       // sum
assert_eq!(a.iter().fold(1, |n, i| n*i), 151200);   // product

// max
assert_eq!(a.iter().fold(i32::min_value(), |m, &i| std::cmp::max(m, i)),
           10);

The fold method’s signature is as follows:

fn fold<A, F>(self, init: A, f: F) -> A
    where Self: Sized, F: FnMut(A, Self::Item) -> A;

Here, A is the accumulator type. The init argument is an A, as is the closure’s first argument and return value, and the return value of fold itself.

Note that the accumulator values are moved into and out of the closure, so you can use fold with non-Copy accumulator types:

let a = ["Pack ", "my ", "box ", "with ",
         "five ", "dozen ", "liquor ", "jugs"];

let pangram = a.iter().fold(String::new(),
                            |mut s, &w| { s.push_str(w); s });
assert_eq!(pangram, "Pack my box with five dozen liquor jugs");

nth

The nth method takes an index n, skips that many items from the iterator, and returns the next item, or None if the sequence ends before that point. Calling .nth(0) is equivalent to .next().

It doesn’t take ownership of the iterator the way an adapter would, so you can call it many times.

let mut squares = (0..10).map(|i| i*i);

assert_eq!(squares.nth(4), Some(16));
assert_eq!(squares.nth(0), Some(25));
assert_eq!(squares.nth(6), None);

Its signature is shown here:

fn nth(&mut self, n: usize) -> Option<Self::Item>
    where Self: Sized;

last

The last method consumes items until the iterator returns None, and then returns the last item. If the iterator produces no items, then last returns None. Its signature is as follows:

fn last(self) -> Option<Self::Item>;

For example:

let squares = (0..10).map(|i| i*i);
assert_eq!(squares.last(), Some(81));

This consumes all the iterator’s items starting from the front, even if the iterator is reversible. If you have a reversible iterator and don’t need to consume all its items, you should instead just write iter.rev().next().

find

The find method draws items from an iterator, returning the first item for which the given closure returns true, or None if the sequence ends before a suitable item is found. Its signature is:

fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where Self: Sized,
          P: FnMut(&Self::Item) -> bool;

For example, using the table of cities and populations from “max_by_key, min_by_key”, you could write:

assert_eq!(populations.iter().find(|&(_name, &pop)| pop > 1_000_000),
           None);
assert_eq!(populations.iter().find(|&(_name, &pop)| pop > 500_000),
           Some((&"Portland", &583_776)));

None of the cities in the table have a population above a million, but there is one city with half a million people.

Building Collections: collect and FromIterator

Throughout the book, we’ve been using the collect method to build vectors holding an iterator’s items. For example, in Chapter 2, we called std::env::args() to get an iterator over the program’s command-line arguments, and then called that iterator’s collect method to gather them into a vector:

let args: Vec<String> = std::env::args().collect();

But collect isn’t specific to vectors: in fact, it can build any kind of collection from Rust’s standard library, as long as the iterator produces a suitable item type:

use std::collections::{HashSet, BTreeSet, LinkedList, HashMap, BTreeMap};

let args: HashSet<String> = std::env::args().collect();
let args: BTreeSet<String> = std::env::args().collect();
let args: LinkedList<String> = std::env::args().collect();

// Collecting a map requires (key, value) pairs, so for this example,
// zip the sequence of strings with a sequence of integers.
let args: HashMap<String, usize> = std::env::args().zip(0..).collect();
let args: BTreeMap<String, usize> = std::env::args().zip(0..).collect();

// and so on

Naturally, collect itself doesn’t know how to construct all these types. Rather, when some collection type like Vec or HashMap knows how to construct itself from an iterator, it implements the std::iter::FromIterator trait, for which collect is just a convenient veneer:

trait FromIterator<A>: Sized {
    fn from_iter<T: IntoIterator<Item=A>>(iter: T) -> Self;
}

If a collection type implements FromIterator<A>, then its static method from_iter builds a value of that type from an iterable producing items of type A.

In the simplest case, the implementation could simply construct an empty collection, and then add the items from the iterator one by one. For example, std::collections::LinkedList’s implementation of FromIterator works this way.

However, some types can do better than that. For example, constructing a vector from some iterator iter could be as simple as:

let mut vec = Vec::new();
for item in iter {
    vec.push(item)
}
vec

But this isn’t ideal: as the vector grows, it may need to expand its buffer, requiring a call to the heap allocator and a copy of the extant elements. Vectors do take algorithmic measures to keep this overhead low, but if there were some way to simply allocate an initial buffer of the right size to begin with, there would be no need to resize at all.

This is where the Iterator trait’s size_hint method comes in:

trait Iterator {
    ...
    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, None)
    }
}

This method returns a lower bound and optional upper bound on the number of items the iterator will produce. The default definition returns zero as the lower bound and declines to name an upper bound, saying, in effect, “I have no idea,” but many iterators can do better than this. An iterator over a Range, for example, knows exactly how many items it will produce, as does an iterator over a Vec or HashMap. Such iterators provide their own specialized definitions for size_hint.

These bounds are exactly the information that Vec’s implementation of FromIterator needs to size the new vector’s buffer correctly from the start. Insertions still check that the buffer is large enough, so even if the hint is incorrect, only performance is affected, not safety. Other types can take similar steps: for example, HashSet and HashMap also use Iterator::size_hint to choose an appropriate initial size for their hash table.

One note about type inference: at the top of this section, it’s a bit strange to see the same call, std::env::args().collect(), produce four different kinds of collections depending on its context. The return type of collect is its type parameter, so the first two calls are equivalent to the following:

let args = std::env::args().collect::<String>();
let args = std::env::args().collect::<HashSet<String>>();

But as long as there’s only one type that could possibly work as collect’s argument, Rust’s type inference will supply it for you. When you spell out the type of args, you ensure this is the case.

The Extend Trait

If a type implements the std::iter::Extend trait, then its extend method adds an iterable’s items to the collection:

let mut v: Vec<i32> = (0..5).map(|i| 1 << i).collect();
v.extend(&[31, 57, 99, 163]);
assert_eq!(v, &[1, 2, 4, 8, 16, 31, 57, 99, 163]);

All of the standard collections implement Extend, so they all have this method; so does String. Arrays and slices, which have a fixed length, do not.

The trait’s definition is as follows:

trait Extend<A> {
    fn extend<T>(&mut self, iter: T)
        where T: IntoIterator<Item=A>;
}

Obviously, this is very similar to std::iter::FromIterator: that creates a new collection, whereas Extend extends an existing collection. In fact, several implementations of FromIterator in the standard library simply create a new empty collection, and then call extend to populate it. For example, the implementation of FromIterator for std::collections::LinkedList works this way:

impl<T> FromIterator<T> for LinkedList<T> {
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
        let mut list = Self::new();
        list.extend(iter);
        list
    }
}

partition

The partition method divides an iterator’s items among two collections, using a closure to decide where each item belongs:

let things = ["doorknob", "mushroom", "noodle", "giraffe", "grapefruit"];

// Amazing fact: the name of a living thing always starts with an
// odd-numbered letter.
let (living, nonliving): (Vec<&str>, _)
    = things.iter().partition(|name| name.as_bytes()[0] & 1 != 0);

assert_eq!(living,    vec!["mushroom", "giraffe", "grapefruit"]);
assert_eq!(nonliving, vec!["doorknob", "noodle"]);

Like collect, partition can make any sort of collection you like (although both must be of the same type). And like collect, you’ll need to specify the return type: the preceding example writes out the type of living and nonliving, and lets type inference choose the right type parameters for the call.

The signature of partition is as follows:

fn partition<B, F>(self, f: F) -> (B, B)
    where Self: Sized,
          B: Default + Extend<Self::Item>,
          F: FnMut(&Self::Item) -> bool;

Whereas collect requires its result type to implement FromIterator, partition instead requires std::default::Default, which all Rust collections implement by returning an empty collection, and std::default::Extend.

Why doesn’t partition just split the iterator into two iterators, instead of building two collections? One reason is that items drawn from the underlying iterator but not yet drawn from the appropriate partitioned iterator would need to be buffered somewhere; you would end up building a collection of some sort internally, anyway. But the more fundamental reason has to do with safety. Partitioning one iterator into two would require the two partitions to share the same underlying iterator. Iterators must be mutable to be used; so the underlying iterator would necessarily be shared, mutable state, which Rust’s safety depends on avoiding.

Implementing Your Own Iterators

You can implement the IntoIterator and Iterator traits for your own types, making all the adapters and consumers shown in this chapter available for use, along with lots of other library and crate code written to work with the standard iterator interface. In this section, we’ll show a simple iterator over a range type, and then a more complex iterator over a binary tree type.

Suppose we have the following range type (simplified from the standard library’s std::ops::Range<T> type):

struct I32Range {
    start: i32,
    end: i32
}

Iterating over an I32Range requires two pieces of state: the current value, and the limit at which the iteration should end. This happens to be a nice fit for the I32Range type itself, using start as the next value, and end as the limit. So you can implement Iterator like so:

impl Iterator for I32Range {
    type Item = i32;
    fn next(&mut self) -> Option<i32> {
        if self.start >= self.end {
            return None;
        }
        let result = Some(self.start);
        self.start += 1;
        result
    }
}

This iterator produces i32 items, so that’s the Item type. If the iteration is complete, next returns None; otherwise, it produces the next value, and updates its state to prepare for the next call.

Of course, a for loop uses IntoIterator::into_iter to convert its operand into an iterator. But the standard library provides a blanket implementation of IntoIterator for every type that implements Iterator, so I32Range is ready for use:

let mut pi = 0.0;
let mut numerator = 1.0;

for k in (I32Range { start: 0, end: 14 }) {
    pi += numerator / (2*k + 1) as f64;
    numerator /= -3.0;
}
pi *= f64::sqrt(12.0);

// IEEE 754 specifies this result exactly.
assert_eq!(pi as f32, std::f32::consts::PI);

But I32Range is a special case, in that the iterable and iterator are the same type. Many cases aren’t so simple. For example, here’s the binary tree type from Chapter 10:

enum BinaryTree<T> {
    Empty,
    NonEmpty(Box<TreeNode<T>>)
}

struct TreeNode<T> {
    element: T,
    left: BinaryTree<T>,
    right: BinaryTree<T>
}

The classic way to walk a binary tree is to recurse, using the stack of function calls to keep track of your place in the tree and the nodes yet to be visited. But when implementing Iterator for BinaryTree<T>, each call to next must produce exactly one value and return. To keep track of the tree nodes it has yet to produce, the iterator must maintain its own stack. Here’s one possible iterator type for BinaryTree:

use self::BinaryTree::*;

// The state of an in-order traversal of a `BinaryTree`.
struct TreeIter<'a, T: 'a> {
    // A stack of references to tree nodes. Since we use `Vec`'s
    // `push` and `pop` methods, the top of the stack is the end of the
    // vector.
    //
    // The node the iterator will visit next is at the top of the stack,
    // with those ancestors still unvisited below it. If the stack is empty,
    // the iteration is over.
    unvisited: Vec<&'a TreeNode<T>>
}

It turns out that pushing the nodes running down the left edge of a subtree is a common operation, so we’ll define a method on TreeIter to do that:

impl<'a, T: 'a> TreeIter<'a, T> {
    fn push_left_edge(&mut self, mut tree: &'a BinaryTree<T>) {
        while let NonEmpty(ref node) = *tree {
            self.unvisited.push(node);
            tree = &node.left;
        }
    }
}

With this helper method in place, we can give BinaryTree an iter method that returns an iterator over the tree:

impl<T> BinaryTree<T> {
    fn iter(&self) -> TreeIter<T> {
        let mut iter = TreeIter { unvisited: Vec::new() };
        iter.push_left_edge(self);
        iter
    }
}

The iter method constructs an empty TreeIter, and then calls push_left_edge to set up the initial stack. The leftmost node ends up on the top, as required by the unvisited stack’s rules.

Following the standard library’s practices, we can then implement IntoIterator on a shared reference to a tree with a call to BinaryTree::iter:

impl<'a, T: 'a> IntoIterator for &'a BinaryTree<T> {
    type Item = &'a T;
    type IntoIter = TreeIter<'a, T>;
    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

The IntoIter definition establishes TreeIter as the iterator type for a &BinaryTree.

Finally, in the Iterator implementation, we get to actually walk the tree. Like BinaryTree’s iter method, the iterator’s next method is guided by the stack’s rules:

impl<'a, T> Iterator for TreeIter<'a, T> {
    type Item = &'a T;
    fn next(&mut self) -> Option<&'a T> {
        // Find the node this iteration must produce,
        // or finish the iteration.
        let node = match self.unvisited.pop() {
            None => return None,
            Some(n) => n
        };

        // The next node after this one is the leftmost child of
        // this node's right child, so push the path from here down.
        self.push_left_edge(&node.right);

        // Produce a reference to this node's value.
        Some(&node.element)
    }
}

If the stack is empty, the iteration is complete. Otherwise, node is a reference to the node to visit now; this call will return a reference to its element field. But first, we must advance the iterator’s state to the next node. If this node has a right subtree, the next node to visit is the subtree’s leftmost node, and we can use push_left_edge to push it, and its unvisited ancestors, onto the stack. But if this node has no right subtree, push_left_edge has no effect, which is just what we want: we can count on the new top of the stack to be node’s first unvisited ancestor, if any.

With IntoIterator and Iterator implementations in place, we can finally use a for loop to iterate over a BinaryTree by reference:

fn make_node<T>(left: BinaryTree<T>, element: T, right: BinaryTree<T>)
           -> BinaryTree<T>
{
    NonEmpty(Box::new(TreeNode { left, element, right }))
}

// Build a small tree.
let subtree_l = make_node(Empty, "mecha", Empty);
let subtree_rl = make_node(Empty, "droid", Empty);
let subtree_r = make_node(subtree_rl, "robot", Empty);
let tree = make_node(subtree_l, "Jaeger", subtree_r);

// Iterate over it.
let mut v = Vec::new();
for kind in &tree {
    v.push(*kind);
}
assert_eq!(v, ["mecha", "Jaeger", "droid", "robot"]);

Figure 15-1 shows how the unvisited stack behaves as we iterate through a sample tree. At every step, the next node to be visited is at the top of the stack, with all its unvisited ancestors below it.

Figure 15-1. Iterating over a binary tree

All the usual iterator adapters and consumers are ready for use on our trees:

assert_eq!(tree.iter()
           .map(|name| format!("mega-{}", name))
           .collect::<Vec<_>>(),
           vec!["mega-mecha", "mega-Jaeger",
                "mega-droid", "mega-robot"]);

¹ Rust RFC 1522 will add syntax to the language very much like our some Iterator notation. As of Rust 1.17, it is not yet included in the language by default.

² In fact, since Option is an iterable behaving like a sequence of zero or one items, iterator.filter_map​(closure) is equivalent to iterator.flat_map(closure), assuming closure returns an Option<T>.

Chapter 16. Collections

We all behave like Maxwell’s demon. Organisms organize. In everyday experience lies the reason sober physicists across two centuries kept this cartoon fantasy alive. We sort the mail, build sand castles, solve jigsaw puzzles, separate wheat from chaff, rearrange chess pieces, collect stamps, alphabetize books, create symmetry, compose sonnets and sonatas, and put our rooms in order, and all this we do requires no great energy, as long as we can apply intelligence.

James Gleick, The Information: A History, a Theory, a Flood

The Rust standard library contains several collections, generic types for storing data in memory. We’ve already been using collections, such as Vec and HashMap, throughout this book. In this chapter, we’ll cover the methods of these two types in detail, along with the other half-dozen standard collections. Before we begin, let’s address a few systematic differences between Rust’s collections and those in other languages.

First, moves and borrowing are everywhere. Rust uses moves to avoid deep-copying values. That’s why the method Vec<T>::push(item) takes its argument by value, not by reference. The value is moved into the vector. The diagrams in Chapter 4 show how this works out in practice: pushing a Rust String to a Vec<String> is quick, because Rust doesn’t have to copy the string’s character data, and ownership of the string is always clear.

Second, Rust doesn’t have invalidation errors—the kind of dangling-pointer bug where a collection is resized, or otherwise changed, while the program is holding a pointer to data inside it. Invalidation errors are another source of undefined behavior in C++, and they cause the occasional ConcurrentModificationException even in memory-safe languages. Rust’s borrow checker rules them out at compile time.

Finally, Rust does not have null, so we’ll see Options in places where other languages would use null.

Apart from these differences, Rust’s collections are about what you’d expect. If you’re an experienced programmer in a hurry, you can skim here, but don’t miss “Entries”.

Overview

Table 16-1 shows Rust’s eight standard collections. All of them are generic types.

Table 16-1. Summary of the standard collections

Collection
Description
Similar collection type in...

C++
Java
Python

Vec<T>
Growable array
vector
ArrayList
list

VecDeque<T>
Double-ended queue
(growable ring buffer)
deque
ArrayDeque
collections​.deque

LinkedList<T>
Doubly linked list
list
LinkedList
—

BinaryHeap<T>
where T: Ord
Max heap
priority_queue
PriorityQueue
heapq

HashMap<K, V>
where K: Eq + Hash
Key-value hash table
unordered_map
HashMap
dict

BTreeMap<K, V>
where K: Ord
Sorted key-value table
map
TreeMap
—

HashSet<T>
where T: Eq + Hash
Hash table
unordered_set
HashSet
set

BTreeSet<T>
where T: Ord
Sorted set
set
TreeSet
—

Vec<T>, HashMap<K, V>, and HashSet<T> are the most generally useful collection types. The rest have niche uses. This chapter discusses each collection type in turn:

Vec<T> is a growable, heap-allocated array of values of type T. About half of this chapter is dedicated to Vec and its many useful methods.

VecDeque<T> is like Vec<T>, but better for use as a first-in-first-out queue. It supports efficiently adding and removing values at the front of the list as well as the back. This comes at the cost of making all other operations slightly slower.

LinkedList<T> supports fast access to the front and back of the list, like VecDeque<T>, and adds fast concatenation. However, in general, LinkedList<T> is slower than Vec<T> and VecDeque<T>.

BinaryHeap<T> is a priority queue. The values in a BinaryHeap are organized so that it’s always efficient to find and remove the maximum value.

HashMap<K, V> is a table of key-value pairs. Looking up a value by its key is fast. The entries are stored in an arbitrary order.

BTreeMap<K, V> is like HashMap<K, V>, but it keeps the entries sorted by key. A BTreeMap<String, i32> stores its entries in String comparison order. Unless you need the entries to stay sorted, a HashMap is faster.

HashSet<T> is a set of values of type T. Adding and removing values is fast, and it’s fast to ask whether a given value is in the set or not.

BTreeSet<T> is like HashSet<T>, but it keeps the elements sorted by value. Again, unless you need the data sorted, a HashSet is faster.

Vec<T>

We’ll assume some familiarity with Vec, since we’ve been using it throughout the book. For an introduction, see “Vectors”. Here we’ll finally describe its methods and its inner workings in depth.

The easiest way to create a vector is to use the vec! macro:

// Create an empty vector
let mut numbers: Vec<i32> = vec![];

// Create a vector with given contents
let words = vec!["step", "on", "no", "pets"];
let mut buffer = vec![0u8; 1024];  // 1024 zeroed-out bytes

As described in Chapter 4, a vector has three fields: the length, the capacity, and a pointer to a heap allocation where the elements are stored. Figure 16-1 shows how the vectors created above would appear in memory. The empty vector, numbers, initially has a capacity of 0. No heap memory is allocated for it until the first element is added.

Like all collections, Vec implements std::iter::FromIterator, so you can create a vector from any iterator using the iterator’s .collect() method, as described in “Building Collections: collect and FromIterator”:

// Convert another collection to a vector.
let my_vec = my_set.into_iter().collect::<Vec<String>>();

Figure 16-1. Vector layout in memory. Each element of words is a &str value consisting of a pointer and a length.

Accessing Elements

Getting elements of an array, slice, or vector by index is straightforward:

// Get a reference to an element
let first_line = &lines[0];

// Get a copy of an element
let fifth_number = numbers[4];       // requires Copy
let second_line = lines[1].clone();  // requires Clone

// Get a reference to a slice
let my_ref = &buffer[4..12];

// Get a copy of a slice
let my_copy = buffer[4..12].to_vec();  // requires Clone

All of these forms panic if an index is out of bounds.

Rust is picky about numeric types, and it makes no exceptions for vectors. Vector lengths and indices are of type usize. Trying to use a u32, u64, or isize as a vector index is an error. You can use an n as usize cast to convert as needed; see “Type Casts”.

Several methods provide easy access to particular elements of a vector or slice (note that all slice methods are available on arrays and vectors too):

slice.first() returns a reference to the first element of slice, if any.

The return type is Option<&T>, so the return value is None if slice is empty and Some(&slice[0]) if it’s not empty.

if let Some(item) = v.first() {
    println!("We got one! {}", item);
}

slice.last() is similar but returns a reference to the last element.

slice.get(index) returns Some reference to slice[index], if it exists. If slice has fewer than index+1 elements, this returns None.

let slice = [0, 1, 2, 3];
assert_eq!(slice.get(2), Some(&2));
assert_eq!(slice.get(4), None);

slice.first_mut(), slice.last_mut(), and slice.get_mut(index) are variations of these that borrow mut references.

let mut slice = [0, 1, 2, 3];
{
    let last = slice.last_mut().unwrap();   // type of last: &mut i32
    assert_eq!(*last, 3);
    *last = 100;
}
assert_eq!(slice, [0, 1, 2, 100]);

Because returning a T by value would mean moving it, methods that access elements in place typically return those elements by reference.

An exception is the .to_vec() method, which makes copies:

slice.to_vec() clones a whole slice, returning a new vector.

let v = [1, 2, 3, 4, 5, 6, 7, 8, 9];
assert_eq!(v.to_vec(),
           vec![1, 2, 3, 4, 5, 6, 7, 8, 9]);
assert_eq!(v[0..6].to_vec(),
           vec![1, 2, 3, 4, 5, 6]);

This method is available only if the elements are cloneable, that is, where T: Clone.

Iteration

Vectors and slices are iterable, either by value or by reference, following the pattern described in “IntoIterator Implementations”:

Iterating over a Vec<T> produces items of type T. The elements are moved out of the vector one by one, consuming it.

Iterating over a value of type &[T; N], &[T], or &Vec<T>—that is, a reference to an array, slice, or vector—produces items of type &T, references to the individual elements, which are not moved.

Iterating over a value of type &mut [T; N], &mut [T], or &mut Vec<T> produces items of type &mut T.

Arrays, slices, and vectors also have .iter() and .iter_mut() methods (described in “iter and iter_mut Methods”) for creating iterators that produce references to their elements.

We’ll cover some fancier ways to iterate over a slice in “Splitting”.

Growing and Shrinking Vectors

The length of an array, slice, or vector is the number of elements it contains.

slice.len() returns a slice’s length, as a usize.

slice.is_empty() is true if slice contains no elements (that is, slice.len() == 0).

The remaining methods in this section are about growing and shrinking vectors. They are not present on arrays and slices, which can’t be resized once created.

All of a vector’s elements are stored in a contiguous, heap-allocated chunk of memory. The capacity of a vector is the maximum number of elements that would fit in this chunk. Vec normally manages the capacity for you, automatically allocating a larger buffer and moving the elements into it when more space is needed. There are also a few methods for managing capacity explicitly:

Vec::with_capacity(n) creates a new, empty vector with capacity n.

vec.capacity() returns vec’s capacity, as a usize. It’s always true that vec.capacity() >= vec.len().

vec.reserve(n) makes sure the vector has at least enough spare capacity for n more elements: that is, vec.capacity() is at least vec.len() + n. If there’s already enough room, this does nothing. If not, this allocates a larger buffer and moves the vector’s contents into it.

vec.reserve_exact(n) is like vec.reserve(n), but tells vec not to allocate any extra capacity for future growth, beyond n. Afterward, vec.capacity() is exactly vec.len() + n.

vec.shrink_to_fit() tries to free up the extra memory if vec.capacity() is greater than vec.len().

Vec<T> has many methods that add or remove elements, changing the vector’s length. Each of these takes its self argument by mut reference.

These two methods add or remove a single value at the end of a vector:

vec.push(value) adds the given value to the end of vec.

vec.pop() removes and returns the last element. The return type is Option<T>. This returns Some(x) if the popped element is x and None if the vector was already empty.

Note that .push() takes its argument by value, not by reference. Likewise, .pop() returns the popped value, not a reference. The same is true of most of the remaining methods in this section. They move values in and out of vectors.

These two methods add or remove a value anywhere in a vector:

vec.insert(index, value) inserts the given value at vec[index], sliding any existing values in vec[index..] one spot to the right to make room.

Panics if index > vec.len().

vec.remove(index) removes and returns vec[index], sliding any existing values in vec[index+1..] one spot to the left to close the gap.

Panics if index >= vec.len(), since in that case there is no element vec[index] to remove.

The longer the vector, the slower this operation gets. If you find yourself doing vec.remove(0) a lot, consider using a VecDeque (explained in “VecDeque<T>”) instead of a Vec.

Both .insert() and .remove() are slower the more elements have to be shifted.

Three methods change the length of a vector to a specific value:

vec.resize(new_len, value) sets vec’s length to new_len. If this increases vec’s length, copies of value are added to fill the new space. The element type must implement the Clone trait.

vec.truncate(new_len) reduces the length of vec to new_len, dropping any elements that were in the range vec[new_len..].

If vec.len() is already less than or equal to new_len, nothing happens.

vec.clear() removes all elements from vec. It’s the same as vec.truncate(0).

Four methods add or remove many values at once:

vec.extend(iterable) adds all items from the given iterable value at the end of vec, in order. It’s like a multivalue version of .push(). The iterable argument can be anything that implements IntoIterator<Item=T>.

This method is so useful that there’s a standard trait for it, the Extend trait, which all standard collections implement. Unfortunately, this causes rustdoc to lump .extend() with other trait methods in a big pile at the bottom of the generated HTML, so it’s hard to find when you need it. You just have to remember it’s there! See “The Extend Trait” for more.

vec.split_off(index) is like vec.truncate(index), except that it returns a Vec<T> containing the values removed from the end of vec. It’s like a multivalue version of .pop().

vec.append(&mut vec2), where vec2 is another vector of type Vec<T>, moves all elements from vec2 into vec. Afterward, vec2 is empty.

This is like vec.extend(vec2) except that vec2 still exists afterward, with its capacity unaffected.

vec.drain(range), where range is a range value, like .. or 0..4, removes the range vec[range] from vec and returns an iterator over the removed elements.

There are also a few oddball methods for selectively removing some of a vector’s elements:

vec.retain(test) removes all elements that don’t pass the given test. The test argument is a function or closure that implements FnMut(&T) -> bool. For each element of vec, this calls test(&element), and if it returns false, the element is removed from the vector and dropped.

Apart from performance, this is like writing:

vec = vec.into_iter().filter(test).collect();

vec.dedup() drops repeated elements. It’s like the Unix uniq shell utility. It scans vec for places where adjacent elements are equal and drops the extra equal values, so that only one is left:

let mut byte_vec = b"Misssssssissippi".to_vec();
byte_vec.dedup();
assert_eq!(&byte_vec, b"Misisipi");

Note that there are still two 's'’s in the output. This method only removes adjacent duplicates. To eliminate all duplicates, you have three options: sort the vector before calling .dedup(); move the data into a set; or (to keep the elements in their original order) use this .retain() trick:

let mut byte_vec = b"Misssssssissippi".to_vec();

let mut seen = HashSet::new();
byte_vec.retain(|r| seen.insert(*r));

assert_eq!(&byte_vec, b"Misp");

This works because .insert() returns false when the set already contains the item we’re inserting.

vec.dedup_by(same) is the same as vec.dedup(), but it uses the function or closure same(&mut elem1, &mut elem2), instead of the == operator, to check whether two elements should be considered equal.

vec.dedup_by_key(key) is the same as vec.dedup(), but it treats two elements as equal if key(&mut elem1) == key(&mut elem2).

For example, if errors is a Vec<Box<Error>>, you can write:

// Remove errors with redundant messages.
errors.dedup_by_key(|err| err.description().to_string());

Of all the methods covered in this section, only .resize() ever clones values. The others work by moving values from one place to another.

Joining

Two methods work on arrays of arrays, by which we mean any array, slice, or vector whose elements are themselves arrays, slices, or vectors.

slices.concat() returns a new vector made by concatenating all the slices.

assert_eq!([[1, 2], [3, 4], [5, 6]].concat(),
           vec![1, 2, 3, 4, 5, 6]);

slices.join(&separator) is the same, except a copy of the value separator is inserted between slices:

assert_eq!([[1, 2], [3, 4], [5, 6]].join(&0),
           vec![1, 2, 0, 3, 4, 0, 5, 6]);

Splitting

It’s easy to get many non-mut references into an array, slice, or vector at once:

let v = vec![0, 1, 2, 3];
let a = &v[i];
let b = &v[j];

let mid = v.len() / 2;
let front_half = &v[..mid];
let back_half = &v[mid..];

Getting multiple mut references is not so easy:

let mut v = vec![0, 1, 2, 3];
let a = &mut v[i];
let b = &mut v[j];  // error: cannot borrow `v` as mutable
                    //        more than once at a time

Rust forbids this because if i == j, then a and b would be two mut references to the same integer, in violation of Rust’s safety rules. (See “Sharing Versus Mutation”.)

Rust has several methods that can borrow mut references to two or more parts of an array, slice, or vector at once. Unlike the code above, these methods are safe, because by design, they split the data into nonoverlapping regions. Many of these methods are also handy for working with non-mut slices, so there are mut and non-mut versions of each.

Figure 16-2 illustrates these methods. None of them directly modify an array, slice, or vector; they merely return new references to parts of the data inside.

slice.iter() and slice.iter_mut() produce a reference to each element of slice. We covered them in “Iteration”.

slice.split_at(index) and slice.split_at_mut(index) break a slice in two, returning a pair. slice.split_at(index) is equivalent to (&slice[..index], &slice[index..]). These methods panic if index is out of bounds.

slice.split_first() and slice.split_first_mut() also return a pair: a reference to the first element (slice[0]) and a slice reference to all the rest (slice[1..]).

The return type of .split_first() is Option<(&T, &[T])>; the result is None if slice is empty.

slice.split_last() and slice.split_last_mut() are analogous but split off the last element rather than the first.

The return type of .split_last() is Option<(&[T], &T)>.

slice.split(is_sep) and slice.split_mut(is_sep) split slice into one or more subslices, using the function or closure is_sep to figure out where to split. They return an iterator over the subslices.

As you consume the iterator, it calls is_sep(&element) for each element in the slice. If is_sep(&element) is true, the element is a separator. Separators are not included in any output subslice.

The output always contains at least one subslice, plus one per separator. Empty subslices are included whenever separators appear adjacent to each other or to the ends of slice.

slice.splitn(n, is_sep) and slice.splitn_mut(n, is_sep) are the same, but they produce at most n subslices. After the first n-1 slices are found, is_sep is not called again. The last subslice contains all the remaining elements.

slice.rsplitn(n, is_sep) and slice.rsplitn_mut(n, is_sep) are just like .splitn() and .splitn_mut() except that the slice is scanned in reverse order. That is, these methods split on the last n-1 separators in the slice, rather than the first, and the subslices are produced starting from the end.

slice.chunks(n) and slice.chunks_mut(n) return an iterator over non-overlapping subslices of length n.

If slice.len() is not a multiple of n, then the last subslice will have a length less than n.

Figure 16-2. Splitting methods illustrated. The little rectangle in the output of slice.split() is an empty slice, caused by the two adjacent separators. Also note that rsplitn produces its output in end-to-start order, unlike all the others.

There’s one more method for iterating over subslices:

slice.windows(n) returns an iterator that behaves like a “sliding window” over the data in slice. It produces subslices that span n consecutive elements of slice. The first value produced is &slice[0..n]; the second is &slice[1..n+1]; and so on.

If n is greater than the length of slice, then no slices are produced. If n is 0, the method panics.

For example, if days.len() == 31, then we can produce all seven-day spans in days by calling days.windows(7).

A sliding window of size 2 is handy for exploring how a data series changes from one data point to the next:

let changes = daily_high_temperatures
                  .windows(2)             // get adjacent days' temps
                  .map(|w| w[1] - w[0])   // how much did it change?
                  .collect::<Vec<_>>();

Because the subslices overlap, there is no variation of this method that returns mut references.

Swapping

There’s a convenience method for swapping two elements:

slice.swap(i, j) swaps the two elements slice[i] and slice[j].

Vectors have a related method for efficiently removing any element:

vec.swap_remove(i) removes and returns vec[i]. This is like vec.remove(i) except that instead of sliding the rest of the vector’s elements over to close the gap, it simply moves vec’s last element into the gap. It’s useful when you don’t care about the order of the items left in the vector.

Sorting and Searching

Slices offer three methods for sorting:

slice.sort() sorts the elements into increasing order. This method is present only when the element type implements Ord.

slice.sort_by(cmp) sorts the elements of slice using a function or closure cmp to specify the sort order. cmp must implement Fn(&T, &T) -> std::cmp::Ordering.

Hand-implementing cmp is a pain, unless you delegate to a .cmp() method:

students.sort_by(|a, b| a.last_name.cmp(&b.last_name));

To sort by one field, using a second field as a tiebreaker, compare tuples:

students.sort_by(|a, b| {
    let a_key = (&a.last_name, &a.first_name);
    let b_key = (&b.last_name, &b.first_name);
    a_key.cmp(&b_key)
});

slice.sort_by_key(key) sorts the elements of slice into increasing order by a sort key, given by the function or closure key. The type of key must implement Fn(&T) -> K where K: Ord.

This is useful when T contains one or more ordered fields, so that it could be sorted multiple ways.

// Sort by grade point average, lowest first.
students.sort_by_key(|s| s.grade_point_average());

Note that these sort-key values are not cached during sorting, so the key function may be called more than n times.

For technical reasons, key(element) can’t return any references borrowed from the element. This won’t work:

students.sort_by_key(|s| &s.last_name);  // error: can't infer lifetime

Rust can’t figure out the lifetimes. But in these cases, it’s easy enough to fall back on .sort_by().

All three methods perform a stable sort.

To sort in reverse order, you can use sort_by with a cmp closure that swaps the two arguments. Taking arguments |b, a| rather than |a, b| effectively produces the opposite order. Or, you can just call the .reverse() method after sorting:

slice.reverse() reverses a slice in place.

Once a slice is sorted, it can be efficiently searched:

slice.binary_search(&value), slice.binary_search_by(&value, cmp), and slice.binary_search_by_key(&value, key) all search for value in the given sorted slice. Note that value is passed by reference.

The return type of these methods is Result<usize, usize>. They return Ok(index) if slice[index] equals value under the specified sort order. If there is no such index, then they return Err(insertion_point) such that inserting value at insertion_point would preserve the order.

Of course, a binary search only works if the slice is in fact sorted in the specified order. Otherwise, the results are arbitrary—garbage in, garbage out.

Since f32 and f64 have NaN values, they do not implement Ord, and can’t be used directly as keys with the sorting and binary search methods. To get similar methods that work on floating-point data, use the ord_subset crate.

There’s one method for searching a vector that is not sorted:

slice.contains(&value) returns true if any element of slice is equal to value. This simply checks each element of the slice until a match is found. Again, value is passed by reference.

To find the location of a value in a slice, like array.indexOf(value) in JavaScript, use an iterator:

slice.iter().position(|x| *x == value)

This returns an Option<usize>.

Comparing Slices

If a type T supports the == and != operators (the PartialEq trait, described in “Equality Tests”), then arrays [T; N], slices [T], and vectors Vec<T> support them too. Two slices are equal if they’re the same length and their corresponding elements are equal. The same goes for arrays and vectors.

If T supports the operators <, <=, >, and >= (the PartialOrd trait, described in “Ordered Comparisons”), then arrays, slices, and vectors of T do too. Slice comparisons are lexicographical.

Two convenience methods perform common slice comparisons:

slice.starts_with(other) returns true if slice starts with a sequence of values that are equal to the elements of the slice other:

assert_eq!([1, 2, 3, 4].starts_with(&[1, 2]), true);
assert_eq!([1, 2, 3, 4].starts_with(&[2, 3]), false);

slice.ends_with(other) is similar but checks the end of slice:

assert_eq!([1, 2, 3, 4].ends_with(&[3, 4]), true);

Random Elements

Random numbers are not built into the Rust standard library. The rand crate, which provides them, offers these two methods for getting random output from an array, slice, or vector:

rng.choose(slice) returns a reference to a random element of a slice. Like slice.first() and slice.last(), this returns an Option<&T> that is None only if the slice is empty.

rng.shuffle(slice) randomly reorders the elements of a slice in place. The slice must be passed by mut reference.

These are methods of the rand::Rng trait, so you need a Rng, a random number generator, in order to call them. Fortunately it’s easy to get one by calling rand::thread_rng(). To shuffle the vector my_vec, we can write:

use rand::{Rng, thread_rng};

thread_rng().shuffle(&mut my_vec);

Rust Rules Out Invalidation Errors

Most mainstream programming languages have collections and iterators, and they all have some variation on this rule: don’t modify a collection while you’re iterating over it. For example, the Python equivalent of a vector is a list:

my_list = [1, 3, 5, 7, 9]

Suppose we try to remove all values greater than 4 from my_list:

for index, val in enumerate(my_list):
    if val > 4:
        del my_list[index]  # bug: modifying list while iterating

print(my_list)

(The enumerate function is Python’s equivalent of Rust’s .enumerate() method, described in “enumerate”.)

This program, surprisingly, prints [1, 3, 7]. But seven is greater than four. How did that slip through? This is an invalidation error: the program modifies data while iterating over it, invalidating the iterator. In Java, the result would be an exception; in C++, undefined behavior. In Python, while the behavior is well-defined, it’s unintuitive: the iterator skips an element. val is never 7.

Let’s try to reproduce this bug in Rust:

fn main() {
    let mut my_vec = vec![1, 3, 5, 7, 9];

    for (index, &val) in my_vec.iter().enumerate() {
        if val > 4 {
            my_vec.remove(index);  // error: can't borrow `my_vec` as mutable
        }
    }
    println!("{:?}", my_vec);
}

Naturally, Rust rejects this program at compile time. When we call my_vec.iter(), it borrows a shared (non-mut) reference to the vector. The reference lives as long as the iterator, to the end of the for loop. We can’t modify the vector by calling my_vec.remove(index) while a non-mut reference exists.

Having an error pointed out to you is nice, but of course, you still need to find a way to get the desired behavior! The easiest fix here is to write:

my_vec.retain(|&val| val <= 4);

Or, you can do what you’d do in Python or any other language: create a new vector using a filter.

VecDeque<T>

Vec supports efficiently adding and removing elements only at the end. When a program needs a place to store values that are “waiting in line,” Vec can be slow.

Rust’s std::collections::VecDeque<T> is a deque (pronounced “deck”), a double-ended queue. It supports efficient add and remove operations at both the front and the back.

deque.push_front(value) adds a value at the front of the queue.

deque.push_back(value) adds a value at the end. (This method is used much more than .push_front(), because the usual convention for queues is that values are added at the back and removed at the front, like people waiting in a line.)

deque.pop_front() removes and returns the front value of the queue, returning an Option<T> that is None if the queue is empty, like vec.pop().

deque.pop_back() removes and returns the value at the back, again returning an Option<T>.

deque.front() and deque.back() work like vec.first() and vec.last(). They return a reference to the front or back element of the queue. The return value is an Option<&T> that is None if the queue is empty.

deque.front_mut() and deque.back_mut() work like vec.first_mut() and vec.last_mut(), returning Option<&mut T>.

The implementation of VecDeque is a ring buffer, as shown in Figure 16-3.

Like a Vec, it has a single heap allocation where elements are stored. Unlike Vec, the data does not always start at the beginning of this region, and it can “wrap around” the end, as shown. The elements of this deque, in order, are ['A', 'B', 'C', 'D', 'E']. VecDeque has private fields, labeled start and stop in the figure, that it uses to remember where in the buffer the data begins and ends.

Adding a value to the queue, on either end, means claiming one of the unused slots, shown in dark gray, wrapping around or allocating a bigger chunk of memory if needed.

VecDeque manages wrapping, so you don’t have to think about it. Figure 16-3 is a behind-the-scenes view of how Rust makes .pop_front() fast.

Figure 16-3. How a VecDeque is stored in memory

Often, when you need a deque, .push_back() and .pop_front() are the only two methods you need. The static methods VecDeque::new() and VecDeque::with​_capacity(n), for creating queues, are just like their counterparts in Vec. Many Vec methods are also implemented for VecDeque: .len() and .is_empty(), .insert(index, value) and .remove(index), .extend(iterable), and so on.

Deques, like vectors, can be iterated by value, by shared reference, or by mut reference. They have the three iterator methods .into_iter(), .iter(), and .iter_mut(). They can be indexed in the usual way: deque[index].

However, because deques don’t store their elements contiguously in memory, they don’t inherit all the methods of slices. One way to perform vector and slice operations on deque data is to convert the VecDeque to a Vec, do the operation, and then change it back:

Vec<T> implements From<VecDeque<T>>, so Vec::from(deque) turns a deque into a vector. This costs O(n) time, since it may require rearranging the elements.

VecDeque<T> implements From<Vec<T>>, so VecDeque::from(vec) turns a vector into a deque. This is also O(n), but it’s usually fast, even if the vector is large, because the vector’s heap allocation can simply be moved to the new deque.

This method makes it easy to create a deque with specified elements, even though there is no standard vec_deque![] macro:

use std::collections::VecDeque;

let v = VecDeque::from(vec![1, 2, 3, 4]);

LinkedList<T>

A linked list is another way to store a sequence of values. Each value is stored in a separate heap allocation, as shown in Figure 16-4.

Figure 16-4. A LinkedList<char> in memory

std::collections::LinkedList<T> is a doubly linked list for Rust. It supports a subset of VecDeque’s methods. The methods that operate on the front and back of the sequence are all there; so are iterator methods, LinkedList::new(), and a few others. Methods that access elements by index, though, are generally omitted, since it’s inherently inefficient to access linked list elements by index.

As of Rust 1.17, Rust’s LinkedList type has no methods for removing a range of elements from a list or inserting elements at specific locations in a list. The API seems incomplete.

For now, the main advantage of LinkedList over VecDeque is that combining two lists is very fast. list.append(&mut list2), the method that moves all elements from one list to another, only involves changing a few pointers, which can be done in constant time. The append methods of Vec and VecDeque sometimes have to move many values from one heap array to another.

BinaryHeap<T>

A BinaryHeap is a collection whose elements are kept loosely organized so that the greatest value always bubbles up to the front of the queue. Here are the three most commonly used BinaryHeap methods:

heap.push(value) adds a value to the heap.

heap.pop() removes and returns the greatest value from the heap. It returns an Option<T> that is None if the heap was empty.

heap.peek() returns a reference to the greatest value in the heap. The return type is Option<&T>.

BinaryHeap also supports a subset of the methods on Vec, including BinaryHeap::new(), .len(), .is_empty(), .capacity(), .clear(), and .append(&mut heap2).

For example, if we populate a BinaryHeap with a bunch of numbers:

use std::collections::BinaryHeap;

let mut heap = BinaryHeap::from(vec![2, 3, 8, 6, 9, 5, 4]);

then the value 9 is at the top of the heap:

assert_eq!(heap.peek(), Some(&9));
assert_eq!(heap.pop(), Some(9));

Removing the value 9 also rearranges the other elements slightly so that 8 is now at the front, and so on:

assert_eq!(heap.pop(), Some(8));
assert_eq!(heap.pop(), Some(6));
assert_eq!(heap.pop(), Some(5));
...

Of course, BinaryHeap is not limited to numbers. It can hold any type of value that implements the Ord built-in trait.

This makes BinaryHeap useful as a work queue. You can define a task struct that implements Ord on the basis of priority, so that higher-priority tasks are Greater than lower-priority tasks. Then, create a BinaryHeap to hold all pending tasks. Its .pop() method will always return the most important item, the task your program should work on next.

Note: BinaryHeap is iterable, and it has an .iter() method, but the iterators produce the heap’s elements in an arbitrary order, not from greatest to least. To consume values from a BinaryHeap in order of priority, use a while loop:

while let Some(task) = heap.pop() {
    handle(task);
}

HashMap<K, V> and BTreeMap<K, V>

A map is a collection of key-value pairs (called entries). No two entries have the same key, and the entries are kept organized so that if you have a key, you can efficiently look up the corresponding value in a map. In short, a map is a lookup table.

Rust offers two map types: HashMap<K, V> and BTreeMap<K, V>. The two share many of the same methods; the difference is in how the two keep entries arranged for fast lookup.

A HashMap stores the keys and values in a hash table, so it requires a key type K that implements Hash and Eq, the standard traits for hashing and equality.

Figure 16-5 shows how a HashMap is arranged in memory. Dark gray regions are unused. All keys, values, and cached hash codes are stored in a single heap-allocated table. Adding entries eventually forces the HashMap to allocate a larger table and move all the data into it.

Figure 16-5. A HashMap in memory

A BTreeMap stores the entries in order by key, in a tree structure, so it requires a key type K that implements Ord. Figure 16-6 shows a BTreeMap. Again, dark gray regions are unused spare capacity.

Figure 16-6. A BTreeMap in memory

A BTreeMap stores its entries in nodes. Most nodes in a BTreeMap contain only key-value pairs. Nonleaf nodes, like the root node shown in this figure, also have room for pointers to child nodes. The pointer between (20, 'q') and (30, 'r') points to a child node containing keys between 20 and 30. Adding entries often requires sliding some of a node’s existing entries to the right, to keep them sorted, and occasionally involves allocating new nodes.

This picture is a bit simplified to fit on the page. For example, real BTreeMap nodes have room for 11 entries, not 4.

The Rust standard library uses B-trees rather than balanced binary trees because B-trees are faster on modern hardware. A binary tree may use fewer comparisons per search than a B-tree, but searching a B-tree has better locality—that is, the memory accesses are grouped together rather than scattered across the whole heap. This makes CPU cache misses rarer. It’s a significant speed boost.

There are several ways to create a map:

HashMap::new() and BTreeMap::new() create new, empty maps.

iter.collect() can be used to create and populate a new HashMap or BTreeMap from key-value pairs. iter must be an Iterator<Item=(K, V)>.

HashMap::with_capacity(n) creates a new, empty hash map with room for at least n entries. HashMaps, like vectors, store their data in a single heap allocation, so they have a capacity and the related methods hash_map.capacity(), hash_map.reserve(additional), and hash_map.shrink_to_fit(). BTreeMaps do not.

HashMaps and BTreeMaps have the same core methods for working with keys and values:

map.len() returns the number of entries.

map.is_empty() returns true if map has no entries.

map.contains_key(&key) returns true if the map has an entry for the given key.

map.get(&key) searches map for an entry with the given key. If a matching entry is found, this returns Some(r), where r is a reference to the corresponding value. Otherwise, this returns None.

map.get_mut(&key) is similar, but it returns a mut reference to the value.

In general, maps let you have mut access to the values stored inside them, but not the keys. The values are yours to modify however you like. The keys belong to the map itself; it needs to ensure that they don’t change, because the entries are organized by their keys. Modifying a key in-place would be a bug.

map.insert(key, value) inserts the entry (key, value) into map. If there’s already an entry for key in the map, the newly inserted value overwrites the old one.

Returns the old value, if any. The return type is Option<V>.

map.extend(iterable) iterates over the (K, V) items of iterable and inserts each of those key-value pairs into map.

map.append(&mut map2) moves all entries from map2 into map. Afterward, map2 is empty.

map.remove(&key) finds and removes any entry with the given key from map.

Returns the removed value, if any. The return type is Option<V>.

map.clear() removes all entries.

A map can also be queried using square brackets: map[&key]. That is, maps implement the Index built-in trait. However, this panics if there is not already an entry for the given key, like an out-of-bounds array access, so use this syntax only if the entry you’re looking up is sure to be populated.

The key argument to .contains_key(), .get(), .get_mut(), and .remove() does not have to have the exact type &K. These methods are generic over types that can be borrowed from K. It’s OK to call fish_map.contains_key("conger") on a HashMap<String, Fish>, even though "conger" isn’t exactly a String, because String implements Borrow<&str>. For details, see “Borrow and BorrowMut”.

Because a BTreeMap<K, V> keeps its entries sorted by key, it supports an additional operation:

btree_map.split_at(&key) splits btree_map in two. Entries with keys less than key are left in btree_map. Returns a new BTreeMap<K, V> containing the other entries.

Entries

Both HashMap and BTreeMap have a corresponding Entry type. The point of entries is to eliminate redundant map lookups. For example, here’s some code to get or create a student record:

// Do we already have a record for this student?
if !student_map.contains_key(name) {
    // No: create one.
    student_map.insert(name.to_string(), Student::new());
}
// Now a record definitely exists.
let record = student_map.get_mut(name).unwrap();
...

This works fine, but it accesses student_map two or three times, doing the same lookup each time.

The idea with entries is that we do the lookup just once, producing an Entry value that is then used for all subsequent operations. This one-liner is equivalent to all the code above, except that it only does the lookup once:

let record = student_map.entry(name.to_string()).or_insert_with(Student::new);

The Entry value returned by student_map.entry(name.to_string()) acts like a mutable reference to a place within the map that’s either occupied by a key-value pair, or vacant, meaning there’s no entry there yet. If vacant, the entry’s .or_insert_with() method inserts a new Student. Most uses of entries are like this: short and sweet.

All Entry values are created by the same method:

map.entry(key) returns an Entry for the given key. If there’s no such key in the map, this returns a vacant Entry.

This method takes its self argument by mut reference and returns an Entry with a matching lifetime:

pub fn entry<'a>(&'a mut self, key: K) -> Entry<'a, K, V>

The Entry type has a lifetime parameter 'a because it’s effectively a fancy kind of borrowed mut reference to the map. As long as the Entry exists, it has exclusive access to the map.

Back in “Structs Containing References”, we saw how to store references in a type, and how that affects lifetimes. Now we’re seeing what that looks like from a user’s perspective. That’s what’s going on with Entry.

Unfortunately, it is not possible to pass a reference of type &str to this method if the map has String keys. The .entry() method, in that case, requires a real String.

Entry values provide two methods for filling in vacant entries:

map.entry(key).or_insert(value) ensures that map contains an entry with the given key, inserting a new entry with the given default value if needed. It returns a mut reference to the new or existing value.

Suppose we need to count votes. We can write:

let mut vote_counts: HashMap<String, usize> = HashMap::new();
for name in ballots {
    let count = vote_counts.entry(name).or_insert(0);
    *count += 1;
}

.or_insert() returns a mut reference, so the type of count is &mut usize.

map.entry(key).or_insert_with(default_fn) is the same, except that if it needs to create a new entry, it calls default_fn() to produce the default value. If there’s already an entry for key in the map, then default_fn is not used.

Suppose we want to know which words appear in which files. We can write:

// This map contains, for each word, the set of files it appears in.
let mut word_occurrence: HashMap<String, HashSet<String>> =
    HashMap::new();
for file in files {
    for word in read_words(file)? {
        let set = word_occurrence
            .entry(word)
            .or_insert_with(HashSet::new);
        set.insert(file.clone());
    }
}

The Entry type is an enum, defined like this for HashMap (and similarly for BTreeMap):

// (in std::collections::hash_map)
pub enum Entry<'a, K: 'a, V: 'a> {
    Occupied(OccupiedEntry<'a, K, V>),
    Vacant(VacantEntry<'a, K, V>)
}

The OccupiedEntry and VacantEntry types have methods for inserting, removing, and accessing entries without repeating the initial lookup. You can find them in the online documentation. The extra methods can occasionally be used to eliminate a redundant lookup or two, but .or_insert() and .or_insert_with() cover the common cases.

Map Iteration

There are several ways to iterate over a map:

Iterating by value (“for (k, v) in map”) produces (K, V) pairs. This consumes the map.

Iterating over a shared reference (“for (k, v) in &map”) produces (&K, &V) pairs.

Iterating over a mut reference (“for (k, v) in &mut map”) produces (&K, &mut V) pairs. (Again, there’s no way to get mut access to keys stored in a map, because the entries are organized by their keys.)

Like vectors, maps have .iter() and .iter_mut() methods which return by-reference iterators, just like iterating over &map or &mut map. In addition:

map.keys() returns an iterator over just the keys, by reference.

map.values() returns an iterator over the values, by reference.

map.values_mut() returns an iterator over the values, by mut reference.

All HashMap iterators visit the map’s entries in an arbitrary order. BTreeMap iterators visit them in order by key.

HashSet<T> and BTreeSet<T>

Sets are collections of values arranged for fast membership testing.

let b1 = large_vector.contains("needle");    // slow, checks every element
let b2 = large_hash_set.contains("needle");  // fast, hash lookup

A set never contains multiple copies of the same value.

Maps and sets have different methods, but behind the scenes, a set is like a map with only keys, rather than key-value pairs. In fact, Rust’s two set types, HashSet<T> and BTreeSet<T>, are implemented as thin wrappers around HashMap<T, ()> and BTreeMap<T, ()>.

HashSet::new() and BTreeSet::new() create new sets.

iter.collect() can be used to create a new set from any iterator. If iter produces any values more than once, the duplicates are dropped.

HashSet::with_capacity(n) creates an empty HashSet with room for at least n values.

HashSet<T> and BTreeSet<T> have all the basic methods in common:

set.len() returns the number of values in set.

set.is_empty() returns true if the set contains no elements.

set.contains(&value) returns true if the set contains the given value.

set.insert(value) adds a value to the set. Returns true if a value was added, false if it was already a member of the set.

set.remove(&value) removes a value from the set. Returns true if a value was removed, false if it already wasn’t a member of the set.

As with maps, the methods that look up a value by reference are generic over types that can be borrowed from T. For details, see “Borrow and BorrowMut”.

Set Iteration

There are two ways to iterate over sets:

Iterating by value (“for v in set”) produces the members of the set (and consumes the set).

Iterating by shared reference (“for v in &set”) produces shared references to the members of the set.

Iterating over a set by mut reference is not supported. There’s no way to get a mut reference to a value stored in a set.

set.iter() returns an iterator over the members of set by reference.

HashSet iterators, like HashMap iterators, produce their values in an arbitrary order. BTreeSet iterators produce values in order, like a sorted vector.

When Equal Values Are Different

Sets have a few odd methods that you need to use only if you care about differences between “equal” values.

Such differences do often exist. Two identical String values, for example, store their characters in different locations in memory:

let s1 = "hello".to_string();
let s2 = "hello".to_string();
println!("{:p}", &s1 as &str);  // 0x7f8b32060008
println!("{:p}", &s2 as &str);  // 0x7f8b32060010

Usually, we don’t care.

But in case you ever do, you can get access to the actual values stored inside a set by using the following methods. Each one returns an Option that’s None if set did not contain a matching value.

set.get(&value) returns a shared reference to the member of set that’s equal to value, if any. Returns an Option<&T>.

set.take(&value) is like set.remove(&value), but it returns the removed value, if any. Returns an Option<T>.

set.replace(value) is like set.insert(value), but if set already contains a value that’s equal to value, this replaces and returns the old value. Returns an Option<T>.

Whole-Set Operations

So far, most of the set methods we’ve seen are focused on a single value in a single set. Sets also have methods that operate on whole sets.

set1.intersection(&set2) returns an iterator over all values that are in both set1 and set2.

For example, if we want to print the names of all students who are taking both brain surgery and rocket science classes, we could write:

for student in brain_class {
    if rocket_class.contains(&student) {
        println!("{}", student);
    }
}

Or, shorter:

for student in brain_class.intersection(&rocket_class) {
    println!("{}", student);
}

Amazingly, there’s an operator for this.

&set1 & &set2 returns a new set that’s the intersection of set1 and set2. This is the binary bitwise AND operator, applied to two references. This finds values that are in both set1 AND set2.

let overachievers = &brain_class & &rocket_class;

set1.union(&set2) returns an iterator over values that are in either set1 or set2, or both.

&set1 | &set2 returns a new set containing all those values. It finds values that are in either set1 OR set2.

set1.difference(&set2) returns an iterator over values that are in set1 but not in set2.

&set1 - &set2 returns a new set containing all those values.

set1.symmetric_difference(&set2) returns an iterator over values that are in either set1 or set2, but not both.

&set1 ^ &set2 returns a new set containing all those values.

And there are three methods for testing relationships between sets:

set1.is_disjoint(set2) is true if set1 and set2 have no values in common—the intersection between them is empty.

set1.is_subset(set2) is true if set1 is a subset of set2—that is, all values in set1 are also in set2.

set1.is_superset(set2) is the reverse: it’s true if set1 is a superset of set2.

Sets also support equality testing with == and !=; two sets are equal if they contain the same values.

Hashing

std::hash::Hash is the standard library trait for hashable types. HashMap keys and HashSet elements must implement both Hash and Eq.

Most built-in types that implement Eq also implement Hash. The integer types, char, and String are all hashable; so are tuples, arrays, slices, and vectors, as long as their elements are hashable.

One principle of the standard library is that a value should have the same hash code regardless of where you store it or how you point to it. Therefore, a reference has the same hash code as the value it refers to, and a Box has the same hash code as the boxed value. A vector vec has the same hash code as the slice containing all its data, &vec[..]. A String has the same hash code as a &str with the same characters.

Structs and enums don’t implement Hash by default, but an implementation can be derived:

/// The ID number for an object in the British Museum's collection.
#[derive(Clone, PartialEq, Eq, Hash)]
enum MuseumNumber {
    ...
}

This works as long as the type’s fields are all hashable.

If you implement PartialEq by hand for a type, you should also implement Hash by hand. For example, suppose we have a type that represents priceless historical treasures:

struct Artifact {
    id: MuseumNumber,
    name: String,
    cultures: Vec<Culture>,
    date: RoughTime,
    ...
}

Two Artifacts are considered equal if they have the same ID:

impl PartialEq for Artifact {
    fn eq(&self, other: &Artifact) -> bool {
        self.id == other.id
    }
}

impl Eq for Artifact {}

Since we compare artifacts purely on the basis of their ID, we must hash them the same way:

impl Hash for Artifact {
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        // Delegate hashing to the MuseumNumber.
        self.id.hash(hasher);
    }
}

(Otherwise, HashSet<Artifact> would not work properly; like all hash tables, it requires that hash(a) == hash(b) if a == b.)

This allows us to create a HashSet of Artifacts:

let mut collection = HashSet::<Artifact>::new();

As this code shows, even when you implement Hash by hand, you don’t need to know anything about hashing algorithms. .hash() receives a reference to a Hasher, which represents the hashing algorithm. You simply feed this Hasher all the data that’s relevant to the == operator. The Hasher computes a hash code from whatever you give it.

Using a Custom Hashing Algorithm

The hash method is generic, so the Hash implementations shown above can feed data to any type that implements Hasher. This is how Rust supports pluggable hashing algorithms. Hash and Hasher are buddy traits, as explained in “Buddy Traits (or How rand::random() Works)”.

A third trait, std::hash::BuildHasher, is the trait for types that represent the initial state of a hashing algorithm. Each Hasher is single-use, like an iterator: you use it once and throw it away. A BuildHasher is reusable.

Every HashMap contains a BuildHasher that it uses each time it needs to compute a hash code. The BuildHasher value contains the key, initial state, or other parameters that the hashing algorithm needs every time it runs.

The complete protocol for computing a hash code looks like this:

use std::hash::{Hash, Hasher, BuildHasher};

fn compute_hash<B, T>(builder: &B, value: &T) -> u64
    where B: BuildHasher, T: Hash
{
    let mut hasher = builder.build_hasher();  // 1. start the algorithm
    value.hash(&mut hasher);                  // 2. feed it data
    hasher.finish()                           // 3. finish, producing a u64
}

HashMap calls these three methods every time it needs to compute a hash code. All the methods are inlineable, so it’s very fast.

Rust’s default hashing algorithm is a well-known algorithm called SipHash-1-3. SipHash is fast, and it’s very good at minimizing hash collisions. In fact, it’s a cryptographic algorithm: there’s no known efficient way to generate SipHash-1-3 collisions. As long as a different, unpredictable key is used for each hash table, Rust is secure against a kind of denial-of-service attack called HashDoS, where attackers deliberately use hash collisions to trigger worst-case performance in a server.

But perhaps you don’t need that for your application. If you’re storing many small keys, such as integers or very short strings, it is possible to implement a faster hash function, at the expense of HashDoS security. The fnv crate implements one such algorithm, the Fowler-Noll-Vo hash. To try it out, add this line to your Cargo.toml:

[dependencies]
fnv = "1.0"

Then import the map and set types from fnv:

extern crate fnv;

use fnv::{FnvHashMap, FnvHashSet};

You can use these two types as drop-in replacements for HashMap and HashSet. A peek inside the fnv source code reveals how they’re defined:

/// A `HashMap` using a default FNV hasher.
pub type FnvHashMap<K, V> = HashMap<K, V, FnvBuildHasher>;

/// A `HashSet` using a default FNV hasher.
pub type FnvHashSet<T> = HashSet<T, FnvBuildHasher>;

The standard HashMap and HashSet collections accept an optional extra type parameter specifying the hashing algorithm; FnvHashMap and FnvHashSet are generic type aliases for HashMap and HashSet, specifying an FNV hasher for that parameter.

Beyond the Standard Collections

Creating a new, custom collection type in Rust is much the same as in any other language. You arrange data by combining the parts the language provides: structs and enums, standard collections, Options, Boxes, and so on. For an example, see the BinaryTree<T> type defined in “Generic Enums”.

If you’re used to implementing data structures in C++, using raw pointers, manual memory management, placement new, and explicit destructor calls to get the best possible performance, you’ll undoubtedly find safe Rust rather limiting. All of those tools are inherently unsafe. They are available in Rust, but only if you opt in to unsafe code. Chapter 21 shows how; it includes an example that uses some unsafe code to implement a safe custom collection.

For now, we’ll just bask in the warm glow of the standard collections and their safe, efficient APIs. Like much of the Rust standard library, they’re designed to ensure that the need to write "unsafe" is as rare as possible.

Chapter 17. Strings and Text

The string is a stark data structure and everywhere it is passed there is much duplication of process. It is a perfect vehicle for hiding information.

Alan Perlis, epigram #34

We’ve been using Rust’s main textual types, String, str, and char, throughout the book. In “String Types”, we described the syntax for character and string literals, and showed how strings are represented in memory. In this chapter, we cover text handling in more detail.

In this chapter:

We give you some background on Unicode that should help you make sense of the standard library’s design.

We describe the char type, representing a single Unicode code point.

We describe the String and str types, representing owned and borrowed sequences of Unicode characters. These have a broad variety of methods for building, searching, modifying, and iterating over their contents.

We cover Rust’s string formatting facilities, like the println! and format! macros. You can write your own macros that work with formatting strings, and extend them to support your own types.

We give an overview of Rust’s regular expression support.

Finally, we talk about why Unicode normalization matters, and show how to do it in Rust.

Some Unicode Background

This book is about Rust, not Unicode, which has entire books devoted to it already. But Rust’s character and string types are designed around Unicode. Here are a few bits of Unicode that help explain Rust.

ASCII, Latin-1, and Unicode

Unicode and ASCII match for all of ASCII’s code points, from 0 to 0x7f: for example, both assign the character '*' the code point 42. Similarly, Unicode assigns 0 through 0xff to the same characters as the ISO/IEC 8859-1 character set, an eight-bit superset of ASCII for use with Western European languages. Unicode calls this range of code points the Latin-1 code block, so we’ll refer to ISO/IEC 8859-1 by the more evocative name Latin-1.

Since Unicode is a superset of Latin-1, converting Latin-1 to Unicode doesn’t even require a table:

fn latin1_to_char(latin1: u8) -> char {
    latin1 as char
}

The reverse conversion is trivial as well, assuming the code points fall in the Latin-1 range:

fn char_to_latin1(c: char) -> Option<u8> {
    if c as u32 <= 0xff {
        Some(c as u8)
    } else {
        None
    }
}

UTF-8

The Rust String and str types represent text using the UTF-8 encoding form. UTF-8 encodes a character as a sequence of one to four bytes (Figure 17-1).

Figure 17-1. The UTF-8 encoding

There are two restrictions on well-formed UTF-8 sequences. First, only the shortest encoding for any given code point is considered well-formed; you can’t spend four bytes encoding a code point that would fit in three. This rule ensures that there is exactly one UTF-8 encoding for a given code point. Second, well-formed UTF-8 must not encode numbers from 0xd800 through 0xdfff or beyond 0x10ffff: those are either reserved for noncharacter purposes, or outside Unicode’s range entirely.

Figure 17-2 shows some examples.

Figure 17-2. UTF-8 examples

Note that, even though the crab emoji has an encoding whose leading byte contributes only zeros to the code point, it still needs a four-byte encoding: three-byte UTF-8 encodings can only convey 16-bit code points, and 0x1f980 is 17 bits long.

Here’s a quick example of a string containing characters with encodings of varying lengths:

assert_eq!("うどん: udon".as_bytes(),
           &[0xe3, 0x81, 0x86, // う
             0xe3, 0x81, 0xa9, // ど
             0xe3, 0x82, 0x93, // ん
             0x3a, 0x20, 0x75, 0x64, 0x6f, 0x6e // : udon
           ]);

The diagram shows some very helpful properties of UTF-8:

Since UTF-8 encodes code points 0 through 0x7f as nothing more than the bytes 0 through 0x7f, a range of bytes holding ASCII text is valid UTF-8. And if a string of UTF-8 includes only characters from ASCII, the reverse is also true: the UTF-8 encoding is valid ASCII.

The same is not true for Latin-1: for example, Latin-1 encodes 'é' as the byte 0xe9, which UTF-8 would interpret as the first byte of a three-byte encoding.

From looking at any byte’s upper bits, you can immediately tell whether it is the start of some character’s UTF-8 encoding, or a byte from the midst of one.

An encoding’s first byte alone tells you the encoding’s full length, via its leading bits.

Since no encoding is longer than four bytes, UTF-8 processing never requires unbounded loops, which is nice when working with untrusted data.

In well-formed UTF-8, you can always tell unambiguously where characters’ encodings begin and end, even if you start from a random point in the midst of the bytes. UTF-8 first bytes and following bytes are always distinct, so one encoding cannot start in the midst of another. The first byte determines the encoding’s total length, so no encoding can be a prefix of another. This has a lot of nice consequences. For example, searching a UTF-8 string for an ASCII delimiter character requires only a simple scan for the delimiter’s byte. It can never appear as any part of a multibyte encoding, so there’s no need to keep track of the UTF-8 structure at all. Similarly, algorithms that search for one byte string in another will work without modification on UTF-8 strings, even though some don’t even examine every byte of the text being searched.

Although variable-width encodings are more complicated than fixed-width encodings, these characteristics make UTF-8 more comfortable to work with than you might expect. The standard library handles most aspects for you.

Text Directionality

Whereas scripts like Latin, Cyrillic, and Thai are written from left to right, other scripts like Hebrew and Arabic are written from right to left. Unicode stores characters in the order in which they would normally be written or read, so the initial bytes of a string holding, say, Hebrew text encode the character that would be written at the right:

assert_eq!("ערב טוב".chars().next(), Some('ע'));

A few method names in the standard library use the terms left and right to mean the start and end of the text. When we describe such functions, we’ll spell out what they actually do.

Characters (char)

A Rust char is a 32-bit value holding a Unicode code point. A char is guaranteed to fall in the range from 0 to 0xd7ff, or in the range 0xe000 to 0x10ffff; all the methods for creating and manipulating char values ensure that this is true. The char type implements Copy and Clone, along with all the usual traits for comparison, hashing, and formatting.

In the descriptions that follow, the variable ch is always of type char.

Classifying Characters

The char type has methods for classifying characters into a few common categories. These all draw their definitions from Unicode, as shown in Table 17-0.

Method
Description
Examples

ch.is_numeric()
A numeric character. This includes the Unicode general categories “Number; digit” and “Number; letter”, but not “Number; other”.
'4'.is_numeric()
'ᛮ'.is_numeric()
!'⑧'.is_numeric()

ch.is_alphabetic()
An alphabetic character: Unicode’s “Alphabetic” derived property.
'q'.is_alphabetic()
'七'.is_alphabetic()

ch.is_alphanumeric()
Either numeric or alphabetic, as defined above.
'9'.is_alphanumeric()
'饂'.is_alphanumeric()
!'*'.is_alphanumeric()

ch.is_whitespace()
A whitespace character: Unicode character property “WSpace=Y”.
' '.is_whitespace()
'\n'.is_whitespace()
'\u{A0}'.is_whitespace()

ch.is_control()
A control character: Unicode’s “Other, control” general category.
'\n'.is_control()
'\u{85}'.is_control()

Handling Digits

For handling digits, you can use the following methods:

ch.to_digit(radix) decides whether ch is a digit in base radix. If it is, it returns Some(num), where num is a u32. Otherwise, it returns None. This recognizes only ASCII digits, not the broader class of characters covered by char::is_numeric. The radix parameter can range from 2 to 36. For radixes larger than 10, ASCII letters of either case are considered digits with values from 10 through 35.

The free function std::char::from_digit(num, radix) converts the u32 digit value num to a char if possible. If num can be represented as a single digit in radix, from_digit returns Some(ch), where ch is the digit. When radix is greater than 10, ch may be a lowercase letter. Otherwise, it returns None.

This is the reverse of to_digit. If std::char::from_digit(num, radix) is Some(ch), then ch.to_digit(radix) is Some(num). If ch is an ASCII digit or lowercase letter, the converse holds as well.

ch.is_digit(radix) returns true if ch is an ASCII digit in base radix. This is equivalent to ch.to_digit(radix) != None.

So, for example:

assert_eq!('F'.to_digit(16), Some(15));
assert_eq!(std::char::from_digit(15, 16), Some('f'));
assert!(char::is_digit('f', 16));

Case Conversion for Characters

For handling character case:

ch.is_lowercase() and ch.is_uppercase() indicate whether ch is a lower- or uppercase alphabetic character. These follow Unicode’s Lowercase and Uppercase derived properties, so they cover non-Latin alphabets like Greek and Cyrillic, and give the expected results for ASCII as well.

ch.to_lowercase() and ch.to_uppercase() return iterators that produce the characters of the lower- and uppercase equivalents of ch, according to the Unicode Default Case Conversion algorithms:

let mut upper = 's'.to_uppercase();
assert_eq!(upper.next(), Some('S'));
assert_eq!(upper.next(), None);

These methods return an iterator instead of a single character because case conversion in Unicode isn’t always a one-to-one process:

// The uppercase form of the German letter "sharp S" is "SS":
let mut upper = 'ß'.to_uppercase();
assert_eq!(upper.next(), Some('S'));
assert_eq!(upper.next(), Some('S'));
assert_eq!(upper.next(), None);

// Unicode says to lowercase Turkish dotted capital 'İ' to 'i'
// followed by `'\u{307}'`, COMBINING DOT ABOVE, so that a
// subsequent conversion back to uppercase preserves the dot.
let ch = 'İ'; // `'\u{130}'`
let mut lower = ch.to_lowercase();
assert_eq!(lower.next(), Some('i'));
assert_eq!(lower.next(), Some('\u{307}'));
assert_eq!(lower.next(), None);

As a convenience, these iterators implement the std::fmt::Display trait, so you can pass them directly to a println! or write! macro.

Conversions to and from Integers

Rust’s as operator will convert a char to any integer type, silently masking off any upper bits:

assert_eq!('B' as u32, 66);
assert_eq!('饂' as u8, 66);   // upper bits truncated
assert_eq!('二' as i8, -116); // same

The as operator will convert any u8 value to a char, and char implements From<u8> as well, but wider integer types can represent invalid code points, so for those you must use std::char::from_u32, which returns Option<char>:

assert_eq!(char::from(66), 'B');
assert_eq!(std::char::from_u32(0x9942), Some('饂'));
assert_eq!(std::char::from_u32(0xd800), None); // reserved for UTF-16

String and str

Rust’s String and str types are guaranteed to hold only well-formed UTF-8. The library ensures this by restricting the ways you can create String and str values and the operations you can perform on them, such that the values are well-formed when introduced, and remain so as you work with them. All their methods protect this guarantee: no safe operation on them can introduce ill-formed UTF-8. This simplifies code that works with the text.

Rust places text-handling methods on either str and String depending on whether the method needs a resizable buffer, or is happy just using the text in place. Since String dereferences to &str, every method defined on str is directly available on String as well. This section presents methods from both types, grouped by rough function.

These methods index text by byte offsets, and measure its length in bytes, rather than characters. In practice, given the nature of Unicode, indexing by character is not as useful as it may seem, and byte offsets are faster and simpler. If you try to use a byte offset that lands in the midst of some character’s UTF-8 encoding, the method panics, so you can’t introduce ill-formed UTF-8 this way.

A String is implemented as a wrapper around a Vec<u8> that ensures the vector’s contents are always well-formed UTF-8. Rust will never change String to use a more complicated representation, so you can assume that String shares Vec’s performance characteristics.

In these explanations, the following variables have the given types:

Variable | Presumed type
string | String
slice | &str or something that dereferences to one, like String or Rc<String>
ch | char
n | usize, a length
i, j | usize, a byte offset
range | A range of usize byte offsets, either fully bounded like i..j, or partly bounded like i.., ..j, or ..
pattern | Any pattern type: char, String, &str, &[char], or FnMut(char) -> bool

We describe pattern types in “Patterns for Searching Text”.

Creating String Values

There are a few common ways to create String values:

String::new() returns a fresh, empty string. This has no heap-allocated buffer, but will allocate one as needed.

String::with_capacity(n) returns a fresh, empty string with a buffer pre-allocated to hold at least n bytes. If you know the length of the string you’re building in advance, this constructor lets you get the buffer sized correctly from the start, instead of resizing the buffer as you build the string. The string will still grow its buffer as needed if its length exceeds n bytes. Like vectors, strings have capacity, reserve, and shrink_to_fit methods, but usually the default allocation logic is fine.

slice.to_string() allocates a fresh String whose contents are a copy of slice. We’ve been using expressions like "literal text".to_string() throughout the book to make Strings from string literals.

iter.collect() constructs a string by concatenating an iterator’s items, which can be char, &str, or String values. For example, to remove all spaces from a string, you can write:

let spacey = "man hat tan";
let spaceless: String =
    spacey.chars().filter(|c| !c.is_whitespace()).collect();
assert_eq!(spaceless, "manhattan");

Using collect this way takes advantage of String’s implementation of the std::iter::FromIterator trait.

The &str type cannot implement Clone: the trait requires clone on a &T to return a T value, but str is unsized. However, &str does implement ToOwned, which lets the implementer specify its owned equivalent, so slice.to_owned() returns a copy of slice as a freshly allocated String.

Simple Inspection

These methods get basic information from string slices:

slice.len() is the length of slice, in bytes.

slice.is_empty() is true if slice.len() == 0.

slice[range] returns a slice borrowing the given portion of slice. Partially bounded and unbounded ranges are OK: For example:

let full = "bookkeeping";
assert_eq!(&full[..4], "book");
assert_eq!(&full[5..], "eeping");
assert_eq!(&full[2..4], "ok");
assert_eq!(full[..].len(), 11);
assert_eq!(full[5..].contains("boo"), false);

You cannot index a string slice with a single position, like slice[i]. Fetching a single character at a given byte offset is a bit clumsy: you must produce a chars iterator over the slice, and ask it to parse one character’s UTF-8:

let parenthesized = "Rust (饂)";
assert_eq!(parenthesized[6..].chars().next(), Some('饂'));

However, you should rarely need to do this. Rust has much nicer ways to iterate over slices, which we describe in “Iterating over Text”.

slice.split_at(i) returns a tuple of two shared slices borrowed from slice: the portion up to byte offset i, and the portion after it. In other words, this returns (slice[..i], slice[i..]).

slice.is_char_boundary(i) is true if the byte offset i falls between character boundaries, and is thus suitable as an offset into slice.

Naturally, slices can be compared for equality, ordered, and hashed. Ordered comparison simply treats the string as a sequence of Unicode code points and compares them in lexicographic order.

Appending and Inserting Text

The following methods add text to a String:

string.push(ch) appends the character ch to the end string.

string.push_str(slice) appends the full contents of slice.

string.extend(iter) appends the items produced by the iterator iter to the string. The iterator can produce char, str, or String values. These are String’s implementations of std::iter::Extend:

let mut also_spaceless = "con".to_string();
also_spaceless.extend("tri but ion".split_whitespace());
assert_eq!(also_spaceless, "contribution");

string.insert(i, ch) inserts the single character ch at byte offset i in string. This entails shifting over any characters after i to make room for ch, so building up a string this way can require time quadratic in the length of the string.

string.insert_str(i, slice) does the same for slice, with the same performance caveat.

String implements std::fmt::Write, meaning that the write! and writeln! macros can append formatted text to Strings:

use std::fmt::Write;

let mut letter = String::new();
writeln!(letter, "Whose {} these are I think I know", "rutabagas")?;
writeln!(letter, "His house is in the village though;")?;
assert_eq!(letter, "Whose rutabagas these are I think I know\n\
                    His house is in the village though;\n");

Since write! and writeln! are designed for writing to output streams, they return a Result, which Rust complains if you ignore. This code uses the ? operator to handle it, but writing to a String is actually infallible, so in this case calling .unwrap() would be OK too.

Since String implements Add<&str> and AddAssign<&str>, you can write code like this:

let left = "partners".to_string();
let mut right = "crime".to_string();
assert_eq!(left + " in " + &right, "partners in crime");

right += " doesn't pay";
assert_eq!(right, "crime doesn't pay");

When applied to strings, the + operator takes its left operand by value, so it can actually reuse that String as the result of the addition. As a consequence, if the left operand’s buffer is large enough to hold the result, no allocation is needed.

In an unfortunate lack of symmetry, the left operand of + cannot be a &str, so you cannot write:

let parenthetical = "(" + string + ")";

You must instead write:

let parenthetical = "(".to_string() + string + ")";

However, this restriction does discourage building up strings from the end backward. This approach performs poorly because the text must be repeatedly shifted toward the end of the buffer.

Building strings from beginning to end by appending small pieces, however, is efficient. A String behaves the way a vector does, always at least doubling its buffer’s size when it needs more capacity. As explained in “Building Vectors Element by Element”, this keeps recopying overhead proportional to the final size. Even so, using String::with_capacity to create strings with the right buffer size to begin with avoids resizing at all, and can reduce the number of calls to the heap allocator.

Removing Text

String has a few methods for removing text (these do not affect the string’s capacity; use shrink_to_fit if you need to free memory):

string.clear() resets string to the empty string.

string.truncate(n) discards all characters after the byte offset n, leaving string with a length of at most n. If string is shorter than n bytes, this has no effect.

string.pop() removes the last character from string, if any, and returns it as an Option<char>.

string.remove(i) removes the character at byte offset i from string and returns it, shifting any following characters toward the front. This takes time linear in the number of following characters.

string.drain(range) returns an iterator over the given range of byte indices, and removes the characters once the iterator is dropped. Characters after the range are shifted toward the front:

let mut choco = "chocolate".to_string();
assert_eq!(choco.drain(3..6).collect::<String>(), "col");
assert_eq!(choco, "choate");

If you just want to remove the range, you can just drop the iterator immediately, without drawing any items from it:

let mut winston = "Churchill".to_string();
winston.drain(2..6);
assert_eq!(winston, "Chill");

Conventions for Searching and Iterating

Rust’s standard library functions for searching text and iterating over text follow some naming conventions to make them easier to remember:

Most operations process text from start to end, but operations with names starting with r work from end to start. For example, rsplit is the end-to-start version of split. In some cases changing direction can affect not only the order in which values are produced but also the values themselves. See the diagram in Figure 17-3 for an example of this.

Iterators with names ending in n limit themselves to a given number of matches.

Iterators with names ending in _indices produce, together with their usual iteration values, the byte offsets in the slice at which they appear.

The standard library doesn’t provide all combinations for every operation. For example, many operations don’t need an n variant, as it’s easy enough to simply end the iteration early.

Patterns for Searching Text

When a standard library function needs to search, match, split, or trim text, it accepts several different types to represent what to look for:

let haystack = "One fine day, in the middle of the night";

assert_eq!(haystack.find(','), Some(12));
assert_eq!(haystack.find("night"), Some(35));
assert_eq!(haystack.find(char::is_whitespace), Some(3));

These types are called patterns, and most operations support them:

assert_eq!("## Elephants"
           .trim_left_matches(|ch: char| ch == '#' || ch.is_whitespace()),
           "Elephants");

The standard library supports four main kinds of patterns:

A char as a pattern matches that character.

A String or &str or &&str as a pattern matches a substring equal to the pattern.

A FnMut(char) -> bool closure as a pattern matches a single character for which the closure returns true.

A &[char] as a pattern (not a &str, but a slice of char values) matches any single character that appears in the list. Note that if you write out the list as an array literal, you may need to use an as expression to get the type right:

let code = "\t    function noodle() { ";
assert_eq!(code.trim_left_matches(&[' ', '\t'] as &[char]),
           "function noodle() { ");
// Shorter equivalent: &[' ', '\t'][..]

Otherwise, Rust will be confused by the fixed-size array type &[char; 2], which is unfortunately not a pattern type.

In the library’s own code, a pattern is any type that implements the std::str::Pattern trait. The details of Pattern are not yet stable, so you can’t implement it for your own types in stable Rust, but the door is open to permit regular expressions and other sophisticated patterns in the future. Rust does guarantee that the pattern types supported now will continue to work in the future.

Searching and Replacing

Rust has a few methods for searching for patterns in slices and possibly replacing them with new text:

slice.contains(pattern) returns true if slice contains a match for pattern.

slice.starts_with(pattern) and slice.ends_with(pattern) return true if slice’s initial or final text matches pattern:

assert!("2017".starts_with(char::is_numeric));

slice.find(pattern) and slice.rfind(pattern) return Some(i) if slice contains a match for pattern, where i is the byte offset at which the pattern appears. The find method returns the first match, rfind the last:

let quip = "We also know there are known unknowns";
assert_eq!(quip.find("know"), Some(8));
assert_eq!(quip.rfind("know"), Some(31));
assert_eq!(quip.find("ya know"), None);
assert_eq!(quip.rfind(char::is_uppercase), Some(0));

slice.replace(pattern, replacement) returns a new String formed by replacing all matches for pattern with replacement:

assert_eq!("The only thing we have to fear is fear itself"
           .replace("fear", "spin"),
           "The only thing we have to spin is spin itself");

assert_eq!("`Borrow` and `BorrowMut`"
           .replace(|ch:char| !ch.is_alphanumeric(), ""),
           "BorrowandBorrowMut");

slice.replacen(pattern, replacement, n) does the same, but replaces at most the first n matches.

Iterating over Text

The standard library provides several ways to iterate over a slice’s text. Figure 17-3 shows examples of some.

You can think of the split and match families as being complements of each other: splits are the ranges between matches.

Figure 17-3. Some ways to iterate over a slice

For some kinds of patterns, working from end to start can change the values produced; for an example, see the splits on the pattern "rr" in the figure. Patterns that always match a single character can’t behave this way. When an iterator would produce the same set of items in either direction (that is, when only the order is affected), the iterator is a DoubleEndedIterator, meaning that you can apply its rev method to iterate in the other order, and draw items from either end:

slice.chars() returns an iterator over slice’s characters.

slice.char_indices() returns an iterator over slice’s characters and their byte offsets:

assert_eq!("élan".char_indices().collect::<Vec<_>>(),
           vec![(0, 'é'), // has a two-byte UTF-8 encoding
                (2, 'l'),
                (3, 'a'),
                (4, 'n')]);

Note that this is not equivalent to .chars().enumerate(), since it supplies each character’s byte offset within the slice, instead of just numbering the characters.

slice.bytes() returns an iterator over the individual bytes of slice, exposing the UTF-8 encoding:

assert_eq!("élan".bytes().collect::<Vec<_>>(),
           vec![195, 169, b'l', b'a', b'n']);

slice.lines() returns an iterator over the lines of slice. Lines are terminated by "\n" or "\r\n". Each item produced is a &str borrowing from slice. The items do not include the lines’ terminating characters.

slice.split(pattern) returns an iterator over the portions of slice separated by matches of pattern. This produces empty strings between immediately adjacent matches, as well as for matches at the beginning and end of slice.

The slice.rsplit(pattern) method is the same, but scans slice from end to start, producing matches in that order.

slice.split_terminator(pattern) and slice.rsplit_terminator(pattern) are similar, except that the pattern is treated as a terminator, not a separator: if pattern matches at the right end of slice, the iterators do not produce an empty slice representing the empty string between that match and the end of the slice, as split and rsplit do. For example:

// The ':' characters are separators here. Note the final "".
assert_eq!("jimb:1000:Jim Blandy:".split(':').collect::<Vec<_>>(),
           vec!["jimb", "1000", "Jim Blandy", ""]);

// The '\n' characters are terminators here.
assert_eq!("127.0.0.1  localhost\n\
            127.0.0.1  www.reddit.com\n"
           .split_terminator('\n').collect::<Vec<_>>(),
           vec!["127.0.0.1  localhost",
                "127.0.0.1  www.reddit.com"]);
                // Note, no final ""!

The slice.splitn(n, pattern) and slice.rsplitn(n, pattern) are like split and rsplit, except that they split the string into at most n slices, at the first or last n-1 matches for pattern.

slice.split_whitespace() returns an iterator over the whitespace-separated portions of slice. A run of multiple whitespace characters is considered a single separator. Trailing whitespace is ignored. This uses the same definition of whitespace as char::is_whitespace:

let poem = "This  is  just  to say\n\
            I have eaten\n\
            the plums\n\
            again\n";

assert_eq!(poem.split_whitespace().collect::<Vec<_>>(),
           vec!["This", "is", "just", "to", "say",
                "I", "have", "eaten", "the", "plums",
                "again"]);

slice.matches(pattern) returns an iterator over the matches for pattern in slice. slice.rmatches(pattern) is the same, but iterates from end to start.

slice.match_indices(pattern) and slice.rmatch_indices(pattern) are similar, except that the items produced are (offset, match) pairs, where offset is the byte offset at which the match begins, and match is the matching slice.

Trimming

To trim a string is to remove text, usually whitespace, from the beginning or end of the string. It’s often useful in cleaning up input read from a file where the user might have indented text for legibility, or accidentally left trailing whitespace on a line.

slice.trim() returns a subslice of slice that omits any leading and trailing whitespace. slice.trim_left() omits only leading whitespace, slice.trim_right() only trailing whitespace:

assert_eq!("\t*.rs  ".trim(), "*.rs");
assert_eq!("\t*.rs  ".trim_left(), "*.rs  ");
assert_eq!("\t*.rs  ".trim_right(), "\t*.rs");

slice.trim_matches(pattern) returns a subslice of slice that omits all matches of pattern from the beginning and end. The trim_left_matches and trim_right_matches methods do the same for only leading or trailing matches:

assert_eq!("001990".trim_left_matches('0'), "1990");

Note that the terms left and right in these methods’ names always refer to the start and end of the slice, respectively, regardless of the directionality of the text they hold.

Case Conversion for Strings

The methods slice.to_uppercase() and slice.to_lowercase() return a freshly allocated string holding the text of slice converted to uppercase or lowercase. The result may not be the same length as slice; see “Case Conversion for Characters” for details.

Parsing Other Types from Strings

Rust provides standard traits for both parsing values from strings and producing textual representations of values.

If a type implements the std::str::FromStr trait, then it provides a standard way to parse a value from a string slice:

pub trait FromStr: Sized {
    type Err;
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

All the usual machine types implement FromStr:

use std::str::FromStr;

assert_eq!(usize::from_str("3628800"), Ok(3628800));
assert_eq!(f64::from_str("128.5625"), Ok(128.5625));
assert_eq!(bool::from_str("true"), Ok(true));

assert!(f64::from_str("not a float at all").is_err());
assert!(bool::from_str("TRUE").is_err());

The std::net::IpAddr type, an enum holding either an IPv4 or an IPv6 internet address, implements FromStr too:

use std::net::IpAddr;

let address = IpAddr::from_str("fe80::0000:3ea9:f4ff:fe34:7a50")?;
assert_eq!(address,
           IpAddr::from([0xfe80, 0, 0, 0, 0x3ea9, 0xf4ff, 0xfe34, 0x7a50]));

String slices have a parse method that parses the slice into whatever type you like, assuming it implements FromStr. As with Iterator::collect, you will sometimes need to spell out which type you want, so parse is not always much more legible than calling from_str directly:

let address = "fe80::0000:3ea9:f4ff:fe34:7a50".parse::<IpAddr>()?;

Converting Other Types to Strings

There are three main ways to convert nontextual values to strings:

Types that have a natural human-readable printed form can implement the std::fmt::Display trait, which lets you use the {} format specifier in the format! macro:

assert_eq!(format!("{}, wow", "doge"), "doge, wow");
assert_eq!(format!("{}", true), "true");
assert_eq!(format!("({:.3}, {:.3})", 0.5, f64::sqrt(3.0)/2.0),
           "(0.500, 0.866)");

// Using `address` from above.
let formatted_addr: String = format!("{}", address);
assert_eq!(formatted_addr, "fe80::3ea9:f4ff:fe34:7a50");

All Rust’s machine numeric types implement Display, as do characters, strings, and slices. The smart pointer types Box<T>, Rc<T>, and Arc<T> implement Display if T itself does: their displayed form is simply that of their referent. Containers like Vec and HashMap do not implement Display, as there’s no single natural human-readable form for those types.

If a type implements Display, the standard library automatically implements the std::str::ToString trait for it, whose sole method to_string can be more convenient when you don’t need the flexibility of format!:

// Continued from above.
assert_eq!(address.to_string(), "fe80::3ea9:f4ff:fe34:7a50");

The ToString trait predates the introduction of Display and is less flexible. For your own types, you should generally implement Display instead of ToString.

Every public type in the standard library implements std::fmt::Debug, which takes a value and formats it as a string in a way helpful to programmers. The easiest way to use Debug to produce a string is via the format! macro’s {:?} format specifier:

// Continued from above.
let addresses = vec![address,
                     IpAddr::from_str("192.168.0.1")?];
assert_eq!(format!("{:?}", addresses),
           "[V6(fe80::3ea9:f4ff:fe34:7a50), V4(192.168.0.1)]");

This takes advantage of a blanket implementation of Debug for Vec<T>, for any T that itself implements Debug. All of Rust’s collection types have such implementations.

You should implement Debug for your own types, too. Usually it’s best to let Rust derive an implementation, as we did for the Complex type earlier in the book:

#[derive(Copy, Clone, Debug)]
struct Complex { r: f64, i: f64 }

The Display and Debug formatting traits are just two among several that the format! macro and its relatives use to format values as text. We’ll cover the others, and explain how to implement them all, in “Formatting Values”.

Borrowing as Other Text-Like Types

You can borrow a slice’s contents in several different ways:

Slices and Strings implement AsRef<str>, AsRef<[u8]>, AsRef<Path>, and AsRef<OsStr>. Many standard library functions use these traits as bounds on their parameter types, so you can pass slices and strings to them directly, even when what they really want is some other type. See “AsRef and AsMut” for a more detailed explanation.

Slices and strings also implement the std::borrow::Borrow<str> trait. HashMap and BTreeMap use Borrow to make Strings work nicely as keys in a table, as do functions like [T]::binary_search. See “Borrow and BorrowMut” for details.

Accessing Text as UTF-8

There are two main ways to get at the bytes representing text, depending on whether you want to take ownership of the bytes or just borrow them:

slice.as_bytes() borrows slice’s bytes as a &[u8]. Since this is not a mutable reference, slice can assume its bytes will remain well-formed UTF-8.

string.into_bytes() takes ownership of string and returns a Vec<u8> of the string’s bytes by value. This is a cheap conversion, as it simply hands over the Vec<u8> that the string had been using as its buffer. Since string no longer exists, there’s no need for the bytes to continue to be well-formed UTF-8, and the caller is free to modify the Vec<u8> as it pleases.

Producing Text from UTF-8 Data

If you have a block of bytes that you believe contains UTF-8 data, you have a few options for converting them into Strings or slices, depending on how you want to handle errors:

str::from_utf8(byte_slice) takes a &[u8] slice of bytes and returns a Result: either Ok(&str) if byte_slice contains well-formed UTF-8, or an error otherwise.

String::from_utf8(vec) tries to construct a string from a Vec<u8> passed by value. If vec holds well-formed UTF-8, from_utf8 returns Ok(string), where string has taken ownership of vec for use as its buffer. No heap allocation or copying of the text takes place.

If the bytes are not valid UTF-8, this returns Err(e), where e is a FromUtf8Error error value. The call e.into_bytes() gives you back the original vector vec, so it is not lost when the conversion fails:

let good_utf8: Vec<u8> = vec![0xe9, 0x8c, 0x86];
assert_eq!(String::from_utf8(good_utf8).ok(), Some("錆".to_string()));

let bad_utf8:  Vec<u8> = vec![0x9f, 0xf0, 0xa6, 0x80];
let result = String::from_utf8(bad_utf8);
assert!(result.is_err());
// Since String::from_utf8 failed, it didn't consume the original
// vector, and the error value hands it back to us unharmed.
assert_eq!(result.unwrap_err().into_bytes(),
           vec![0x9f, 0xf0, 0xa6, 0x80]);

String::from_utf8_lossy(byte_slice) tries to construct a String or &str from a &[u8] shared slice of bytes. This conversion always succeeds, replacing any ill-formed UTF-8 with Unicode replacement characters. The return value is a Cow<str> that either borrows a &str directly from byte_slice if it contains well-formed UTF-8, or owns a freshly allocated String with replacement characters substituted for the ill-formed bytes. Hence, when byte_slice is well-formed, no heap allocation or copying takes place. We discuss Cow<str> in more detail in “Putting Off Allocation”.

If you know for a fact that your Vec<u8> contains well-formed UTF-8, then you can call the unsafe function String::from_utf8_unchecked. This simply wraps the Vec<u8> up as a String and returns it, without examining the bytes at all. You are responsible for making sure you haven’t introduced ill-formed UTF-8 into the system, which is why this function is marked unsafe.

Similarly, str::from_utf8_unchecked takes a &[u8] and returns it as a &str, without checking to see if it holds well-formed UTF-8. As with String::from​_utf8_unchecked, you are responsible for making sure this is safe.

Putting Off Allocation

Suppose you want your program to greet the user. On Unix, you could write:

fn get_name() -> String {
    std::env::var("USER") // Windows uses "USERNAME"
        .unwrap_or("whoever you are".to_string())
}

println!("Greetings, {}!", get_name());

For Unix users, this greets them by username. For Windows users and the tragically unnamed, it provides alternative stock text.

The std::env::var function returns a String—and has good reasons to do so that we won’t go into here. But that means the alternative stock text must also be returned as a String. This is disappointing: when get_name returns a static string, no allocation should be necessary at all.

The nub of the problem is that sometimes the return value of name should be an owned String, sometimes it should be a &'static str, and we can’t know which one it will be until we run the program. This dynamic character is the hint to consider using std::borrow::Cow, the clone-on-write type that can hold either owned or borrowed data.

As explained in “Borrow and ToOwned at Work: The Humble Cow”, Cow<'a, T> is an enum with two variants: Owned and Borrowed. Borrowed holds a reference &'a T, and Owned holds the owning version of &T: String for &str, Vec<i32> for &[i32], and so on. Whether Owned or Borrowed, a Cow<'a, T> can always produce a &T for you to use. In fact, Cow<'a, T> dereferences to &T, behaving as a kind of smart pointer.

Changing get_name to return a Cow results in the following:

use std::borrow::Cow;

fn get_name() -> Cow<'static, str> {
    std::env::var("USER")
        .map(|v| Cow::Owned(v))
        .unwrap_or(Cow::Borrowed("whoever you are"))
}

If this succeeds in reading the "USER" environment variable, the map returns the resulting String as a Cow::Owned. If it fails, the unwrap_or returns its static &str as a Cow::Borrowed. The caller can remain unchanged:

println!("Greetings, {}!", get_name());

As long as T implements the std::fmt::Display trait, displaying a Cow<'a, T> produces the same results as displaying a T.

Cow is also useful when you may or may not need to modify some text you’ve borrowed. When no changes are necessary, you can continue to borrow it. But Cows namesake clone-on-write behavior can give you an owned, mutable copy of the value on demand. Cow’s to_mut method makes sure the Cow is Cow::Owned, applying the value’s ToOwned implementation if necessary, and then returns a mutable reference to the value.

So if you find that some of your users, but not all, have titles by which they would prefer to be addressed, you can say:

fn get_title() -> Option<&'static str> { ... }

let mut name = get_name();
if let Some(title) = get_title() {
    name.to_mut().push_str(", ");
    name.to_mut().push_str(title);
}

println!("Greetings, {}!", name);

This might produce output like the following:

$ cargo run
Greetings, jimb, Esq.!
$

What’s nice here is that, if get_name() returns a static string and get_title returns None, the Cow simply carries the static string all the way through to the println!. You’ve managed to put off allocation unless it’s really necessary, while still writing straightforward code.

Since Cow is frequently used for strings, the standard library has some special support for Cow<'a, str>. It provides From and Into conversions from both String and &str, so you can write get_name more tersely:

fn get_name() -> Cow<'static, str> {
    std::env::var("USER")
        .map(|v| v.into())
        .unwrap_or("whoever you are".into())
}

Cow<'a, str> also implements std::ops::Add and std::ops::AddAssign, so to add the title to the name, you could write:

if let Some(title) = get_title() {
    name += ", ";
    name += title;
}

Or, since a String can be a write! macro’s destination:

use std::fmt::Write;

if let Some(title) = get_title() {
    write!(name.to_mut(), ", {}", title).unwrap();
}

As before, no allocation occurs until you try to modify the Cow.

Keep in mind that not every Cow<..., str> must be 'static: you can use Cow to borrow previously computed text until the moment a copy becomes necessary.

Strings as Generic Collections

String implements both std::default::Default and std::iter::Extend: default returns an empty string, and extend can append characters, string slices, or strings to the end of a string. This is the same combination of traits implemented by Rust’s other collection types like Vec and HashMap for generic construction patterns such as collect and partition.

The &str type also implements Default, returning an empty slice. This is handy in some corner cases; for example, it lets you derive Default for structures containing string slices.

Formatting Values

Throughout the book, we’ve been using text formatting macros like println!:

println!("{:.3}µs: relocated {} at {:#x} to {:#x}, {} bytes",
         0.84391, "object",
         140737488346304_usize, 6299664_usize, 64);

That call produces the following output:

0.844µs: relocated object at 0x7fffffffdcc0 to 0x602010, 64 bytes

The string literal serves as a template for the output: each {...} in the template gets replaced by the formatted form of one of the following arguments. The template string must be a constant, so that Rust can check it against the types of the arguments at compile time. Each argument must be used; Rust reports a compile-time error otherwise.

Several standard library features share this little language for formatting strings:

The format! macro uses it to build Strings.

The println! and print! macros write formatted text to the standard output stream.

The writeln! and write! macros write it to a designated output stream.

The panic! macro uses it to build a (hopefully informative) expression of terminal dismay.

Rust’s formatting facilities are designed to be open-ended. You can extend these macros to support your own types by implementing the std::fmt module’s formatting traits. And you can use the format_args! macro and the std::fmt::Arguments type to make your own functions and macros support the formatting language.

Formatting macros always borrow shared references to their arguments; they never take ownership of them or mutate them.

The template’s {...} forms are called format parameters, and have the form {which:how}. Both parts are optional; {} is frequently used.

The which value selects which argument following the template should take the parameter’s place. You can select arguments by index or by name. Parameters with no which value are simply paired with arguments from left to right.

The how value says how the argument should be formatted: how much padding, to which precision, in which numeric radix, and so on. If how is present, the colon before it is required.

Here are some examples:

Template string | Argument list | Result
"number of {}: {}" | "elephants", 19 | "number of elephants: 19"
"from {1} to {0}" | "the grave", "the cradle" | "from the cradle to the grave"
"v = {:?}" | vec![0,1,2,5,12,29] | "v = [0, 1, 2, 5, 12, 29]"
"name = {:?}" | "Nemo" | "name = \"Nemo\""
"{:8.2} km/s" | 11.186 | " 11.19 km/s"
"{:20} {:02x} {:02x}" | "adc #42", 105, 42 | "adc #42 69 2a"
"{1:02x} {2:02x} {0}" | "adc #42", 105, 42 | "69 2a adc #42"
"{lsb:02x} {msb:02x} {insn}" | insn="adc #42", lsb=105, msb=42 | "69 2a adc #42"

If you want to include '{' or '}' characters in your output, double the characters in the template:

assert_eq!(format!("{{a, c}} ⊂ {{a, b, c}}"),
           "{a, c} ⊂ {a, b, c}");

Formatting Text Values

When formatting a textual type like &str or String (char is treated like a single-character string), the how value of a parameter has several parts, all optional.

A text length limit. Rust truncates your argument if it is longer than this. If you specify no limit, Rust uses the full text.

A minimum field width. After any truncation, if your argument is shorter than this, Rust pads it on the right (by default) with spaces (by default) to make a field of this width. If omitted, Rust doesn’t pad your argument.

An alignment. If your argument needs to be padded to meet the minimum field width, this says where your text should be placed within the field. <, ^, and > put your text at the start, middle, and end, respectively.

A padding character to use in this padding process. If omitted, Rust uses spaces. If you specify the padding character, you must also specify the alignment.

Here are some examples showing how to write things out, and their effects. All are using the same eight-character argument, "bookends":

Features in use | Template string | Result
Default | "{}" | "bookends"
Minimum field width | "{:4}" | "bookends"
| "{:12}" | "bookends "
Text length limit | "{:.4}" | "book"
| "{:.12}" | "bookends"
Field width, length limit | "{:12.20}" | "bookends "
| "{:4.20}" | "bookends"
| "{:4.6}" | "booken"
| "{:6.4}" | "book "
Aligned left, width | "{:<12}" | "bookends "
Centered, width | "{:^12}" | " bookends "
Aligned right, width | "{:>12}" | " bookends"
Pad with '=', centered, width | "{:=^12}" | "==bookends=="
Pad '*', aligned right, width, limit | "{:*>12.4}" | "********book"

Rust’s formatter has a naïve understanding of width: it assumes each character occupies one column, with no regard for combining characters, half-width katakana, zero-width spaces, or the other messy realities of Unicode. For example:

assert_eq!(format!("{:4}", "th\u{e9}"),   "th\u{e9} ");
assert_eq!(format!("{:4}", "the\u{301}"), "the\u{301}");

Although Unicode says these strings are both equivalent to "thé", Rust’s formatter doesn’t know that characters like '\u{301}', COMBINING ACUTE ACCENT, need special treatment. It pads the first string correctly, but assumes the second is four columns wide and adds no padding. Although it’s easy to see how Rust could improve in this specific case, true multilingual text formatting for all of Unicode’s scripts is a monumental task, best handled by relying on your platform’s user interface toolkits, or perhaps by generating HTML and CSS and making a web browser sort it all out.

Along with &str and String, you can also pass formatting macros smart pointer types with textual referents, like Rc<String> or Cow<'a, str>, without ceremony.

Since filename paths are not necessarily well-formed UTF-8, std::path::Path isn’t quite a textual type; you can’t pass a std::path::Path directly to a formatting macro. However, a Path’s display method returns a value you can format that sorts things out in a platform-appropriate way:

println!("processing file: {}", path.display());

Formatting Numbers

When the formatting argument has a numeric type like usize or f64, the parameter’s how value has the following parts, all optional:

A padding and alignment, which work as they do with textual types.

A + character, requesting that the number’s sign always be shown, even when the argument is positive.

A # character, requesting an explicit radix prefix like 0x or 0b. See the “notation” bullet point that concludes this list.

A 0 character, requesting that the minimum field width be satisfied by including leading zeros in the number, instead of the usual padding approach.

A minimum field width. If the formatted number is not at least this wide, Rust pads it on the left (by default) with spaces (by default) to make a field of the given width.

A precision for floating-point arguments, indicating how many digits Rust should include after the decimal point. Rust rounds or zero-extends as necessary to produce exactly this many fractional digits. If the precision is omitted, Rust tries to accurately represent the value using as few digits as possible. For arguments of integer type, the precision is ignored.

A notation. For integer types, this can be b for binary, o for octal, or x or X for hexadecimal with lower- or uppercase letters. If you included the # character, these include an explicit Rust-style radix prefix, 0b, 0o, 0x, or 0X. For floating-point types, a radix of e or E requests scientific notation, with a normalized coefficient, using e or E for the exponent. If you don’t specify any notation, Rust formats numbers in decimal.

Some examples of formatting the i32 value 1234:

Features in use | Template string | Result
Default | "{}" | "1234"
Forced sign | "{:+}" | "+1234"
Minimum field width | "{:12}" | " 1234"
| "{:2}" | "1234"
Sign, width | "{:+12}" | " +1234"
Leading zeros, width | "{:012}" | "000000001234"
Sign, zeros, width | "{:+012}" | "+00000001234"
Aligned left, width | "{:<12}" | "1234 "
Centered, width | "{:^12}" | " 1234 "
Aligned right, width | "{:>12}" | " 1234"
Aligned left, sign, width | "{:<+12}" | "+1234 "
Centered, sign, width | "{:^+12}" | " +1234 "
Aligned right, sign, width | "{:>+12}" | " +1234"
Padded with '=', centered, width | "{:=^12}" | "====1234===="
Binary notation | "{:b}" | "10011010010"
Width, octal notation | "{:12o}" | " 2322"
Sign, width, hexadecimal notation | "{:+12x}" | " +4d2"
Sign, width, hex with capital digits | "{:+12X}" | " +4D2"
Sign, explicit radix prefix, width, hex | "{:+#12x}" | " +0x4d2"
Sign, radix, zeros, width, hex | "{:+#012x}" | "+0x0000004d2"
| "{:+#06x}" | "+0x4d2"

As the last two examples show, the minimum field width applies to the entire number, sign, radix prefix, and all.

Negative numbers always include their sign. The results are like those shown in the “forced sign” examples.

When you request leading zeros, alignment and padding characters are simply ignored, since the zeros expand the number to fill the entire field.

Using the argument 1234.5678, we can show effects specific to floating-point types:

Features in use | Template string | Result
Default | "{}" | "1234.5678"
Precision | "{:.2}" | "1234.57"
| "{:.6}" | "1234.567800"
Minimum field width | "{:12}" | " 1234.5678"
Minimum, precision | "{:12.2}" | " 1234.57"
| "{:12.6}" | " 1234.567800"
Leading zeros, minimum, precision | "{:012.6}" | "01234.567800"
Scientific | "{:e}" | "1.2345678e3"
Scientific, precision | "{:.3e}" | "1.235e3"
Scientific, minimum, precision | "{:12.3e}" | " 1.235e3"
| "{:12.3E}" | " 1.235E3"

Formatting Other Types

Beyond strings and numbers, you can format several other standard library types:

Error types can all be formatted directly, making it easy to include them in error messages. Every error type should implement the std::error::Error trait, which extends the default formatting trait std::fmt::Display. As a consequence, any type that implements Error is ready to format.

You can format internet protocol address types like std::net::IpAddr and std::net::SocketAddr.

The Boolean true and false values can be formatted, although these are usually not the best strings to present directly to end users.

You should use the same sorts of format parameters that you would for strings. Length limit, field width, and alignment controls work as expected.

Formatting Values for Debugging

To help with debugging and logging, the {:?} parameter formats any public type in the Rust standard library in a way meant to be helpful to programmers. You can use this to inspect vectors, slices, tuples, hash tables, threads, and hundreds of other types.

For example, you can write the following:

use std::collections::HashMap;
let mut map = HashMap::new();
map.insert("Portland", (45.5237606,-122.6819273));
map.insert("Taipei",   (25.0375167, 121.5637));
println!("{:?}", map);

This prints:

{"Taipei": (25.0375167, 121.5637), "Portland": (45.5237606, -122.6819273)}

The HashMap and (f64, f64) types already know how to format themselves, with no effort required on your part.

If you include the # character in the format parameter, Rust will pretty-print the value. Changing this code to say println!("{:#?}", map) leads to this output:

{
    "Taipei": (
        25.0375167,
        121.5637
    ),
    "Portland": (
        45.5237606,
        -122.6819273
    )
}

These exact forms aren’t guaranteed, and do sometimes change from one Rust release to the next.

As we’ve mentioned, you can use the #[derive(Debug)] syntax to make your own types work with {:?}:

#[derive(Copy, Clone, Debug)]
struct Complex { r: f64, i: f64 }

With this definition in place, we can use a {:?} format to print Complex values:

let third = Complex { r: -0.5, i: f64::sqrt(0.75) };
println!("{:?}", third);

This prints:

Complex { r: -0.5, i: 0.8660254037844386 }

This is fine for debugging, but it might be nice if {} could print them in a more traditional form, like -0.5 + 0.8660254037844386i. In “Formatting Your Own Types”, we’ll show how to do exactly that.

Formatting Pointers for Debugging

Normally, if you pass any sort of pointer to a formatting macro—a reference, a Box, an Rc—the macro simply follows the pointer and formats its referent; the pointer itself is not of interest. But when you’re debugging, it’s sometimes helpful to see the pointer: an address can serve as a rough “name” for an individual value, which can be illuminating when examining structures with cycles or sharing.

The {:p} notation formats references, boxes, and other pointer-like types as addresses:

use std::rc::Rc;
let original = Rc::new("mazurka".to_string());
let cloned = original.clone();
let impostor = Rc::new("mazurka".to_string());
println!("text:     {}, {}, {}",       original, cloned, impostor);
println!("pointers: {:p}, {:p}, {:p}", original, cloned, impostor);

This code prints:

text:     mazurka, mazurka, mazurka
pointers: 0x7f99af80e000, 0x7f99af80e000, 0x7f99af80e030

Of course, the specific pointer values will vary from run to run, but even so, comparing the addresses makes it clear that the first two are references to the same String, whereas the third points to a distinct value.

Addresses do tend to look like hexadecimal soup, so more refined visualizations can be worthwhile, but the {:p} style can still be an effective quick-and-dirty solution.

Referring to Arguments by Index or Name

A format parameter can explicitly select which argument it uses. For example:

assert_eq!(format!("{1},{0},{2}", "zeroth", "first", "second"),
           "first,zeroth,second");

You can include format parameters after a colon:

assert_eq!(format!("{2:#06x},{1:b},{0:=>10}", "first", 10, 100),
           "0x0064,1010,=====first");

You can also select arguments by name. This makes complex templates with many parameters much more legible. For example:

assert_eq!(format!("{description:.<25}{quantity:2} @ {price:5.2}",
                   price=3.25,
                   quantity=3,
                   description="Maple Turmeric Latte"),
           "Maple Turmeric Latte..... 3 @  3.25");

(The named arguments here resemble keyword arguments in Python, but this is just a special feature of the formatting macros, not part of Rust’s function call syntax.)

You can mix indexed, named, and positional (that is, no index or name) parameters together in a single formatting macro use. The positional parameters are paired with arguments from left to right as if the indexed and named parameters weren’t there:

assert_eq!(format!("{mode} {2} {} {}",
                   "people", "eater", "purple", mode="flying"),
           "flying purple people eater");

Named arguments must appear at the end of the list.

Dynamic Widths and Precisions

A parameter’s minimum field width, text length limit, and numeric precision need not always be fixed values; you can choose them at runtime.

We’ve been looking at cases like this expression, which gives you the string content right-justified in a field 20 characters wide:

format!("{:>20}", content)

But if you’d like to choose the field width at runtime, you can write:

format!("{:>1$}", content, get_width())

Writing 1$ for the minimum field width tells format! to use the value of the second argument as the width. The cited argument must be a usize. You can also refer to the argument by name:

format!("{:>width$}", content, width=get_width())

The same approach works for the text length limit as well:

format!("{:>width$.limit$}", content,
        width=get_width(), limit=get_limit())

In place of the text length limit or floating-point precision, you can also write *, which says to take the next positional argument as the precision. The following clips content to at most get_limit() characters:

format!("{:.*}", get_limit(), content)

The argument taken as the precision must be a usize. There is no corresponding syntax for the field width.

Formatting Your Own Types

The formatting macros use a set of traits defined in the std::fmt module to convert values to text. You can make Rust’s formatting macros format your own types by implementing one or more of these traits yourself.

The notation of a format parameter indicates which trait its argument’s type must implement:

Notation | Example | Trait | purpose
none | {} | std::fmt::Display | Text, numbers, errors: the catch-all trait
b | {bits:#b} | std::fmt::Binary | Numbers in binary
o | {:#5o} | std::fmt::Octal | Numbers in octal
x | {:4x} | std::fmt::LowerHex | Numbers in hexadecimal, lower-case digits
X | {:016X} | std::fmt::UpperHex | Numbers in hexadecimal, upper-case digits
e | {:.3e} | std::fmt::LowerExp | Floating-point numbers in scientific notation
E | {:.3E} | std::fmt::UpperExp | Same, upper-case E
? | {:#?} | std::fmt::Debug | Debugging view, for developers
p | {:p} | std::fmt::Pointer | Pointer as address, for developers

When you put the #[derive(Debug)] attribute on a type definition so that you can use the {:?} format parameter, you are simply asking Rust to implement the std::fmt::Debug trait for you.

The formatting traits all have the same structure, differing only in their names. We’ll use std::fmt::Display as a representative:

trait Display {
    fn fmt(&self, dest: &mut std::fmt::Formatter)
        -> std::fmt::Result;
}

The fmt method’s job is to produce a properly formatted representation of self and write its characters to dest. In addition to serving as an output stream, the dest argument also carries details parsed from the format parameter, like the alignment and minimum field width.

For example, earlier in this chapter we suggested that it would be nice if Complex values printed themselves in the usual a + bi form. Here’s a Display implementation that does that:

use std::fmt;

impl fmt::Display for Complex {
    fn fmt(&self, dest: &mut fmt::Formatter) -> fmt::Result {
        let i_sign = if self.i < 0.0 { '-' } else { '+' };
        write!(dest, "{} {} {}i", self.r, i_sign, f64::abs(self.i))
    }
}

This takes advantage of the fact that Formatter is itself an output stream, so the write! macro can do most of the work for us. With this implementation in place, we can write the following:

let one_twenty = Complex { r: -0.5, i: 0.866 };
assert_eq!(format!("{}", one_twenty),
           "-0.5 + 0.866i");

let two_forty = Complex { r: -0.5, i: -0.866 };
assert_eq!(format!("{}", two_forty),
           "-0.5 - 0.866i");

It’s sometimes helpful to display complex numbers in polar form: if you imagine a line drawn on the complex plane from the origin to the number, the polar form gives the line’s length, and its clockwise angle to the positive x-axis. The # character in a format parameter typically selects some alternate display form; the Display implementation could treat it as a request to use polar form:

impl fmt::Display for Complex {
    fn fmt(&self, dest: &mut fmt::Formatter) -> fmt::Result {
        let (r, i) = (self.r, self.i);
        if dest.alternate() {
            let abs = f64::sqrt(r * r + i * i);
            let angle = f64::atan2(i, r) / std::f64::consts::PI * 180.0;
            write!(dest, "{} ∠ {}°", abs, angle)
        } else {
            let i_sign = if i < 0.0 { '-' } else { '+' };
            write!(dest, "{} {} {}i", r, i_sign, f64::abs(i))
        }
    }
}

Using this implementation:

let ninety = Complex { r: 0.0, i: 2.0 };
assert_eq!(format!("{}", ninety),
           "0 + 2i");
assert_eq!(format!("{:#}", ninety),
           "2 ∠ 90°");

Although the formatting traits’ fmt methods return a fmt::Result value (a typical module-specific Result type), you should propagate failures only from operations on the Formatter, as the fmt::Display implementation does with its calls to write!; your formatting functions must never originate errors themselves. This allows macros like format! to simply return a String instead of a Result<String, ...>, since appending the formatted text to a String never fails. It also ensures that any errors you do get from write! or writeln! reflect real problems from the underlying I/O stream, not formatting issues.

Formatter has plenty of other helpful methods, including some for handling structured data like maps, lists, and so on, which we won’t cover here; consult the online documentation for the full details.

Using the Formatting Language in Your Own Code

You can write your own functions and macros that accept format templates and arguments by using Rust’s format_args! macro and the std::fmt::Arguments type. For example, suppose your program needs to log status messages as it runs, and you’d like to use Rust’s text formatting language to produce them. The following would be a start:

fn logging_enabled() -> bool {
    ...
}

use std::fs::OpenOptions;
use std::io::Write;

fn write_log_entry(entry: std::fmt::Arguments) {
    if logging_enabled() {
        // Keep things simple for now, and just
        // open the file every time.
        let mut log_file = OpenOptions::new()
            .append(true)
            .create(true)
            .open("log-file-name")
            .expect("failed to open log file");

        log_file.write_fmt(entry)
            .expect("failed to write to log");
    }
}

You can call write_log_entry like so:

write_log_entry(format_args!("Hark! {:?}\n", mysterious_value));

At compile time, the format_args! macro parses the template string and checks it against the arguments’ types, reporting an error if there are any problems. At runtime, it evaluates the arguments and builds an Arguments value carrying all the information necessary to format the text: a pre-parsed form of the template, along with shared references to the argument values.

Constructing an Arguments value is cheap: it’s just gathering up some pointers. No formatting work takes place yet, only the collection of the information needed to do so later. This can be important: if logging is not enabled, any time spent converting numbers to decimal, padding values, and so on would be wasted.

The File type implements the std::io::Write trait, whose write_fmt method takes an Argument and does the formatting. It writes the results to the underlying stream.

That call to write_log_entry isn’t pretty. This is where a macro can help:

macro_rules! log { // no ! needed after name in macro definitions
    ($format:tt, $($arg:expr),*) => (
        write_log_entry(format_args!($format, $($arg),*))
    )
}

We cover macros in detail in Chapter 20. For now, take it on faith that this defines a new log! macro that passes its arguments along to format_args!, and then calls your write_log_entry function on the resulting Arguments value. The formatting macros like println!, writeln!, and format! are all roughly the same idea.

You can use log! like so:

log!("O day and night, but this is wondrous strange! {:?}\n",
     mysterious_value);

Hopefully, this looks a little better.

Regular Expressions

The external regex crate is Rust’s official regular expression library. It provides the usual searching and matching functions. It has good support for Unicode, but it can search byte strings as well. Although it doesn’t support some features you’ll often find in other regular expression packages, like backreferences and look-around patterns, those simplifications allow regex to ensure that searches take time linear in the size of the expression and in the length of the text being searched. These guarantees, among others, make regex safe to use even with untrusted expressions searching untrusted text.

In this book, we’ll provide only an overview of regex; you should consult its online documentation for details.

Although the regex crate is not in std, it is maintained by the Rust library team, the same group responsible for std. To use regex, put the following line in the [dependencies] section of your crate’s Cargo.toml file:

regex = "0.2.2"

Then place an extern crate item in your crate’s root:

extern crate regex;

In the following sections, we’ll assume that you have these changes in place.

Basic Regex Use

A Regex value represents a parsed regular expression, ready to use. The Regex::new constructor tries to parse a &str as a regular expression, and returns a Result:

use regex::Regex;

// A semver version number, like 0.2.1.
// May contain a pre-release version suffix, like 0.2.1-alpha.
// (No build metadata suffix, for brevity.)
//
// Note use of r"..." raw string syntax, to avoid backslash blizzard.
let semver = Regex::new(r"(\d+)\.(\d+)\.(\d+)(-[-.[:alnum:]]*)?")?;

// Simple search, with a Boolean result.
let haystack = r#"regex = "0.2.5""#;
assert!(semver.is_match(haystack));

The Regex::captures method searches a string for the first match, and returns a regex::Captures value holding match information for each group in the expression:

// You can retrieve capture groups:
let captures = semver.captures(haystack)
    .ok_or("semver regex should have matched")?;
assert_eq!(&captures[0], "0.2.5");
assert_eq!(&captures[1], "0");
assert_eq!(&captures[2], "2");
assert_eq!(&captures[3], "5");

Indexing a Captures value panics if the requested group didn’t match. To test whether a particular group matched, you can call Captures::get, which returns an Option<regex::Match>. A Match value records a single group’s match:

assert_eq!(captures.get(4), None);
assert_eq!(captures.get(3).unwrap().start(), 13);
assert_eq!(captures.get(3).unwrap().end(), 14);
assert_eq!(captures.get(3).unwrap().as_str(), "5");

You can iterate over all the matches in a string:

let haystack = "In the beginning, there was 1.0.0. \
                For a while, we used 1.0.1-beta, \
                but in the end, we settled on 1.2.4.";

let matches: Vec<&str> = semver.find_iter(haystack)
    .map(|match_| match_.as_str())
    .collect();
assert_eq!(matches, vec!["1.0.0", "1.0.1-beta", "1.2.4"]);

The find_iter iterator produces a Match value for each nonoverlapping match of the expression, working from the start of the string to the end. The captures_iter method is similar, but produces Captures values recording all capture groups. Searching is slower when capture groups must be reported, so if you don’t need them, it’s best to use one of the methods that doesn’t return them.

Building Regex Values Lazily

The Regex::new constructor can be expensive: constructing a Regex for a 1200-character regular expression can take almost a millisecond on a fast developer machine, and even a trivial expression takes microseconds. It’s best to keep Regex construction out of heavy computational loops; instead, you should construct your Regex once, and then reuse the same one.

The lazy_static crate provides a nice way to construct static values lazily the first time they are used. To start with, note the dependency in your Cargo.toml file:

[dependencies]
lazy_static = "0.2.8"

This crate provides a macro to declare such variables:

#[macro_use]
extern crate lazy_static;

lazy_static! {
    static ref SEMVER: Regex
        = Regex::new(r"(\d+)\.(\d+)\.(\d+)(-[-.[:alnum:]]*)?")
              .expect("error parsing regex");
}

The macro expands to a declaration of a static variable named SEMVER, but its type is not exactly Regex. Instead, it’s a macro-generated type that implements Deref<Target=Regex> and therefore exposes all the same methods as a Regex. The first time SEMVER is dereferenced, the initializer is evaluated, and the value saved for later use. Since SEMVER is a static variable, not just a local variable, the initializer runs at most once per program execution.

With this declaration in place, using SEMVER is straightforward:

use std::io::BufRead;

let stdin = std::io::stdin();
for line in stdin.lock().lines() {
    let line = line?;
    if let Some(match_) = SEMVER.find(&line) {
        println!("{}", match_.as_str());
    }
}

You can put the lazy_static! declaration in a module, or even inside the function that uses the Regex, if that’s the most appropriate scope. The regular expression is still always compiled only once per program execution.

Normalization

Most users would consider the French word for tea, thé, to be three characters long. However, Unicode actually has two ways to represent this text:

In the composed form, thé comprises the three characters 't', 'h', and 'é', where 'é' is a single Unicode character with code point 0xe9.

In the decomposed form, thé comprises the four characters 't', 'h', 'e', and '\u{301}', where the 'e' is the plain ASCII character, without an accent, and code point 0x301 is the “COMBINING ACUTE ACCENT” character, which adds an acute accent to whatever character it follows.

Unicode does not consider either the composed or the decomposed form of é to be the “correct” one; rather, it considers them both equivalent representations of the same abstract character. Unicode says both forms should be displayed in the same way, and text input methods are permitted to produce either, so users will generally not know which form they are viewing or typing. (Rust lets you use Unicode characters directly in string literals, so you can simply write "thé" if you don’t care which encoding you get. Here we’ll use the \u escapes for clarity.)

However, considered as Rust &str or String values, "th\u{e9}" and "the\u{301}" are completely distinct. They have different lengths, compare as unequal, have different hash values, and order themselves differently with respect to other strings:

assert!("th\u{e9}" != "the\u{301}");
assert!("th\u{e9}" >  "the\u{301}");

// A Hasher is designed to accumulate the hash of a series of values,
// so hashing just one is a bit clunky.
use std::hash::{Hash, Hasher};
use std::collections::hash_map::DefaultHasher;
fn hash<T: ?Sized + Hash>(t: &T) -> u64 {
    let mut s = DefaultHasher::new();
    t.hash(&mut s);
    s.finish()
}

// These values may change in future Rust releases.
assert_eq!(hash("th\u{e9}"),   0x53e2d0734eb1dff3);
assert_eq!(hash("the\u{301}"), 0x90d837f0a0928144);

Clearly, if you intend to compare user-supplied text, or use it as a key in a hash table or B-tree, you will need to put each string in some canonical form first.

Fortunately, Unicode specifies normalized forms for strings. Whenever two strings should be treated as equivalent according to Unicode’s rules, their normalized forms are character-for-character identical. When encoded with UTF-8, they are byte-for-byte identical. This means you can compare normalized strings with ==, use them as keys in a HashMap or HashSet, and so on, and you’ll get Unicode’s notion of equality.

Failure to normalize can even have security consequences. For example, if your website normalizes usernames in some cases but not others, you could end up with two distinct users named bananasflambé, which some parts of your code treat as the same user, but others distinguish, resulting in one’s privileges being extended incorrectly to the other. Of course, there are many ways to avoid this sort of problem, but history shows there are also many ways not to.

Normalization Forms

Unicode defines four normalized forms, each of which is appropriate for different uses. There are two questions to answer:

First, do you prefer characters to be as composed as possible or as decomposed as possible?

For example, the most composed representation of the Vietnamese word Phở is the three-character string "Ph\u{1edf}", where both the tonal mark ̉ and the vowel mark ̛ are applied to the base character “o” in a single Unicode character, '\u{1edf}', which Unicode dutifully names LATIN SMALL LETTER O WITH HORN AND HOOK ABOVE.

The most decomposed representation splits out the base letter and its two marks into three separate Unicode characters: 'o', '\u{31b}' (COMBINING HORN), and '\u{309}' (COMBINING HOOK ABOVE), resulting in "Pho\u{31b}\u{309}". (Whenever combining marks appear as separate characters, rather than as part of a composed character, all normalized forms specify a fixed order in which they must appear, so normalization is well specified even when characters have multiple accents.)

The composed form generally has fewer compatibility problems, since it more closely matches the representations most languages used for their text before Unicode became established. It may also work better with naïve string formatting features like Rust’s format! macro. The decomposed form, on the other hand, may be better for displaying text or searching, since it makes the detailed structure of the text more explicit.

The second question is: if two character sequences represent the same fundamental text, but differ in the way that text should be formatted, do you want to treat them as equivalent, or keep them distinct?

Unicode has separate characters for the ordinary digit '5', the superscript digit '⁵' (or '\u{2075}'), and the circled digit '⑤' (or '\u{2464}'), but declares all three to be compatibility equivalent. Similarly, Unicode has a single character for the ligature ffi ('\u{fb03}'), but declares this to be compatibility equivalent to the three-character sequence "ffi".

Compatibility equivalence makes sense for searches: a search for "difficult", using only ASCII characters, ought to match the string "di\u{fb03}cult", which uses the ffi ligature. Applying compatibility decomposition to the latter string would replace the ligature with the three plain letters "ffi", making the search easier. But normalizing text to a compatibility equivalent form can lose essential information, so it should not be applied carelessly. For example, it would be incorrect in most contexts to store "2⁵" as "25".

The Unicode Normalization Form C and Normalization Form D (NFC and NFD) use the maximally composed and maximally decomposed forms of each character, but do not try to unify compatibility equivalent sequences. The NFKC and NFKD normalization forms are like NFC and NFD, but normalize all compatibility equivalent sequences to some simple representative of their class.

The World Wide Web Consortium’s “Character Model For the World Wide Web” recommends using NFC for all content. The Unicode Identifier and Pattern Syntax annex suggests using NFKC for identifiers in programming languages, and offers principles for adapting the form when necessary.

The unicode-normalization Crate

Rust’s unicode-normalization crate provides a trait that adds methods to &str to put the text in any of the four normalized forms. To use it, add the following line to the [dependencies] section of your Cargo.toml file:

unicode-normalization = "0.1.5"

The top file of your crate needs an extern crate declaration:

extern crate unicode_normalization;

With these declarations in place, a &str has four new methods that return iterators over a particular normalized form of the string:

use unicode_normalization::UnicodeNormalization;

// No matter what representation the lefthand string uses
// (you shouldn't be able to tell just by looking),
// these assertions will hold.
assert_eq!("Phở".nfd().collect::<String>(), "Pho\u{31b}\u{309}");
assert_eq!("Phở".nfc().collect::<String>(), "Ph\u{1edf}");

// The lefthand side here uses the "ffi" ligature character.
assert_eq!("① Di\u{fb03}culty".nfkc().collect::<String>(), "1 Difficulty");

Taking a normalized string and normalizing it again in the same form is guaranteed to return identical text.

Although any substring of a normalized string is itself normalized, the concatenation of two normalized strings is not necessarily normalized: for example, the second string might start with combining characters that should be placed before combining characters at the end of the first string.

As long as a text uses no unassigned code points when it is normalized, Unicode promises that its normalized form will not change in future versions of the standard. This means that normalized forms are generally safe to use in persistent storage, even as the Unicode standard evolves.

Chapter 18. Input and Output

Doolittle: What concrete evidence do you have that you exist?

Bomb #20: Hmmmm...well...I think, therefore I am.

Doolittle: That’s good. That’s very good. But how do you know that anything else exists?

Bomb #20: My sensory apparatus reveals it to me.

Dark Star

Rust’s standard library features for input and output are organized around three traits—Read, BufRead, and Write—and the various types that implement them:

Values that implement Read have methods for byte-oriented input. They’re called readers.

Values that implement BufRead are buffered readers. They support all the methods of Read, plus methods for reading lines of text and so forth.

Values that implement Write support both byte-oriented and UTF-8 text output. They’re called writers.

Figure 18-1 shows these three traits and some examples of reader and writer types.

In this chapter, we’ll show how to use these traits and their methods, the various types that implement them, and other ways to interact with files, the terminal, and the network.

Figure 18-1. Selected reader and writer types from the Rust standard library

Readers and Writers

Readers are values that your program can read bytes from. Examples include:

Files opened using std::fs::File::open(filename)

std::net::TcpStreams, for receiving data over the network

std::io::stdin(), for reading from the process’s standard input stream

std::io::Cursor<&[u8]> values, which are readers that “read” from a byte array that’s already in memory

Writers are values that your program can write bytes to. Examples include:

Files opened using std::fs::File::create(filename)

std::net::TcpStreams, for sending data over the network

std::io::stdout() and std::io:stderr(), for writing to the terminal

std::io::Cursor<&mut [u8]> values, which let you treat any mutable slice of bytes as a file for writing

Vec<u8>, a writer whose write methods append to the vector

Since there are standard traits for readers and writers (std::io::Read and std::io::Write), it’s quite common to write generic code that works across a variety of input or output channels. For example, here’s a function that copies all bytes from any reader to any writer:

use std::io::{self, Read, Write, ErrorKind};

const DEFAULT_BUF_SIZE: usize = 8 * 1024;

pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W)
    -> io::Result<u64>
    where R: Read, W: Write
{
    let mut buf = [0; DEFAULT_BUF_SIZE];
    let mut written = 0;
    loop {
        let len = match reader.read(&mut buf) {
            Ok(0) => return Ok(written),
            Ok(len) => len,
            Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
            Err(e) => return Err(e),
        };
        writer.write_all(&buf[..len])?;
        written += len as u64;
    }
}

This is the implementation of std::io::copy() from Rust’s standard library. Since it’s generic, you can use it to copy data from a File to a TcpStream, from Stdin to an in-memory Vec<u8>, etc.

If the error-handling code here is unclear, revisit Chapter 7. We’ll be using Results constantly in the pages ahead; it’s important to have a good grasp of how they work.

The four std::io traits Read, BufRead, Write, and Seek are so commonly used that there’s a prelude module containing only those traits:

use std::io::prelude::*;

You’ll see this once or twice in this chapter. We also make a habit of importing the std::io module itself:

use std::io::{self, Read, Write, ErrorKind};

The self keyword here declares io as an alias to the std::io module. That way, std::io::Result and std::io::Error can be written more concisely as io::Result and io::Error, and so on.

Readers

std::io::Read has several methods for reading data. All of them take the reader itself by mut reference.

reader.read(&mut buffer) reads some bytes from the data source and stores them in the given buffer. The type of the buffer argument is &mut [u8]. This reads up to buffer.len() bytes.

The return type is io::Result<u64>, which is a type alias for Result<u64, io::Error>. On success, the u64 value is the number of bytes read—which may be equal to or less than buffer.len(), even if there’s more data to come, at the whim of the data source. Ok(0) means there is no more input to read.

On error, .read() returns Err(err), where err is an io::Error value. An io::Error is printable, for the benefit of humans; for programs, it has a .kind() method that returns an error code of type io::ErrorKind. The members of this enum have names like PermissionDenied and ConnectionReset. Most indicate serious errors that can’t be ignored, but one kind of error should be handled specially. io::ErrorKind::Interrupted corresponds to the Unix error code EINTR, which means the read happened to be interrupted by a signal. Unless the program is designed to do something clever with signals, it should just retry the read. The code for copy(), in the preceding section, shows an example of this.

As you can see, the .read() method is very low-level, even inheriting quirks of the underlying operating system. If you’re implementing the Read trait for a new type of data source, this gives you a lot of leeway. If you’re trying to read some data, it’s a pain. Therefore, Rust provides several higher-level convenience methods. All of them have default implementations in terms of .read(). They all handle ErrorKind::Interrupted, so you don’t have to.

reader.read_to_end(&mut byte_vec) reads all remaining input from this reader, appending it to byte_vec, which is a Vec<u8>. Returns io::Result<()>.

There is no limit on the amount of data this method will pile into the vector, so don’t use it on an untrusted source. (You can impose a limit using the .take() method, described below.)

reader.read_to_string(&mut string) is the same, but append the data to the given String. If the stream isn’t valid UTF-8, this returns an ErrorKind::InvalidData error.

In some languages, byte input and character input are handled by different types. These days, UTF-8 is so dominant that Rust acknowledges this de facto standard and supports UTF-8 everywhere. Other character sets are supported with the open source encoding crate.

reader.read_exact(&mut buf) reads exactly enough data to fill the given buffer. The argument type is &[u8]. If the reader runs out of data before reading buf.len() bytes, this returns an ErrorKind::UnexpectedEof error.

Those are the main methods of the Read trait. In addition, there are four adapter methods that take the reader by value, transforming it into an iterator or a different reader:

reader.bytes() returns an iterator over the bytes of the input stream. The item type is io::Result<u8>, so an error check is required for every byte. Furthermore, this calls reader.read() once per byte, which will be very inefficient if the reader is not buffered.

reader.chars() is the same, but iterates over characters, treating the input as UTF-8. Invalid UTF-8 causes an InvalidData error.

reader.chain(reader2) returns a new reader that produces all the input from reader, followed by all the input from reader2.

reader.take(n) returns a new reader that reads from the same source as reader, but is limited to n bytes of input.

There is no method for closing a reader. Readers and writers typically implement Drop so that they are closed automatically.

Buffered Readers

For efficiency, readers and writers can be buffered, which simply means they have a chunk of memory (a buffer) that holds some input or output data in memory. This saves on system calls, as shown in Figure 18-2. The application reads data from the BufReader, in this example by calling its .read_line() method. The BufReader in turn gets its input in larger chunks from the operating system.

Figure 18-2. A buffered file reader

This picture is not to scale. The actual default size of a BufReader’s buffer is several kilobytes, so a single system read can serve hundreds of .read_line() calls. This matters because system calls are slow.

(As the picture shows, the operating system has a buffer too, for the same reason: system calls are slow, but reading data from a disk is slower.)

Buffered readers implement both Read and a second trait, BufRead, which adds the following methods:

reader.read_line(&mut line) reads a line of text and appends it to line, which is a String. The newline character '\n' at the end of the line is included in line. If the input has Windows-style line endings, "\r\n", both characters are included in line.

The return value is an io::Result<usize>, the number of bytes read, including the line ending, if any.

If the reader is at the end of the input, this leaves line unchanged and returns Ok(0).

reader.lines() returns an iterator over the lines of the input. The item type is io::Result<String>. Newline characters are not included in the strings. If the input has Windows-style line endings, "\r\n", both characters are stripped.

This method is almost always what you want for text input. The next two sections show some examples of its use.

reader.read_until(stop_byte, &mut byte_vec) and reader.split(stop​_byte) are just like .read_line() and .lines(), but byte-oriented, producing Vec<u8>s instead of Strings. You choose the delimiter stop_byte.

BufRead also provides a pair of low-level methods, .fill_buf() and .consume(n), for direct access to the reader’s internal buffer. For more about these methods, see the online documentation.

The next two sections cover buffered readers in more detail.

Reading Lines

Here is a function that implements the Unix grep utility. It searches many lines of text, typically piped in from another command, for a given string:

use std::io;
use std::io::prelude::*;

fn grep(target: &str) -> io::Result<()> {
    let stdin = io::stdin();
    for line_result in stdin.lock().lines() {
        let line = line_result?;
        if line.contains(target) {
            println!("{}", line);
        }
    }
    Ok(())
}

Since we want to call .lines(), we need a source of input that implements BufRead. In this case, we call io::stdin() to get the data that’s being piped to us. However, the Rust standard library protects stdin with a mutex. We call .lock() to lock stdin for the current thread’s exclusive use; it returns a StdinLock value that implements BufRead. At the end of the loop, the StdinLock is dropped, releasing the mutex. (Without a mutex, two threads trying to read from stdin at the same time would cause undefined behavior. C has the same issue and solves it the same way: all of the C standard input and output functions obtain a lock behind the scenes. The only difference is that in Rust, the lock is part of the API.)

The rest of the function is straightforward: it calls .lines() and loops over the resulting iterator. Because this iterator produces Result values, we use the ? operator to check for errors.

Suppose we want to take our grep program a step further and add support for searching files on disk. We can make this function generic:

fn grep<R>(target: &str, reader: R) -> io::Result<()>
    where R: BufRead
{
    for line_result in reader.lines() {
        let line = line_result?;
        if line.contains(target) {
            println!("{}", line);
        }
    }
    Ok(())
}

Now we can pass it either a StdinLock or a buffered File:

let stdin = io::stdin();
grep(&target, stdin.lock())?;  // ok

let f = File::open(file)?;
grep(&target, BufReader::new(f))?;  // also ok

Note that a File is not automatically buffered. File implements Read but not BufRead. However, it’s easy to create a buffered reader for a File, or any other unbuffered reader. BufReader::new(reader) does this. (To set the size of the buffer, use BufReader::with_capacity(size, reader).)

In most languages, files are buffered by default. If you want unbuffered input or output, you have to figure out how to turn buffering off. In Rust, File and BufReader are two separate library features, because sometimes you want files without buffering, and sometimes you want buffering without files (for example, you may want to buffer input from the network).

The full program, including error handling and some crude argument parsing, is shown here:

// grep - Search stdin or some files for lines matching a given string.

use std::error::Error;
use std::io::{self, BufReader};
use std::io::prelude::*;
use std::fs::File;
use std::path::PathBuf;

fn grep<R>(target: &str, reader: R) -> io::Result<()>
    where R: BufRead
{
    for line_result in reader.lines() {
        let line = line_result?;
        if line.contains(target) {
            println!("{}", line);
        }
    }
    Ok(())
}

fn grep_main() -> Result<(), Box<Error>> {
    // Get the command-line arguments. The first argument is the
    // string to search for; the rest are filenames.
    let mut args = std::env::args().skip(1);
    let target = match args.next() {
        Some(s) => s,
        None => Err("usage: grep PATTERN FILE...")?
    };
    let files: Vec<PathBuf> = args.map(PathBuf::from).collect();

    if files.is_empty() {
        let stdin = io::stdin();
        grep(&target, stdin.lock())?;
    } else {
        for file in files {
            let f = File::open(file)?;
            grep(&target, BufReader::new(f))?;
        }
    }

    Ok(())
}

fn main() {
    let result = grep_main();
    if let Err(err) = result {
        let _ = writeln!(io::stderr(), "{}", err);
    }
}

Collecting Lines

Several reader methods, including .lines(), return iterators that produce Result values. The first time you want to collect all the lines of a file into one big vector, you’ll run into a problem getting rid of the Results.

// ok, but not what you want
let results: Vec<io::Result<String>> = reader.lines().collect();

// error: can't convert collection of Results to Vec<String>
let lines: Vec<String> = reader.lines().collect();

The second try doesn’t compile: what would happen to the errors? The straightforward solution is to write a for loop and check each item for errors:

let mut lines = vec![];
for line_result in reader.lines() {
    lines.push(line_result?);
}

Not bad; but it would be nice to use .collect() here, and it turns out that we can. We just have to know which type to ask for:

let lines = reader.lines().collect::<io::Result<Vec<String>>>()?;

How does this work? The standard library contains an implementation of FromIterator for Result—easy to overlook in the online documentation—that makes this possible:

impl<T, E, C> FromIterator<Result<T, E>> for Result<C, E>
    where C: FromIterator<T>
{
    ...
}

This says: if you can collect items of type T into a collection of type C (“where C: FromIterator<T>”) then you can collect items of type Result<T, E> into a result of type Result<C, E> (“FromIterator<Result<T, E>> for Result<C, E>”).

In other words, io::Result<Vec<String>> is a collection type, so the .collect() method can create and populate values of that type.

Writers

As we’ve seen, input is mostly done using methods. Output is a bit different.

Throughout the book, we’ve used println!() to produce plain-text output.

println!("Hello, world!");

println!("The greatest common divisor of {:?} is {}",
         numbers, d);

There’s also a print!() macro, which does not add a newline character at the end. The formatting codes for print!() and println!() are the same as those for the format! macro, described in “Formatting Values”.

To send output to a writer, use the write!() and writeln!() macros. They are the same as print!() and println!(), except for two differences.

writeln!(io::stderr(), "error: world not helloable")?;

writeln!(&mut byte_vec, "The greatest common divisor of {:?} is {}",
         numbers, d)?;

One difference is that the write macros each take an extra first argument, a writer. The other is that they return a Result, so errors must be handled. That’s why we used the ? operator at the end of each line.

The print macros don’t return a Result; they simply panic if the write fails. Since they write to the terminal, this is rare.

The Write trait has these methods:

writer.write(&buf) writes some of the bytes in the slice buf to the underlying stream. It returns an io::Result<usize>. On success, this gives the number of bytes written, which may be less than buf.len(), at the whim of the stream.

Like Reader::read(), this is a low-level method that you should avoid using directly.

writer.write_all(&buf) writes all the bytes in the slice buf. Returns Result<()>.

writer.flush() flushes any buffered data to the underlying stream. Returns Result<()>.

Like readers, writers are closed automatically when they are dropped.

Just as BufReader::new(reader) adds a buffer to any reader, BufWriter::new(writer) adds a buffer to any writer.

let file = File::create("tmp.txt")?;
let writer = BufWriter::new(file);

To set the size of the buffer, use BufWriter::with_capacity(size, writer).

When a BufWriter is dropped, all remaining buffered data is written to the underlying writer. However, if an error occurs during this write, the error is ignored. (Since this happens inside BufWriter’s .drop() method, there is no useful place to report the error.) To make sure your application notices all output errors, manually .flush() buffered writers before dropping them.

Files

We’ve already seen two ways to open a file:

File::open(filename) opens an existing file for reading. It returns an io::Result<File>, and it’s an error if the file doesn’t exist.

File::create(filename) creates a new file for writing. If a file exists with the given filename, it is truncated.

Note that the File type is in the filesystem module, std::fs, not std::io.

When neither of these fits the bill, you can use OpenOptions to specify the exact desired behavior:

use std::fs::OpenOptions;

let log = OpenOptions::new()
    .append(true)  // if file exists, add to the end
    .open("server.log")?;

let file = OpenOptions::new()
    .write(true)
    .create_new(true)  // fail if file exists
    .open("new_file.txt")?;

The methods .append(), .write(), .create_new(), and so on are designed to be chained like this: each one returns self. This method-chaining design pattern is common enough to have a name in Rust: it’s called a builder. std::process::Command is another example. For more details on OpenOptions, see the online documentation.

Once a File has been opened, it behaves like any other reader or writer. You can add a buffer if needed. The File will be closed automatically when you drop it.

Seeking

File also implements the Seek trait, which means you can hop around within a File rather than reading or writing in a single pass from the beginning to the end. Seek is defined like this:

pub trait Seek {
    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64>;
}

pub enum SeekFrom {
    Start(u64),
    End(i64),
    Current(i64)
}

Thanks to the enum, the seek method is nicely expressive: use file.seek(SeekFrom::Start(0)) to rewind to the beginning, file.seek(SeekFrom::Current(-8)) to go back a few bytes, and so on.

Seeking within a file is slow. Whether you’re using a hard disk or a solid-state drive (SSD), a seek takes as long as reading several megabytes of data.

Other Reader and Writer Types

Earlier in this chapter, we gave a few examples of types other than File that implement Read and Write. Here, we’ll give a few more details about these types.

io::stdin() returns a reader for the standard input stream. Its type is io::Stdin. Since this is shared by all threads, each read acquires and releases a mutex.

Stdin has a .lock() method that acquires the mutex and returns an io::StdinLock, a buffered reader that holds the mutex until it’s dropped. Individual operations on the StdinLock therefore avoid the mutex overhead. We showed example code using this method in “Reading Lines”.

For technical reasons, io::stdin().lock() doesn’t work. The lock holds a reference to the Stdin value, and that means the Stdin value must be stored somewhere so that it lives long enough:

let stdin = io::stdin();
let lines = stdin.lock().lines();  // ok

io::stdout() and io::stderr() return writers for the standard output and standard error streams. These too have mutexes and .lock() methods.

Vec<u8> implements Write. Writing to a Vec<u8> extends the vector with the new data.

(String, however, does not implement Write. To build a string using Write, first write to a Vec<u8>, then use String::from_utf8(vec) to convert the vector to a string.)

Cursor::new(buf) creates a Cursor, a buffered reader that reads from buf. This is how you create a reader that reads from a String. The argument buf can be any type that implements AsRef<[u8]>, so you can also pass a &[u8], &str, or Vec<u8>.

Cursors are trivial internally. They have just two fields: buf itself; and an integer, the offset in buf where the next read will start. The position is initially 0.

Cursors implement Read, BufRead, and Seek. If the type of buf is &mut [u8] or Vec<u8>, then the Cursor also implements Write. Writing to a cursor overwrites bytes in buf starting at the current position. If you try to write past the end of a &mut [u8], you’ll get a partial write or an io::Error. Using a cursor to write past the end of a Vec<u8> is fine, though: it grows the vector. Cursor<&mut [u8]> and Cursor<Vec<u8>> thus implement all four of the std::io::prelude traits.

std::net::TcpStream represents a TCP network connection. Since TCP enables two-way communication, it’s both a reader and a writer.

The static method TcpStream::connect(("hostname", PORT)) tries to connect to a server and returns an io::Result<TcpStream>.

std::process::Command supports spawning a child process and piping data to its standard input, like so:

use std::process::{Command, Stdio};

let mut child =
    Command::new("grep")
    .arg("-e")
    .arg("a.*e.*i.*o.*u")
    .stdin(Stdio::piped())
    .spawn()?;

let mut to_child = child.stdin.take().unwrap();
for word in my_words {
    writeln!(to_child, "{}", word)?;
}
drop(to_child);  // close grep's stdin, so it will exit
child.wait()?;

The type of child.stdin is Option<std::process::ChildStdin>; here we’ve used .stdin(Stdio::piped()) when setting up the child process, so child.stdin is definitely populated when .spawn() succeeds. If we hadn’t, child.stdin would be None.

Command also has similar methods .stdout() and .stderr(), which can be used to request readers in child.stdout and child.stderr.

The std::io module also offers a handful of functions that return trivial readers and writers.

io::sink() is the no-op writer. All the write methods return Ok, but the data is just discarded.

io::empty() is the no-op reader. Reading always succeeds, but returns end-of-input.

io::repeat(byte) returns a reader that repeats the given byte endlessly.

Binary Data, Compression, and Serialization

Many open source crates build on the std::io framework to offer extra features.

The byteorder crate offers ReadBytesExt and WriteBytesExt traits that add methods to all readers and writers for binary input and output:

use byteorder::{ReadBytesExt, WriteBytesExt, LittleEndian};

let n = reader.read_u32::<LittleEndian>()?;
writer.write_i64::<LittleEndian>(n as i64)?;

The flate2 crate provides adapter methods for reading and writing gzipped data:

use flate2::FlateReadExt;

let file = File::open("access.log.gz")?;
let mut gzip_reader = file.gz_decode()?;

The serde crate is for serialization and deserialization: it converts back and forth between Rust structs and bytes. We mentioned this once before, in “Traits and Other People’s Types”. Now we can take a closer look.

Suppose we have some data—the map for a text adventure game—stored in a HashMap:

type RoomId = String;                       // each room has a unique name
type RoomExits = Vec<(char, RoomId)>;       // ...and a list of exits
type RoomMap = HashMap<RoomId, RoomExits>;  // room names and exits, simple

// Create a simple map.
let mut map = RoomMap::new();
map.insert("Cobble Crawl".to_string(),
           vec![('W', "Debris Room".to_string())]);
map.insert("Debris Room".to_string(),
           vec![('E', "Cobble Crawl".to_string()),
                ('W', "Sloping Canyon".to_string())]);
...

Turning this data into JSON for output is just a few lines of code:

use std::io;
use serde::Serialize;
use serde_json::Serializer;

let mut serializer = Serializer::new(io::stdout());
map.serialize(&mut serializer)?;

This code uses the serialize method of the serde::Serialize trait. The library attaches this trait to all types that it knows how to serialize, and that includes all of the types that appear in our data: strings, characters, tuples, vectors, and HashMaps.

serde is flexible. In this program, the output is JSON data, because we chose the serde_json serializer. Other formats, like MessagePack, are also available. Likewise, you could send this output to a file, a Vec<u8>, or any other writer. The code above prints the data on stdout. Here it is:

{"Debris Room":[["E","Cobble Crawl"],["W","Sloping Canyon"]],"Cobble Crawl":
[["W","Debris Room"]]}

serde also includes support for deriving the two key serde traits:

#[derive(Serialize, Deserialize)]
struct Player {
    location: String,
    items: Vec<String>,
    health: u32
}

As of Rust 1.17, this #[derive] attribute requires a few extra steps when setting up your project. We won’t cover that here; see the serde documentation for details. In short, the build system autogenerates implementations of serde::Serialize and serde::Deserialize for Player, so that serializing a Player value is simple:

player.serialize(&mut serializer)?;

The output looks like this:

{"location":"Cobble Crawl","items":["a wand"],"health":3}

Files and Directories

The next few sections cover Rust’s features for working with files and directories, which live in the std::path and std::fs modules. All of these features involve working with filenames, so we’ll start with the filename types.

OsStr and Path

Inconveniently, your operating system does not force filenames to be valid Unicode. Here are two Linux shell commands that create text files. Only the first uses a valid UTF-8 filename.

$ echo "hello world" > ô.txt
$ echo "O brave new world, that has such filenames in't" > $'\xf4'.txt

Both commands pass without comment, because the Linux kernel doesn’t know UTF-8 from Ogg Vorbis. To the kernel, any string of bytes (excluding null bytes and slashes) is an acceptable filename. It’s a similar story on Windows: almost any string of 16-bit “wide characters” is an acceptable filename, even strings that are not valid UTF-16. The same is true of other strings the operating system handles, like command-line arguments and environment variables.

Rust strings are always valid Unicode. Filenames are almost always Unicode in practice, but Rust has to cope somehow with the rare case where they aren’t. This is why Rust has std::ffi::OsStr and OsString.

OsStr is a string type that’s a superset of UTF-8. Its job is to be able to represent all filenames, command-line arguments, and environment variables on the current system, whether they’re valid Unicode or not. On Unix, an OsStr can hold any sequence of bytes. On Windows, an OsStr is stored using an extension of UTF-8 that can encode any sequence of 16-bit-values, including unmatched surrogates.

So we have two string types: str for actual Unicode strings; and OsStr for whatever nonsense your operating system can dish out. We’ll introduce one more: std::path::Path, for filenames. This one is purely a convenience. Path is exactly like OsStr, but it adds many handy filename-related methods, which we’ll cover in the next section. Use Path for both absolute and relative paths. For an individual component of a path, use OsStr.

Lastly, for each string type, there’s a corresponding owning type: a String owns a heap-allocated str, a std::ffi::OsString owns a heap-allocated OsStr, and a std::path::PathBuf owns a heap-allocated Path.

| str | OsStr | Path
Unsized type, always passed by reference | Yes | Yes | Yes
Can contain any Unicode text | Yes | Yes | Yes
Looks just like UTF-8, normally | Yes | Yes | Yes
Can contain non-Unicode data | No | Yes | Yes
Text processing methods | Yes | No | No
Filename-related methods | No | No | Yes
Owned, growable, heap-allocated equivalent | String | OsString | PathBuf
Convert to owned type | .to_string() | .to_os_string() | .to_path_buf()

All three of these types implement a common trait, AsRef<Path>, so we can easily declare a generic function that accepts “any filename type” as an argument. This uses a technique we showed in “AsRef and AsMut”:

use std::path::Path;
use std::io;

fn swizzle_file<P>(path_arg: P) -> io::Result<()>
   where P: AsRef<Path>
{
    let path = path_arg.as_ref();
    ...
}

All the standard functions and methods that take path arguments use this technique, so you can freely pass string literals to any of them.

Path and PathBuf Methods

Path offers the following methods, among others:

Path::new(str) converts a &str or &OsStr to a &Path. This doesn’t copy the string: the new &Path points to the same bytes as the original &str or &OsStr.

use std::path::Path;
let home_dir = Path::new("/home/fwolfe");

(The similar method OsStr::new(str) converts a &str to a &OsStr.)

path.parent() returns the path’s parent directory, if any. The return type is Option<&Path>.

This doesn’t copy the path: the parent directory of path is always a substring of path.

assert_eq!(Path::new("/home/fwolfe/program.txt").parent(),
           Some(Path::new("/home/fwolfe")));

path.file_name() returns the last component of path, if any. The return type is Option<&OsStr>.

In the typical case, where path consists of a directory, then a slash, then a filename, this returns the filename.

assert_eq!(Path::new("/home/fwolfe/program.txt").file_name(),
           Some(OsStr::new("program.txt")));

path.is_absolute() and path.is_relative() tell whether the file is absolute, like the Unix path /usr/bin/advent or the Windows path C:\Program Files; or relative, like src/main.rs.

path1.join(path2) joins two paths, returning a new PathBuf.

let path1 = Path::new("/usr/share/dict");
assert_eq!(path1.join("words"),
           Path::new("/usr/share/dict/words"));

If path2 is an absolute path, this just returns a copy of path2, so this method can be used to convert any path to an absolute path:

let abs_path = std::env::current_dir()?.join(any_path);

path.components() returns an iterator over the components of the given path, from left to right. The item type of this iterator is std::path::Component, an enum that can represent all the different pieces that can appear in filenames:

pub enum Component<'a> {
    Prefix(PrefixComponent<'a>),  // Windows-only: a drive letter or share
    RootDir,                      // the root directory, `/` or `\`
    CurDir,                       // the `.` special directory
    ParentDir,                    // the `..` special directory
    Normal(&'a OsStr)             // plain file and directory names
}

For example, the Windows path \\venice\Music\A Love Supreme\04-Psalm.mp3 consists of a Prefix representing \\venice\Music, followed by a RootDir, and then two Normal components representing A Love Supreme and 04-Psalm.mp3.

For details, see the online documentation.

These methods work on strings in memory. Paths also have some methods that query the filesystem: .exists(), .is_file(), .is_dir(), .read_dir(), ​.canonicalize(), and so on. See the online documentation to learn more.

There are three methods for converting Paths to strings. Each one allows for the possibility of invalid UTF-8 in the Path.

path.to_str() converts a Path to a string, as an Option<&str>. If path isn’t valid UTF-8, this returns None.

if let Some(file_str) = path.to_str() {
    println!("{}", file_str);
}  // ...otherwise skip this weirdly named file

path.to_string_lossy() is basically the same thing, but it manages to return some sort of string in all cases. If path isn’t valid UTF-8, these methods make a copy, replacing each invalid byte sequence with the Unicode replacement character, U+FFFD (‘�’).

The return type is std::borrow::Cow<str>: an either-borrowed-or-owned string. To get a String from this value, use its .to_owned() method. (For more about Cow, see “Borrow and ToOwned at Work: The Humble Cow”.)

path.display() is for printing paths:

println!("Download found. You put it in: {}", dir_path.display());

The value this returns isn’t a string, but it implements std::fmt::Display, so it can be used with format!(), println!(), and friends. If the path isn’t valid UTF-8, the output may contain the � character.

Filesystem Access Functions

Table 18-1 shows some of the functions in std::fs and their approximate equivalents on Unix and Windows. All of these functions return io::Result values. They are Result<()> unless otherwise noted.

Table 18-1. Summary of filesystem access functions

| Rust function | Unix | Windows
Creating and deleting | create_dir(path) | mkdir() | CreateDirectory()
create_dir_all(path) | like mkdir -p | like mkdir
remove_dir(path) | rmdir() | RemoveDirectory()
remove_dir_all(path) | like rm -r | like rmdir /s
remove_file(path) | unlink() | DeleteFile()
Copying, moving, and linking | copy(src_path, dest_path) -> Result<u64> | like cp -p | CopyFileEx()
rename(src_path, dest_path) | rename() | MoveFileEx()
hard_link(src_path, dest_path) | link() | CreateHardLink()
Inspecting | canonicalize(path) -> Result<PathBuf> | realpath() | GetFinalPathNameByHandle()
metadata(path) -> Result<Metadata> | stat() | GetFileInformationByHandle()
symlink_metadata(path) -> Result<Metadata> | lstat() | GetFileInformationByHandle()
read_dir(path) -> Result<ReadDir> | opendir() | FindFirstFile()
read_link(path) -> Result<PathBuf> | readlink() | FSCTL_GET_REPARSE_POINT
Permissions | set_permissions(path, perm) | chmod() | SetFileAttributes()

(The number returned by copy() is the size of the copied file, in bytes. For creating symbolic links, see “Platform-Specific Features”.)

As you can see, Rust strives to provide portable functions that work predictably on Windows as well as macOS, Linux, and other Unix systems.

A full tutorial on filesystems is beyond the scope of this book, but if you’re curious about any of these functions, you can easily find more about them online. We’ll show some examples in the next section.

All of these functions are implemented by calling out to the operating system. For example, std::fs::canonicalize(path) does not merely use string processing to eliminate . and .. from the given path. It resolves relative paths using the current working directory, and it chases symbolic links. It’s an error if the path doesn’t exist.

The Metadata type produced by std::fs::metadata(path) and std::fs::symlink_metadata(path) contains such information as the file type and size, permissions, and timestamps. As always, consult the documentation for details.

As a convenience, the Path type has a few of these built in as methods: path.metadata(), for example, is the same thing as std::fs::metadata(path).

Reading Directories

To list the contents of a directory, use std::fs::read_dir, or equivalently, the .read_dir() method of a Path:

for entry_result in path.read_dir()? {
    let entry = entry_result?;
    println!("{}", entry.file_name().to_string_lossy());
}

Note the two uses of ? in this code. The first line checks for errors opening the directory. The second line checks for errors reading the next entry.

The type of entry is std::fs::DirEntry, and it’s a struct with just a few methods:

entry.file_name() is the name of the file or directory, as an OsString.

entry.path() is the same, but with the original path joined to it, producing a new PathBuf. If the directory we’re listing is "/home/jimb", and entry.file_name() is ".emacs", then entry.path() would return PathBuf::from("/home/jimb/.emacs").

entry.file_type() returns an io::Result<FileType>. FileType has .is_file(), .is_dir(), and .is_symlink() methods.

entry.metadata() gets the rest of the metadata about this entry.

The special directories . and .. are not listed when reading a directory.

Here’s a more substantial example. The following code recursively copies a directory tree from one place to another on disk:

use std::fs;
use std::io;
use std::path::Path;

/// Copy the existing directory `src` to the target path `dst`.
fn copy_dir_to(src: &Path, dst: &Path) -> io::Result<()> {
    if !dst.is_dir() {
        fs::create_dir(dst)?;
    }

    for entry_result in src.read_dir()? {
        let entry = entry_result?;
        let file_type = entry.file_type()?;
        copy_to(&entry.path(), &file_type, &dst.join(entry.file_name()))?;
    }

    Ok(())
}

A separate function, copy_to, copies individual directory entries:

/// Copy whatever is at `src` to the target path `dst`.
fn copy_to(src: &Path, src_type: &fs::FileType, dst: &Path) -> io::Result<()> {
    if src_type.is_file() {
        fs::copy(src, dst)?;
    } else if src_type.is_dir() {
        copy_dir_to(src, dst)?;
    } else {
        return Err(io::Error::new(io::ErrorKind::Other,
                                  format!("don't know how to copy: {}",
                                          src.display())));
    }
    Ok(())
}

Platform-Specific Features

So far, our copy_to function can copy files and directories. Suppose we also want to support symbolic links on Unix.

There is no portable way to create symbolic links that works on both Unix and Windows, but the standard library offers a Unix-specific symlink function,

use std::os::unix::fs::symlink;

and with this, our job is easy. We need only add a branch to the if-expression in copy_to:

...
} else if src_type.is_symlink() {
    let target = src.read_link()?;
    symlink(target, dst)?;
...

This will work as long as we compile our program only for Unix systems, such as Linux and macOS.

The std::os module contains various platform-specific features, like symlink. The actual body of std::os in the standard library looks like this (taking some poetic license):

//! OS-specific functionality.

#[cfg(unix)]                    pub mod unix;
#[cfg(windows)]                 pub mod windows;
#[cfg(target_os = "ios")]       pub mod ios;
#[cfg(target_os = "linux")]     pub mod linux;
#[cfg(target_os = "macos")]     pub mod macos;
...

The #[cfg] attribute indicates conditional compilation: each of these modules exists only on some platforms. This is why our modified program, using std::os::unix, will successfully compile only for Unix: on other platforms, std::os::unix doesn’t exist.

If we want our code to compile on all platforms, with support for symbolic links on Unix, we must use #[cfg] in our program as well. In this case, it’s easiest to import symlink on Unix, while defining our own symlink stub on other systems:

#[cfg(unix)]
use std::os::unix::fs::symlink;

/// Stub implementation of `symlink` for platforms that don't provide it.
#[cfg(not(unix))]
fn symlink<P: AsRef<Path>, Q: AsRef<Path>>(src: P, _dst: Q)
    -> std::io::Result<()>
{
    Err(io::Error::new(io::ErrorKind::Other,
                       format!("can't copy symbolic link: {}",
                               src.as_ref().display())))
}

As of this writing, the online documentation at https://doc.rust-lang.org/std is generated by running rustdoc on the standard library—on Linux. This means that system-specific functionality for macOS, Windows, and other platforms does not show up in the online documentation. The best way to find it is to use rustup doc to see the HTML documentation for your platform. Of course, another option is to consult the source code, which is available online.

It turns out that symlink is something of a special case. Most Unix-specific features are not standalone functions but rather extension traits that add new methods to standard library types. (We covered extension traits in “Traits and Other People’s Types”.) There’s a prelude module that can be used to enable all of these extensions at once:

use std::os::unix::prelude::*;

For example, on Unix, this adds a .mode() method to std::fs::Permissions, providing access to the underlying u32 value that represents permissions on Unix. Similarly, it extends std::fs::Metadata with accessors for the fields of the underlying struct stat value—such as .uid(), the user ID of the file’s owner.

All told, what’s in std::os is pretty basic. Much more platform-specific functionality is available via third-party crates, like winreg for accessing the Windows registry.

Networking

A tutorial on networking is well beyond the scope of this book. However, if you already know a bit about network programming, this section will help you get started with networking in Rust.

For low-level networking code, start with the std::net module, which provides cross-platform support for TCP and UDP networking. Use the native_tls crate for SSL/TLS support.

These modules provide the building blocks for straightforward, blocking input and output over the network. You can write a simple server in a few lines of code, using std::net and spawning a thread for each connection. For example, here’s an “echo” server:

use std::net::TcpListener;
use std::io;
use std::thread::spawn;

/// Accept connections forever, spawning a thread for each one.
fn echo_main(addr: &str) -> io::Result<()> {
    let listener = TcpListener::bind(addr)?;
    println!("listening on {}", addr);
    loop {
        // Wait for a client to connect.
        let (mut stream, addr) = listener.accept()?;
        println!("connection received from {}", addr);

        // Spawn a thread to handle this client.
        let mut write_stream = stream.try_clone()?;
        spawn(move || {
            // Echo everything we receive from `stream` back to it.
            io::copy(&mut stream, &mut write_stream)
                .expect("error in client thread: ");
            println!("connection closed");
        });
    }
}

fn main() {
    echo_main("127.0.0.1:17007").expect("error: ");
}

An echo server simply repeats back everything you send to it. This kind of code is not so different from what you’d write in Java or Python. (We’ll cover std::thread::spawn() in the next chapter.)

However, for high-performance servers, you’ll need to use asynchronous input and output. The mio crate provides the needed support. MIO is very low-level. It provides a simple event loop and asynchronous methods for reading, writing, connecting, and accepting connections—basically an asynchronous copy of the whole networking API. Whenever an asynchronous operation completes, MIO passes an event to an event handler method that you write.

There’s also the experimental tokio crate, which wraps the mio event loop in a futures-based API, reminiscent of JavaScript promises.

Higher-level protocols are supported by third-party crates. For example, the reqwest crate offers a beautiful API for HTTP clients. Here is a complete command-line program that fetches any document with an http: or https: URL and dumps it to your terminal. This code was written using reqwest = "0.5.1".

extern crate reqwest;

use std::error::Error;
use std::io::{self, Write};

fn http_get_main(url: &str) -> Result<(), Box<Error>> {
    // Send the HTTP request and get a response.
    let mut response = reqwest::get(url)?;
    if !response.status().is_success() {
        Err(format!("{}", response.status()))?;
    }

    // Read the response body and write it to stdout.
    let stdout = io::stdout();
    io::copy(&mut response, &mut stdout.lock())?;

    Ok(())
}

fn main() {
    let args: Vec<String> = std::env::args().collect();
    if args.len() != 2 {
        writeln!(io::stderr(), "usage: http-get URL").unwrap();
        return;
    }

    if let Err(err) = http_get_main(&args[1]) {
        writeln!(io::stderr(), "error: {}", err).unwrap();
    }
}

The iron framework for HTTP servers offers high-level touches such as the BeforeMiddleware and AfterMiddleware traits, which help you compose an app from pluggable parts. The websocket crate implements the WebSocket protocol. And so on. Rust is a young language with a busy open source ecosystem. Support for networking is rapidly expanding.

Chapter 19. Concurrency

In the long run it is not advisable to write large concurrent programs in machine-oriented languages that permit unrestricted use of store locations and their addresses. There is just no way we will be able to make such programs reliable (even with the help of complicated hardware mechanisms).

Per Brinch Hansen (1977)

Patterns for communication are patterns for parallelism.

Whit Morriss

If your attitude toward concurrency has changed over the course of your career, you’re not alone. It’s a common story.

At first, writing concurrent code is easy and fun. The tools—threads, locks, queues, and so on—are a snap to pick up and use. There are a lot of pitfalls, it’s true, but fortunately you know what they all are, and you are careful not to make mistakes.

At some point, you have to debug someone else’s multithreaded code, and you’re forced to conclude that some people really should not be using these tools.

Then at some point you have to debug your own multithreaded code.

Experience inculcates a healthy skepticism, if not outright cynicism, toward all multithreaded code. This is helped along by the occasional article explaining in mind-numbing detail why some obviously correct multithreading idiom does not work at all. (It has to do with “the memory model.”) But you eventually find one approach to concurrency that you think you can realistically use without constantly making mistakes. You can shoehorn pretty much everything into that idiom, and (if you’re really good) you learn to say “no” to added complexity.

Of course, there are rather a lot of idioms. Approaches that systems programmers commonly use include the following:

A background thread that has a single job and periodically wakes up to do it.

General-purpose worker pools that communicate with clients via task queues.

Pipelines where data flows from one thread to the next, with each thread doing a little of the work.

Data parallelism, where it is assumed (rightly or wrongly) that the whole computer will mainly just be doing one large computation, which is therefore split into n pieces and run on n threads in the hopes of putting all n of the machine’s cores to work at once.

A sea of synchronized objects, where multiple threads have access to the same data, and races are avoided using ad hoc locking schemes based on low-level primitives like mutexes. (Java includes built-in support for this model, which was quite popular during the 1990s and 2000s.)

Atomic integer operations allow multiple cores to communicate by passing information through fields the size of one machine word. (This is even harder to get right than all the others, unless the data being exchanged is literally just integer values. In practice, it’s usually pointers.)

In time, you may come to be able to use several of these approaches and combine them safely. You are a master of the art. And things would be great, if only nobody else were ever allowed to modify the system in any way. Programs that use threads well are full of unwritten rules.

Rust offers a better way to use concurrency, not by forcing all programs to adopt a single style (which for systems programmers would be no solution at all), but by supporting multiple styles safely. The unwritten rules are written down—in code—and enforced by the compiler.

You’ve heard that Rust lets you write safe, fast, concurrent programs. This is the chapter where we show you how it’s done. We’ll cover three ways to use Rust threads:

Fork-join parallelism

Channels

Shared mutable state

Along the way, you’re going to use everything you’ve learned so far about the Rust language. The care Rust takes with references, mutability, and lifetimes is valuable enough in single-threaded programs, but it is in concurrent programming that the true significance of those rules becomes apparent. They make it possible to expand your toolbox, to hack multiple styles of multithreaded code quickly and correctly—without skepticism, without cynicism, without fear.

Fork-Join Parallelism

The simplest use cases for threads arise when we have several completely independent tasks that we’d like to do at once.

For example, suppose we’re doing natural language processing on a large corpus of documents. We could write a loop:

fn process_files(filenames: Vec<String>) -> io::Result<()> {
    for document in filenames {
        let text = load(&document)?;  // read source file
        let results = process(text);  // compute statistics
        save(&document, results)?;    // write output file
    }
    Ok(())
}

The program would run as shown in Figure 19-1.

Figure 19-1. Single-threaded execution of process_files() 

Since each document is processed separately, it’s relatively easy to speed this task up by splitting the corpus into chunks and processing each chunk on a separate thread, as shown in Figure 19-2.

This pattern is called fork-join parallelism. To fork is to start a new thread, and to join a thread is to wait for it to finish. We’ve already seen this technique: we used it to speed up the Mandelbrot program in Chapter 2.

Figure 19-2. Multithreaded file processing using a fork-join approach

Fork-join parallelism is attractive for a few reasons:

It’s dead simple. Fork-join is easy to implement, and Rust makes it easy to get right.

It avoids bottlenecks. There’s no locking of shared resources in fork-join. The only time any thread has to wait for another is at the end. In the meantime, each thread can run freely. This helps keep task-switching overhead low.

The performance math is straightforward. In the best case, by starting four threads, we can finish our work in a quarter of the time. Figure 19-2 shows one reason we shouldn’t expect this ideal speed-up: we might not be able to distribute the work evenly across all threads. Another reason for caution is that sometimes fork-join programs must spend some time after the threads join, combining the results computed by the threads. That is, isolating the tasks completely may make some extra work. Still, apart from those two things, any CPU-bound program with isolated units of work can expect a significant boost.

It’s easy to reason about program correctness. A fork-join program is deterministic as long as the threads are really isolated, like the compute threads in the Mandelbrot program. The program always produces the same result, regardless of variations in thread speed. It’s a concurrency model without race conditions.

The main disadvantage of fork-join is that it requires isolated units of work. Later in this chapter, we’ll consider some problems that don’t split up so cleanly.

For now, let’s stick with the natural language processing example. We’ll show a few ways of applying the fork-join pattern to the process_files function.

spawn and join

The function std::thread::spawn starts a new thread.

spawn(|| {
    println!("hello from a child thread");
})

It takes one argument, a FnOnce closure or function. Rust starts a new thread to run the code of that closure or function. The new thread is a real operating system thread with its own stack, just like threads in C++, C#, and Java.

Here’s a more substantial example, using spawn to implement a parallel version of the process_files function from before:

use std::thread::spawn;

fn process_files_in_parallel(filenames: Vec<String>) -> io::Result<()> {
    // Divide the work into several chunks.
    const NTHREADS: usize = 8;
    let worklists = split_vec_into_chunks(filenames, NTHREADS);

    // Fork: Spawn a thread to handle each chunk.
    let mut thread_handles = vec![];
    for worklist in worklists {
        thread_handles.push(
            spawn(move || process_files(worklist))
        );
    }

    // Join: Wait for all threads to finish.
    for handle in thread_handles {
        handle.join().unwrap()?;
    }

    Ok(())
}

Let’s take this function line by line.

fn process_files_in_parallel(filenames: Vec<String>) -> io::Result<()> {

Our new function has the same type signature as the original process_files, making it a handy drop-in replacement.

    // Divide the work into several chunks.
    const NTHREADS: usize = 8;
    let worklists = split_vec_into_chunks(filenames, NTHREADS);

We use a utility function split_vec_into_chunks, not shown here, to divide up the work. The result, worklists, is a vector of vectors. It contains eight evenly sized slices of the original vector filenames.

    // Fork: Spawn a thread to handle each chunk.
    let mut thread_handles = vec![];
    for worklist in worklists {
        thread_handles.push(
            spawn(move || process_files(worklist))
        );
    }

We spawn a thread for each worklist. spawn() returns a value called a JoinHandle, which we’ll use later. For now, we put all the JoinHandles into a vector.

Note how we get the list of filenames into the worker thread:

worklist is defined and populated by the for loop, in the parent thread.

As soon as the move closure is created, worklist is moved into the closure.

spawn then moves the closure (including the worklist vector) over to the new child thread.

These moves are cheap. Like the Vec<String> moves we discussed in Chapter 4, the Strings are not cloned. In fact, nothing is allocated or freed. The only data moved is the Vec itself: three machine words.

Most every thread you create needs both code and data to get started. Rust closures, conveniently, contain whatever code you want and whatever data you want.

Moving on:

    // Join: Wait for all threads to finish.
    for handle in thread_handles {
        handle.join().unwrap()?;
    }

We use the .join() method of the JoinHandles we collected earlier to wait for all eight threads to finish. Joining threads is often necessary for correctness, because a Rust program exits as soon as main returns, even if other threads are still running. Destructors are not called; the extra threads are just killed. If this isn’t what you want, be sure to join any threads you care about before returning from main.

If we manage to get through this loop, it means all eight child threads finished successfully. Our function therefore ends by returning Ok(()):

    Ok(())
}

Error Handling Across Threads

The code we used to join the child threads in our example is trickier than it looks, because of error handling. Let’s revisit that line of code:

handle.join().unwrap()?;

The .join() method does two neat things for us.

First, handle.join() returns a std::thread::Result that’s an error if the child thread panicked. This makes threading in Rust dramatically more robust than in C++. In C++, an out-of-bounds array access is undefined behavior, and there’s no protecting the rest of the system from the consequences. In Rust, panic is safe and per thread. The boundaries between threads serve as a firewall for panic; panic doesn’t automatically spread from one thread to the threads that depend on it. Instead, a panic in one thread is reported as an error Result in other threads. The program as a whole can easily recover.

In our program, though, we don’t attempt any fancy panic handling. Instead, we immediately use .unwrap() on this Result, asserting that it is an Ok result and not an Err result. If a child thread did panic, then this assertion would fail, so the parent thread would panic too. We’re explicitly propagating panic from the child threads to the parent thread.

Second, handle.join() passes the return value from the child thread back to the parent thread. The closure we passed to spawn has a return type of io::Result<()>. because that’s what process_files returns. This return value isn’t discarded. When the child thread is finished, its return value is saved, and JoinHandle::join() transfers that value back to the parent thread.

The full type returned by handle.join() in this program is std::thread::Result<std::io::Result<()>>. The thread::Result is part of the spawn/join API; the io::Result is part of our app.

In our case, after unwrapping the thread::Result, we use the ? operator on the io::Result, explicitly propagating I/O errors from the child threads to the parent thread.

All of this may seem rather intricate. But consider that it’s just one line of code, and then compare this with other languages. The default behavior in Java and C# is for exceptions in child threads to be dumped to the terminal and then forgotten. In C++, the default is to abort the process. In Rust, errors are Result values (data) instead of exceptions (control flow). They’re delivered across threads just like any other value. Any time you use low-level threading APIs, you end up having to write careful error-handling code, but given that you have to write it, Result is very nice to have around.

Sharing Immutable Data Across Threads

Suppose the analysis we’re doing requires a large database of English words and phrases:

// before
fn process_files(filenames: Vec<String>)

// after
fn process_files(filenames: Vec<String>, glossary: &GigabyteMap)

This glossary is going to be big, so we’re passing it in by reference. How can we update process_files_in_parallel to pass the glossary through to the worker threads?

The obvious change does not work:

fn process_files_in_parallel(filenames: Vec<String>,
                             glossary: &GigabyteMap)
    -> io::Result<()>
{
    ...
    for worklist in worklists {
        thread_handles.push(
            spawn(move || process_files(worklist, glossary))  // error
        );
    }
    ...
}

We’ve simply added a glossary argument to our function and passed it along to process_files. Rust complains:

error[E0477]: the type `[closure@...]` does not fulfill the required lifetime
  --> concurrency_spawn_lifetimes.rs:35:13
   |
35 |             spawn(move || process_files(worklist, glossary))  // error
   |             ^^^^^
   |
   = note: type must satisfy the static lifetime

Rust is complaining about the lifetime of the closure we’re passing to spawn.

spawn launches independent threads. Rust has no way of knowing how long the child thread will run, so it assumes the worst: it assumes the child thread may keep running even after the parent thread has finished and all values in the parent thread are gone. Obviously, if the child thread is going to last that long, the closure it’s running needs to last that long too. But this closure has a bounded lifetime: it depends on the reference glossary, and references don’t last forever.

Note that Rust is right to reject this code! The way we’ve written this function, it is possible for one thread to hit an I/O error, causing process_files_in_parallel to bail out before the other threads are finished. Child threads could end up trying to use the glossary after the main thread has freed it. It would be a race—with undefined behavior as the prize, if the main thread should win. Rust can’t allow this.

It seems spawn is too open-ended to support sharing references across threads. Indeed, we already saw a case like this, in “Closures That Steal”. There, our solution was to transfer ownership of the data to the new thread, using a move closure. That won’t work here, since we have many threads that all need to use the same data. One safe alternative is to clone the whole glossary for each thread, but since it’s large, we want to avoid that. Fortunately, the standard library provides another way: atomic reference counting.

We described Arc in “Rc and Arc: Shared Ownership”. It’s time to put it to use:

use std::sync::Arc;

fn process_files_in_parallel(filenames: Vec<String>,
                             glossary: Arc<GigabyteMap>)
    -> io::Result<()>
{
    ...
    for worklist in worklists {
        // This call to .clone() only clones the Arc and bumps the
        // reference count. It does not clone the GigabyteMap.
        let glossary_for_child = glossary.clone();
        thread_handles.push(
            spawn(move || process_files(worklist, &glossary_for_child))
        );
    }
    ...
}

We have changed the type of glossary: to run the analysis in parallel, the caller must pass in an Arc<GigabyteMap>, a smart pointer to a GigabyteMap that’s been moved into the heap, by doing Arc::new(giga_map).

When we call glossary.clone(), we are making a copy of the Arc smart pointer, not the whole GigabyteMap. This amounts to incrementing a reference count.

With this change, the program compiles and runs, because it no longer depends on reference lifetimes. As long as any thread owns an Arc<GigabyteMap>, it will keep the map alive, even if the parent thread bails out early. There won’t be any data races, because data in an Arc is immutable.

Rayon

The standard library’s spawn function is an important primitive, but it’s not designed specifically for fork-join parallelism. Better fork-join APIs have been built on top of it. For example, in Chapter 2 we used the Crossbeam library to split some work across eight threads. Crossbeam’s scoped threads support fork-join parallelism quite naturally.

The Rayon library, by Niko Matsakis, is another example. It provides two ways of running tasks concurrently:

extern crate rayon;
use rayon::prelude::*;

// "do 2 things in parallel"
let (v1, v2) = rayon::join(fn1, fn2);

// "do N things in parallel"
giant_vector.par_iter().for_each(|value| {
    do_thing_with_value(value);
});

rayon::join(fn1, fn2) simply calls both functions and returns both results. The .par_iter() method creates a ParallelIterator, a value with map, filter, and other methods, much like a Rust Iterator. In both cases, Rayon uses its own pool of worker threads to spread out the work when possible. You simply tell Rayon what tasks can be done in parallel; Rayon manages threads and distributes the work as best it can.

The diagrams in Figure 19-3 illustrate two ways of thinking about the call giant_vector.par_iter().for_each(...). (a) Rayon acts as though it spawns one thread per element in the vector. (b) Behind the scenes, Rayon has one worker thread per CPU core, which is more efficient. This pool of worker threads is shared by all your program’s threads. When thousands of tasks come in at once, Rayon divides the work.

Figure 19-3. Rayon in theory and practice

Here’s a version of process_files_in_parallel using Rayon:

extern crate rayon;

use rayon::prelude::*;

fn process_files_in_parallel(filenames: Vec<String>, glossary: &GigabyteMap)
    -> io::Result<()>
{
    filenames.par_iter()
        .map(|filename| process_file(filename, glossary))
        .reduce_with(|r1, r2| {
            if r1.is_err() { r1 } else { r2 }
        })
        .unwrap_or(Ok(()))
}

This code is shorter and less tricky than the version using std::thread::spawn. Let’s look at it line by line:

First, we use filenames.par_iter() to create a parallel iterator.

We use .map() to call process_file on each filename. This produces a ParallelIterator over a sequence of io::Result<()> values.

We use .reduce_with() to combine the results. Here we’re keeping the first error, if any, and discarding the rest. If we wanted to accumulate all the errors, or print them, we could do that here.

The .reduce_with() method is also handy when you pass a .map() closure that returns a useful value on success. Then you can pass .reduce_with() a closure that knows how to combine two success results.

reduce_with returns an Option that is None only if filenames was empty. We use the Option’s .unwrap_or() method to make the result Ok(()) in that case.

Behind the scenes, Rayon balances workloads across threads dynamically, using a technique called work-stealing. It will typically do a better job keeping all the CPUs busy than we can do by manually dividing the work in advance, as in “spawn and join”.

As a bonus, Rayon supports sharing references across threads. Any parallel processing that happens behind the scenes is guaranteed to be finished by the time reduce_with returns. This explains why we were able to pass glossary to process_file even though that closure will be called on multiple threads.

(Incidentally, it’s no coincidence that we’ve used a map method and a reduce method. The MapReduce programming model, popularized by Google and by Apache Hadoop, has a lot in common with fork-join. It can be seen as a fork-join approach to querying distributed data.)

Revisiting the Mandelbrot Set

Back in Chapter 2, we used fork-join concurrency to render the Mandelbrot set. This made rendering four times as fast—impressive, but not as impressive as it could be, considering that we had the program spawn eight worker threads and ran it on an eight-core machine!

The problem is that we didn’t distribute the workload evenly. Computing one pixel of the image amounts to running a loop (see “What the Mandelbrot Set Actually Is”). It turns out that the pale gray parts of the image, where the loop quickly exits, are much faster to render than the black parts, where the loop runs the full 255 iterations. So although we split the area into equal-sized horizontal bands, we were creating unequal workloads, as Figure 19-4 shows.

Figure 19-4. Uneven work distribution in the Mandelbrot program

This is easy to fix using Rayon. We can just fire off a parallel task for each row of pixels in the output. This creates several hundred tasks that Rayon can distribute across its threads. Thanks to work-stealing, it won’t matter that the tasks vary in size. Rayon will balance the work as it goes.

Here is the code. The first line and the last line are part of the main function we showed back in “A Concurrent Mandelbrot Program”, but we’ve changed the rendering code, which is everything in between.

let mut pixels = vec![0; bounds.0 * bounds.1];

// Scope of slicing up `pixels` into horizontal bands.
{
    let bands: Vec<(usize, &mut [u8])> = pixels
        .chunks_mut(bounds.0)
        .enumerate()
        .collect();

    bands.into_par_iter()
        .weight_max()
        .for_each(|(i, band)| {
            let top = i;
            let band_bounds = (bounds.0, 1);
            let band_upper_left = pixel_to_point(bounds, (0, top),
                                                 upper_left, lower_right);
            let band_lower_right = pixel_to_point(bounds, (bounds.0, top + 1),
                                                  upper_left, lower_right);
            render(band, band_bounds, band_upper_left, band_lower_right);
        });
}

write_bitmap(&args[1], &pixels, bounds).expect("error writing PNG file");

First, we create bands, the collection of tasks that we will be passing to Rayon. Each task is just a tuple of type (usize, &mut [u8]): the row number, since the computation requires that; and the slice of pixels to fill in. We use the chunks_mut method to break the image buffer into rows, enumerate to attach a row number to each row, and collect to slurp all the number-slice pairs into a vector. (We need a vector because Rayon creates parallel iterators only out of arrays and vectors.)

Next, we turn bands into a parallel iterator, call .weight_max() as a hint to Rayon that these tasks are very CPU-intensive, and then use the .for_each() method to tell Rayon what work we want done.

Since we’re using Rayon, we must add these lines to main.rs:

extern crate rayon;
use rayon::prelude::*;

and to Cargo.toml:

[dependencies]
rayon = "0.4"

With these changes, the program now uses about 7.75 cores on an 8-core machine. It’s 75% faster than before, when we were dividing the work manually. And the code is a little shorter, reflecting the benefits of letting a crate do a job (work distribution) rather than doing it ourselves.

Channels

A channel is a one-way conduit for sending values from one thread to another. In other words, it’s a thread-safe queue.

Figure 19-5 illustrates how channels are used. They’re something like Unix pipes: one end is for sending data, and the other is for receiving. The two ends are typically owned by two different threads. But whereas Unix pipes are for sending bytes, channels are for sending Rust values. sender.send(item) puts a single value into the channel; receiver.recv() removes one. Ownership is transferred from the sending thread to the receiving thread. If the channel is empty, receiver.recv() blocks until a value is sent.

Figure 19-5. A channel for Strings. Ownership of the string msg is transferred from thread 1 to thread 2.

With channels, threads can communicate by passing values to one another. It’s a very simple way for threads to work together without using locking or shared memory.

This is not a new technique. Erlang has had isolated processes and message passing for 30 years now. Unix pipes have been around for almost 50 years. We tend to think of pipes as providing flexibility and composability, not concurrency, but in fact, they do all of the above. An example of a Unix pipeline is shown in Figure 19-6. It is certainly possible for all three programs to be working at the same time.

Figure 19-6. Execution of a Unix pipeline

Rust channels are faster than Unix pipes. Sending a value moves it rather than copying it, and moves are fast even when you’re moving data structures that contain many megabytes of data.

Sending Values

Over the next few sections, we’ll use channels to build a concurrent program that creates an inverted index, one of the key ingredients of a search engine. Every search engine works on a particular collection of documents. The inverted index is the database that tells which words appear where.

We’ll show the parts of the code that have to do with threads and channels. The complete program can be found at https://github.com/ProgrammingRust/fingertips. It’s short, about a thousand lines of code all told.

Our program is structured as a pipeline, as shown in Figure 19-7. Pipelines are only one of the many ways to use channels—we’ll discuss a few other uses later—but they’re a straightforward way to introduce concurrency into an existing single-threaded program.

Figure 19-7. The index builder pipeline. The arrows represent values sent via a channel from one thread to another. Disk I/O is not shown.

We’ll use a total of five threads, each doing a distinct task. Each thread produces output continually over the lifetime of the program. The first thread, for example, simply reads the source documents from disk into memory, one by one. (We want a thread to do this because we’ll be writing the simplest possible code here, using File::open and read_to_string, which are blocking APIs. We don’t want the CPU to sit idle whenever the disk is working.) The output of this stage is one long String per document, so this thread is connected to the next thread by a channel of Strings.

Our program will begin by spawning the thread that reads files. Suppose documents is a Vec<PathBuf>, a vector of filenames. The code to start our file-reading thread looks like this:

use std::fs::File;
use std::io::prelude::*;  // for `Read::read_to_string`
use std::thread::spawn;
use std::sync::mpsc::channel;

let (sender, receiver) = channel();

let handle = spawn(move || {
    for filename in documents {
        let mut f = File::open(filename)?;
        let mut text = String::new();
        f.read_to_string(&mut text)?;

        if sender.send(text).is_err() {
            break;
        }
    }
    Ok(())
});

Channels are part of the std::sync::mpsc module. We’ll explain what this name means later; first, let’s look at how this code works. We start by creating a channel:

let (sender, receiver) = channel();

The channel function returns a pair of values: a sender and a receiver. The underlying queue data structure is an implementation detail that the standard library does not expose.

Channels are typed. We’re going to use this channel to send the text of each file, so we have a sender of type Sender<String> and a receiver of type Receiver<String>. We could have explicitly asked for a channel of strings, by writing channel::<String>(). Instead, we let Rust’s type inference figure it out.

let handle = spawn(move || {

As before, we’re using std::thread::spawn to start a thread. Ownership of sender (but not receiver) is transferred to the new thread via this move closure.

The next few lines of code simply read files from disk:

    for filename in documents {
        let mut f = File::open(filename)?;
        let mut text = String::new();
        f.read_to_string(&mut text)?;

After successfully reading a file, we send its text into the channel:

        if sender.send(text).is_err() {
            break;
        }
    }

sender.send(text) moves the value text into the channel. Ultimately, it will be moved again to whoever receives the value. Whether text contains 10 lines of text or 10 megabytes, this operation copies three machine words (the size of a String), and the corresponding receiver.recv() call will also copy three machine words.

The send and recv methods both return Results, but these methods fail only if the other end of the channel has been dropped. A send call fails if the Receiver has been dropped, because otherwise the value would sit in the channel forever: without a Receiver, there’s no way for any thread to receive it. Likewise, a recv call fails if there are no values waiting in the channel and the Sender has been dropped, because otherwise recv would wait forever: without a Sender, there’s no way for any thread to send the next value. Dropping your end of a channel is the normal way of “hanging up,” closing the connection when you’re done with it.

In our code, sender.send(text) will fail only if the receiver’s thread has exited early. This is typical for code that uses channels. Whether that happened deliberately or due to an error, it’s OK for our reader thread to quietly shut itself down.

When that happens, or the thread finishes reading all the documents, it returns Ok(()):

    Ok(())
});

Note that this closure returns a Result. If the thread encounters an I/O error, it exits immediately, and the error is stored in the thread’s JoinHandle.

Of course, just like any other programming language, Rust admits many other possibilities when it comes to error handling. When an error happens, we could just print it out using println! and move on to the next file. We could pass errors along via the same channel that we’re using for data, making it a channel of Results—or create a second channel just for errors. The approach we’ve chosen here is both lightweight and responsible: we get to use the ? operator, so there’s not a bunch of boilerplate code, or even an explicit try/catch as you might see in Java; and yet errors won’t pass silently.

For convenience, our program wraps all of this code in a function that returns both the receiver (which we haven’t used yet) and the new thread’s JoinHandle:

fn start_file_reader_thread(documents: Vec<PathBuf>)
    -> (Receiver<String>, JoinHandle<io::Result<()>>)
{
    let (sender, receiver) = channel();

    let handle = spawn(move || {
        ...
    });

    (receiver, handle)
}

Note that this function launches the new thread and immediately returns. We’ll write a function like this for each stage of our pipeline.

Receiving Values

Now we have a thread running a loop that sends values. We can spawn a second thread running a loop that calls receiver.recv():

while let Ok(text) = receiver.recv() {
    do_something_with(text);
}

But Receivers are iterable, so there’s a nicer way to write this:

for text in receiver {
    do_something_with(text);
}

These two loops are equivalent. Either way we write it, if the channel happens to be empty when control reaches the top of the loop, the receiving thread will block until some other thread sends a value. The loop will exit normally when the channel is empty and the Sender has been dropped. In our program, that happens naturally when the reader thread exits. That thread is running a closure that owns the variable sender; when the closure exits, sender is dropped.

Now we can write code for the second stage of the pipeline:

fn start_file_indexing_thread(texts: Receiver<String>)
    -> (Receiver<InMemoryIndex>, JoinHandle<()>)
{
    let (sender, receiver) = channel();

    let handle = spawn(move || {
        for (doc_id, text) in texts.into_iter().enumerate() {
            let index = InMemoryIndex::from_single_document(doc_id, text);
            if sender.send(index).is_err() {
                break;
            }
        }
    });

    (receiver, handle)
}

This function spawns a thread that receives String values from one channel (texts) and sends InMemoryIndex values to another channel (sender/receiver). This thread’s job is to take each of the files loaded in the first stage and turn each document into a little one-file, in-memory inverted index.

The main loop of this thread is straightforward. All the work of indexing a document is done by the function make_single_file_index. We won’t show its source code here, but it’s a simple matter of splitting the input string along word boundaries, and then producing a map from words to lists of positions.

This stage doesn’t perform I/O, so it doesn’t have to deal with io::Errors. Instead of an io::Result<()>, it returns ().

Running the Pipeline

The remaining three stages are similar in design. Each one consumes a Receiver created by the previous stage. Our goal for the rest of the pipeline is to merge all the small indexes into a single large index file on disk. The fastest way we found to do this is in three stages. We won’t show the code here, just the type signatures of these three functions. The full source is online.

First, we merge indexes in memory until they get unwieldy (stage 3):

fn start_in_memory_merge_thread(file_indexes: Receiver<InMemoryIndex>)
    -> (Receiver<InMemoryIndex>, JoinHandle<()>)

We write these large indexes to disk (stage 4):

fn start_index_writer_thread(big_indexes: Receiver<InMemoryIndex>,
                             output_dir: &Path)
    -> (Receiver<PathBuf>, JoinHandle<io::Result<()>>)

Finally, if we have multiple large files, we merge them using a file-based merging algorithm (stage 5):

fn merge_index_files(files: Receiver<PathBuf>, output_dir: &Path)
    -> io::Result<()>

This last stage does not return a Receiver, because it’s the end of the line. It produces a single output file on disk. It doesn’t return a JoinHandle, because we don’t bother spawning a thread for this stage. The work is done on the caller’s thread.

Now we come to the code that launches the threads and checks for errors:

fn run_pipeline(documents: Vec<PathBuf>, output_dir: PathBuf)
    -> io::Result<()>
{
    // Launch all five stages of the pipeline.
    let (texts,   h1) = start_file_reader_thread(documents);
    let (pints,   h2) = start_file_indexing_thread(texts);
    let (gallons, h3) = start_in_memory_merge_thread(pints);
    let (files,   h4) = start_index_writer_thread(gallons, &output_dir);
    let result = merge_index_files(files, &output_dir);

    // Wait for threads to finish, holding on to any errors that they encounter.
    let r1 = h1.join().unwrap();
    h2.join().unwrap();
    h3.join().unwrap();
    let r4 = h4.join().unwrap();

    // Return the first error encountered, if any.
    // (As it happens, h2 and h3 can't fail: those threads
    // are pure in-memory data processing.)
    r1?;
    r4?;
    result
}

As before, we use .join().unwrap() to explicitly propagate panics from child threads to the main thread. The only other unusual thing here is that instead of using ? right away, we set aside the io::Result values until we’ve joined all four threads.

This pipeline is 40% faster than the single-threaded equivalent. That’s not bad for an afternoon’s work, but paltry-looking next to the 675% boost we got for the Mandelbrot program. We clearly haven’t saturated either the system’s I/O capacity or all the CPU cores. What’s going on?

Pipelines are like assembly lines in a manufacturing plant: performance is limited by the throughput of the slowest stage. A brand-new, untuned assembly line may be as slow as unit production, but assembly lines reward targeted tuning. In our case, measurement shows that the second stage is the bottleneck. Our indexing thread uses .to_lowercase() and .is_alphanumeric(), so it spends a lot of time poking around in Unicode tables. The other stages downstream from indexing spend most of their time asleep in Receiver::recv, waiting for input.

This means we should be able to go faster. As we address the bottlenecks, the degree of parallelism will rise. Now that you know how to use channels and our program is made of isolated pieces of code, it’s easy to see ways to address this first bottleneck. We could hand-optimize the code for the second stage, just like any other code; break up the work into two or more stages; or run multiple file-indexing threads at once.

Channel Features and Performance

The mpsc part of std::sync::mpsc stands for multi-producer, single-consumer, a terse description of the kind of communication Rust’s channels provide.

The channels in our sample program carry values from a single sender to a single receiver. This is a fairly common case. But Rust channels also support multiple senders, in case you need, say, a single thread that handles requests from many client threads, as shown in Figure 19-8.

Figure 19-8. A single channel receiving requests from many senders

Sender<T> implements the Clone trait. To get a channel with multiple senders, simply create a regular channel and clone the sender as many times as you like. You can move each Sender value to a different thread.

A Receiver<T> can’t be cloned, so if you need to have multiple threads receiving values from the same channel, you need a Mutex. We’ll show how to do it later in this chapter.

Rust channels are carefully optimized. When a channel is first created, Rust uses a special “one-shot” queue implementation. If you only ever send one object through the channel, the overhead is minimal. If you send a second value, Rust switches to a different queue implementation. It’s settling in for the long haul, really, preparing the channel to transfer many values while minimizing allocation overhead. And if you clone the Sender, Rust must fall back on yet another implementation, one that is safe when multiple threads are trying to send values at once. But even the slowest of these three implementations is a lock-free queue, so sending or receiving a value is at most a few atomic operations and a heap allocation, plus the move itself. System calls are needed only when the queue is empty and the receiving thread therefore needs to put itself to sleep. In this case, of course, traffic through your channel is not maxed out anyway.

Despite all that optimization work, there is one mistake that’s very easy for applications to make around channel performance: sending values faster than they can be received and processed. This causes an ever-growing backlog of values to accumulate in the channel. For example, in our program, we found that the file reader thread (stage 1) could load files much faster than the file indexing thread (stage 2) could index them. The result is that hundreds of megabytes of raw data would be read from disk and stuffed in the queue at once.

This kind of misbehavior costs memory and hurts locality. Even worse, the sending thread keeps running, using up CPU and other system resources to send ever more values just when those resources are most needed on the receiving end.

Here Rust again takes a page from Unix pipes. Unix uses an elegant trick to provide some backpressure, so that fast senders are forced to slow down: each pipe on a Unix system has a fixed size, and if a process tries to write to a pipe that’s momentarily full, the system simply blocks that process until there’s room in the pipe. The Rust equivalent is called a synchronous channel.

use std::sync::mpsc::sync_channel;

let (sender, receiver) = sync_channel(1000);

A synchronous channel is exactly like a regular channel except that when you create it, you specify how many values it can hold. For a synchronous channel, sender.send(value) is potentially a blocking operation. After all, the idea is that blocking is not always bad. In our example program, changing the channel in start_file_reader_thread to a sync_channel with room for 32 values cut memory usage by two-thirds on our benchmark data set, without decreasing throughput.

Thread Safety: Send and Sync

So far we’ve been acting as though all values can be freely moved and shared across threads. This is mostly true, but Rust’s full thread-safety story hinges on two built-in traits, std::marker::Send and std::marker::Sync.

Types that implement Send are safe to pass by value to another thread. They can be moved across threads.

Types that implement Sync are safe to pass by non-mut reference to another thread. They can be shared across threads.

By safe here, we mean the same thing we always mean: free from data races and other undefined behavior.

For example, in the process_files_in_parallel example , we used a closure to pass a Vec<String> from the parent thread to each child thread. We didn’t point it out at the time, but this means the vector and its strings are allocated in the parent thread, but freed in the child thread. The fact that Vec<String> implements Send is an API promise that this is OK: the allocator used internally by Vec and String is thread-safe.

(If you were to write your own Vec and String types with fast but non-thread-safe allocators, you’d have to implement them using types that are not Send, such as unsafe pointers. Rust would then infer that your NonThreadSafeVec and NonThreadSafeString types are not Send and restrict them to single-threaded use. But that’s a rare case.)

As Figure 19-9 illustrates, most types are both Send and Sync. You don’t even have to use #[derive] to get these traits on structs and enums in your program. Rust does it for you. A struct or enum is Send if its fields are Send, and Sync if its fields are Sync.

Figure 19-9. Send and Sync types

The few types that are not Send and Sync are mostly those that use mutability in a way that isn’t thread-safe. For example, consider std::rc::Rc<T>, the type of reference-counting smart pointers.

What would happen if you could share an Rc<String> across threads? If both threads happen to try to clone the Rc at the same time, as shown in Figure 19-10, we have a data race as both threads increment the shared reference count. The reference count could become inaccurate, leading to a use-after-free or double free later—undefined behavior.

Figure 19-10. Why Rc<String> is neither Sync nor Send

Of course, Rust prevents this. Here’s the code to set up this data race:

use std::thread::spawn;
use std::rc::Rc;

fn main() {
    let rc1 = Rc::new("hello threads".to_string());
    let rc2 = rc1.clone();
    spawn(move || {  // error
        rc2.clone();
    });
    rc1.clone();
}

Rust refuses to compile it, giving a detailed error message:

error[E0277]: the trait bound `Rc<String>: std::marker::Send` is not satisfied
              in `[closure@...]`
  --> concurrency_send_rc.rs:10:5
   |
10 |     spawn(move || {  // error
   |     ^^^^^ within `[closure@...]`, the trait `std::marker::Send` is not
   |           implemented for `Rc<String>`
   |
   = note: `Rc<String>` cannot be sent between threads safely
   = note: required because it appears within the type `[closure@...]`
   = note: required by `std::thread::spawn`

Now you can see how Send and Sync help Rust enforce thread safety. They appear as bounds in the type signature of functions like spawn that transfer data across thread boundaries. When you spawn a thread, the closure you pass must be Send, which means all the values it contains must be Send. Similarly, if you try to want to send values through a channel to another thread, the values must be Send.

Piping Almost Any Iterator to a Channel

Our inverted index builder is built as a pipeline. The code is clear enough, but it has us manually setting up channels and launching threads. By contrast, the iterator pipelines we built in Chapter 15 seemed to pack a lot more work into just a few lines of code. Can we build something like that for thread pipelines?

In fact, it would be nice if we could unify iterator pipelines and thread pipelines. Then our index builder could be written as an iterator pipeline. It might start like this:

documents.into_iter()
    .map(read_whole_file)
    .errors_to(error_sender)   // filter out error results
    .off_thread()              // spawn a thread for the above work
    .map(make_single_file_index)
    .off_thread()              // spawn another thread for stage 2
    ...

Traits allow us to add methods to standard library types, so we can actually do this. We start by writing a trait that declares the method we want:

use std::sync::mpsc;

pub trait OffThreadExt: Iterator {
    /// Transform this iterator into an off-thread iterator: the
    /// `next()` calls happen on a separate worker thread, so the
    /// iterator and the body of your loop run concurrently.
    fn off_thread(self) -> mpsc::IntoIter<Self::Item>;
}

Then we implement this trait for iterator types. It helps that mpsc::Receiver is already iterable.

use std::thread::spawn;

impl<T> OffThreadExt for T
    where T: Iterator + Send + 'static,
          T::Item: Send + 'static
{
    fn off_thread(self) -> mpsc::IntoIter<Self::Item> {
        // Create a channel to transfer items from the worker thread.
        let (sender, receiver) = mpsc::sync_channel(1024);

        // Move this iterator to a new worker thread and run it there.
        spawn(move || {
            for item in self {
                if sender.send(item).is_err() {
                    break;
                }
            }
        });

        // Return an iterator that pulls values from the channel.
        receiver.into_iter()
    }
}

The where clause in this code was determined via a process much like the one described in “Reverse-Engineering Bounds”. At first, we just had this:

impl<T: Iterator> OffThreadExt for T

That is, we wanted the implementation to work for all iterators. Rust was having none of it. Because we’re using spawn to move an iterator of type T to a new thread, we must specify T: Iterator + Send + 'static. Because we’re sending the items back over a channel, we must specify T::Item: Send + 'static. With these changes, Rust was satisfied.

This is Rust’s character in a nutshell: we’re free to add a concurrency power tool to almost every iterator in the language—but not without first understanding and documenting the restrictions that make it safe to use.

Beyond Pipelines

In this section, we used pipelines as our examples because pipelines are a nice, obvious way to use channels. Everyone understands them. They’re concrete, practical, and deterministic. Channels are useful for more than just pipelines, though. They’re also a quick, easy way to offer any asynchronous service to other threads in the same process.

For example, suppose you’d like to do logging on its own thread, as in Figure 19-8. Other threads could send log messages to the logging thread over a channel; since you can clone the channel’s Sender, many client threads can have senders that ship log messages to the same logging thread.

Running a service like logging on its own thread has advantages. The logging thread can rotate log files whenever it needs to. It doesn’t have to do any fancy coordination with the other threads. Those threads won’t be blocked. Messages will accumulate harmlessly in the channel for a moment until the logging thread gets back to work.

Channels can also be used for cases where one thread sends a request to another thread and needs to get some sort of response back. The first thread’s request can be a struct or tuple that includes a Sender, a sort of self-addressed envelope that the second thread uses to send its reply. This doesn’t mean the interaction must be synchronous. The first thread gets to decide whether to block and wait for the response or use the .try_recv() method to poll for it.

The tools we’ve presented so far—fork-join for highly parallel computation, channels for loosely connecting components—are sufficient for a wide range of applications. But we’re not done.

Shared Mutable State

In the months since you published the fern_sim crate in Chapter 8, your fern simulation software has really taken off. Now you’re creating a multiplayer real-time strategy game in which eight players compete to grow mostly authentic period ferns in a simulated Jurassic landscape. The server for this game is a massively parallel app, with requests pouring in on many threads. How can these threads coordinate to start a game as soon as eight players are available?

The problem to be solved here is that many threads need access to a shared list of players who are waiting to join a game. This data is necessarily both mutable and shared across all threads. If Rust doesn’t have shared mutable state, where does that leave us?

You could solve this by creating a new thread whose whole job is to manage this list. Other threads would communicate with it via channels. Of course, this costs a thread, which has some operating system overhead.

Another option is to use the tools Rust provides for safely sharing mutable data. Such things do exist. They’re low-level primitives that will be familiar to any system programmer who’s worked with threads. In this section, we’ll cover mutexes, read/write locks, condition variables, and atomic integers. Lastly, we’ll show how to implement global mutable variables in Rust.

What Is a Mutex?

A mutex (or lock) is used to force multiple threads to take turns when accessing certain data. We’ll introduce Rust’s mutexes in the next section. First, it makes sense to recall what mutexes are like in other languages. A simple use of a mutex in C++ might look like this:

// C++ code, not Rust
void FernEngine::JoinWaitingList(PlayerId player) {
    mutex.Acquire();

    waitingList.push_back(player);

    // Start a game if we have enough players waiting.
    if (waitingList.length() >= GAME_SIZE) {
        vector<PlayerId> players;
        waitingList.swap(players);
        StartGame(players);
    }

    mutex.Release();
}

The calls mutex.Acquire() and mutex.Release() mark the beginning and end of a critical section in this code. For each mutex in a program, only one thread can be running inside a critical section at a time. If one thread is in a critical section, all other threads that call mutex.Acquire() will block until the first thread reaches mutex.Release().

We say that the mutex protects the data: in this case, mutex protects waitingList. It is the programmer’s responsibility, though, to make sure every thread always acquires the mutex before accessing the data, and releases it afterward.

Mutexes are helpful for several reasons:

They prevent data races, situations where racing threads concurrently read and write the same memory. Data races are undefined behavior in C++ and Go. Managed languages like Java and C# promise not to crash, but the results of data races are still (to summarize) nonsense.

Even if data races didn’t exist, even if all reads and writes happened one by one in program order, without a mutex the actions of different threads could interleave in arbitrary ways. Imagine trying to write code that works even if other threads modify its data while it’s running. Imagine trying to debug it. It would be like your program was haunted.

Mutexes support programming with invariants, rules about the protected data that are true by construction when you set it up and maintained by every critical section.

Of course, all of these are really the same reason: uncontrolled race conditions make programming intractable. Mutexes bring some order to the chaos (though not as much order as channels or fork-join).

However, in most languages, mutexes are very easy to mess up. In C++, as in most languages, the data and the lock are separate objects. Ideally, comments explain that every thread must acquire the mutex before touching the data:

class FernEmpireApp {
    ...

private:
    // List of players waiting to join a game. Protected by `mutex`.
    vector<PlayerId> waitingList;

    // Lock to acquire before reading or writing `waitingList`.
    Mutex mutex;
    ...
};

But even with such nice comments, the compiler can’t enforce safe access here. When a piece of code neglects to acquire the mutex, we get undefined behavior. In practice, this means bugs that are extremely hard to reproduce and fix.

Even in Java, where there is some notional association between objects and mutexes, the relationship does not run very deep. The compiler makes no attempt to enforce it, and in practice, the data protected by a lock is rarely exactly the associated object’s fields. It often includes data in several objects. Locking schemes are still tricky. Comments are still the main tool for enforcing them.

Mutex<T>

Now we’ll show an implementation of the waiting list in Rust. In our Fern Empire game server, each player has a unique ID:

type PlayerId = u32;

The waiting list is just a collection of players:

const GAME_SIZE: usize = 8;

/// A waiting list never grows to more than GAME_SIZE players.
type WaitingList = Vec<PlayerId>;

The waiting list is stored as a field of the FernEmpireApp, a singleton that’s set up in an Arc during server startup. Each thread has an Arc pointing to it. It contains all the shared configuration and other flotsam our program needs. Most of that is read-only. Since the waiting list is both shared and mutable, it must be protected by a Mutex:

use std::sync::Mutex;

/// All threads have shared access to this big context struct.
struct FernEmpireApp {
    ...
    waiting_list: Mutex<WaitingList>,
    ...
}

Unlike C++, in Rust the protected data is stored inside the Mutex. Setting up the Mutex looks like this:

let app = Arc::new(FernEmpireApp {
    ...
    waiting_list: Mutex::new(vec![]),
    ...
});

Creating a new Mutex looks like creating a new Box or Arc, but while Box and Arc signify heap allocation, Mutex is solely about locking. If you want your Mutex to be allocated in the heap, you have to say so, as we’ve done here by using Arc::new for the whole app and Mutex::new just for the protected data. These types are commonly used together: Arc is handy for sharing things across threads, and Mutex is handy for mutable data that’s shared across threads.

Now we can implement the join_waiting_list method that uses the mutex:

impl FernEmpireApp {
    /// Add a player to the waiting list for the next game.
    /// Start a new game immediately if enough players are waiting.
    fn join_waiting_list(&self, player: PlayerId) {
        // Lock the mutex and gain access to the data inside.
        // The scope of `guard` is a critical section.
        let mut guard = self.waiting_list.lock().unwrap();

        // Now do the game logic.
        guard.push(player);
        if guard.len() == GAME_SIZE {
            let players = guard.split_off(0);
            self.start_game(players);
        }
    }
}

The only way to get at the data is to call the .lock() method:

let mut guard = self.waiting_list.lock().unwrap();

self.waiting_list.lock() blocks until the mutex can be obtained. The MutexGuard<WaitingList> value returned by this method call is a thin wrapper around a &mut WaitingList. Thanks to deref coercions, discussed , we can call WaitingList methods directly on the guard:

guard.push(player);

The guard even lets us borrow direct references to the underlying data. Rust’s lifetime system ensures those references can’t outlive the guard itself. There is no way to access the data in a Mutex without holding the lock.

When guard is dropped, the lock is released. Ordinarily that happens at the end of the block, but you can also drop it manually:

if guard.len() == GAME_SIZE {
    let players = guard.split_off(0);
    drop(guard);  // don't keep the list locked while starting a game
    self.start_game(players);
}

mut and Mutex

It may seem odd—certainly it seemed odd to us at first—that our join_waiting_list method doesn’t take self by mut reference. Its type signature is:

fn join_waiting_list(&self, player: PlayerId)

The underlying collection, Vec<PlayerId>, does require a mut reference when you call its push method. Its type signature is:

pub fn push(&mut self, item: T)

And yet this code compiles and runs fine. What’s going on here?

In Rust, mut means exclusive access. Non-mut means shared access.

We’re used to types passing mut access along from the parent to the child, from the container to the contents. You only expect to be able to call mut methods on starships[id].engine if you have a mut reference to starships to begin with (or you own starships, in which case congratulations on being Elon Musk). That’s the default, because if you don’t have exclusive access to the parent, Rust generally has no way of ensuring that you have exclusive access to the child.

But Mutex does have a way: the lock. In fact, a mutex is little more than a way to do exactly this, to provide exclusive (mut) access to the data inside, even though many threads may have shared (non-mut) access to the Mutex itself.

Rust’s type system is telling us what Mutex does. It dynamically enforces exclusive access, something that’s usually done statically, at compile time, by the Rust compiler.

(You may recall that std::cell::RefCell does the same, except without trying to support multiple threads. Mutex and RefCell are both flavors of interior mutability, which we covered .)

Why Mutexes Are Not Always a Good Idea

Before we started on mutexes, we presented some approaches to concurrency that might have seemed weirdly easy to use correctly if you’re coming from C++. This is no coincidence: these approaches are designed to provide strong guarantees against the most confusing aspects of concurrent programming. Programs that exclusively use fork-join parallelism are deterministic and can’t deadlock. Programs that use channels are almost as well-behaved. Those that use channels exclusively for pipelining, like our index builder, are deterministic: the timing of message delivery can vary, but it won’t affect the output. And so on. Guarantees about multithreaded programs are nice!

The design of Rust’s Mutex will almost certainly have you using mutexes more systematically and more sensibly than you ever have before. But it’s worth pausing and thinking about what Rust’s safety guarantees can and can’t help with.

Safe Rust code cannot trigger a data race, a specific kind of bug where multiple threads read and write the same memory concurrently, producing meaningless results. This is great: data races are always bugs, and they are not rare in real multithreaded programs.

However, threads that use mutexes are subject to some other problems that Rust doesn’t fix for you:

Valid Rust programs can’t have data races, but they can still have other race conditions—situations where a program’s behavior depends on timing among threads and may therefore vary from run to run. Some race conditions are benign. Some manifest as general flakiness and incredibly hard-to-fix bugs. Using mutexes in an unstructured way invites race conditions. It’s up to you to make sure they’re benign.

Shared mutable state also affects program design. Where channels serve as an abstraction boundary in your code, making it easy to separate isolated components for testing, mutexes encourage a “just-add-a-method” way of working that can lead to a monolithic blob of interrelated code.

Lastly, mutexes are just not as simple as they seem at first, as the next two sections will show.

All of these problems are inherent in the tools. Use a more structured approach when you can; use a Mutex when you must.

Deadlock

A thread can deadlock itself by trying to acquire a lock that it’s already holding:

let mut guard1 = self.waiting_list.lock().unwrap();
let mut guard2 = self.waiting_list.lock().unwrap();  // deadlock

Suppose the first call to self.waiting_list.lock() succeeds, taking the lock. The second call sees that the lock is held, so it blocks, waiting for it to be released. It will be waiting forever. The waiting thread is the one that’s holding the lock.

To put it another way, the lock in a Mutex is not a recursive lock.

Here the bug is obvious. In a real program, the two lock() calls might be in two different methods, one of which calls the other. The code for each method, taken separately, would look fine. There are other ways to get deadlock, too, involving multiple threads that each acquire multiple mutexes at once. Rust’s borrow system can’t protect you from deadlock. The best protection is to keep critical sections small: get in, do your work, and get out.

It’s also possible to get deadlock with channels. For example, two threads might block, each one waiting to receive a message from the other. However, again, good program design can give you high confidence that this won’t happen in practice. In a pipeline, like our inverted index builder, data flow is acyclic. Deadlock is as unlikely in such a program as in a Unix shell pipeline.

Poisoned Mutexes

Mutex::lock() returns a Result, for the same reason that JoinHandle::join() does: to fail gracefully if another thread has panicked. When we write handle.join().unwrap(), we’re telling Rust to propagate panic from one thread to another. The idiom mutex.lock().unwrap() is similar.

If a thread panics while holding a Mutex, Rust marks the Mutex as poisoned. Any subsequent attempt to lock the poisoned Mutex will get an error result. Our .unwrap() call tells Rust to panic if that happens, propagating panic from the other thread to this one.

How bad is it to have a poisoned mutex? Poison sounds deadly, but this scenario is not necessarily fatal. As we said in Chapter 7, panic is safe. One panicking thread leaves the rest of the program in a safe state.

The reason mutexes are poisoned on panic, then, is not for fear of undefined behavior. Rather, the concern is that you’ve probably been programming with invariants. Since your program panicked and bailed out of a critical section without finishing what it was doing, perhaps having updated some fields of the protected data but not others, it’s possible that the invariants are now broken. Rust poisons the mutex to prevent other threads from blundering unwittingly into this broken situation and making it worse. You can still lock a poisoned mutex and access the data inside, with mutual exclusion fully enforced; see the documentation for PoisonError::into_inner(). But you won’t do it by accident.

Multi-producer Channels Using Mutexes

We mentioned earlier that Rust’s channels are multiple-producer, single-consumer. Or to put it more concretely, a channel only has one Receiver. We can’t have a thread pool where many threads use a single mpsc channel as a shared worklist.

However, it turns out there is a very simple workaround, using only standard library pieces. We can add a Mutex around the Receiver and share it anyway. Here is a module that does so:

pub mod shared_channel {
    use std::sync::{Arc, Mutex};
    use std::sync::mpsc::{channel, Sender, Receiver};

    /// A thread-safe wrapper around a `Receiver`.
    #[derive(Clone)]
    pub struct SharedReceiver<T>(Arc<Mutex<Receiver<T>>>);

    impl<T> Iterator for SharedReceiver<T> {
        type Item = T;

        /// Get the next item from the wrapped receiver.
        fn next(&mut self) -> Option<T> {
            let guard = self.0.lock().unwrap();
            guard.recv().ok()
        }
    }

    /// Create a new channel whose receiver can be shared across threads.
    /// This returns a sender and a receiver, just like the stdlib's
    /// `channel()`, and sometimes works as a drop-in replacement.
    pub fn shared_channel<T>() -> (Sender<T>, SharedReceiver<T>) {
        let (sender, receiver) = channel();
        (sender, SharedReceiver(Arc::new(Mutex::new(receiver))))
    }
}

We’re using an Arc<Mutex<Receiver<T>>>. The generics have really piled up. This happens more often in Rust than in C++. It might seem this would get confusing, but often, as in this case, just reading off the names gives the meaning in plain English:

Figure 19-11.  

Read/Write Locks (RwLock<T>)

Now let’s move on from mutexes to the other thread synchronization tools provided in Rust’s standard library toolkit, std::sync. We’ll move quickly, since a complete discussion of these tools is beyond the scope of this book.

Server programs often have configuration information that is loaded once and rarely ever changes. Most threads only query the configuration, but since the configuration can change—it may be possible to ask the server to reload its configuration from disk, for example—it must be protected by a lock anyway. In cases like this, a mutex can work, but it’s an unnecessary bottleneck. Threads shouldn’t have to take turns querying the configuration if it’s not changing. This is a case for a read/write lock, or RwLock.

Whereas a mutex has a single lock method, a read/write lock has two locking methods, read and write. The RwLock::write method is like Mutex::lock. It waits for exclusive, mut access to the protected data. The RwLock::read method provides non-mut access, with the advantage that it is less likely to have to wait, because many threads can safely read at once. With a mutex, at any given moment, the protected data has only one reader or writer (or none). With a read/write lock, it can have either one writer or many readers, much like Rust references generally.

FernEmpireApp might have a struct for configuration, protected by a RwLock:

use std::sync::RwLock;

struct FernEmpireApp {
    ...
    config: RwLock<AppConfig>,
    ...
}

Methods that read the configuration would use RwLock::read():

/// True if experimental fungus code should be used.
fn mushrooms_enabled(&self) -> bool {
    let config_guard = self.config.read().unwrap();
    config_guard.mushrooms_enabled
}

The method to reload the configuration would use RwLock::write():

fn reload_config(&self) -> io::Result<()> {
    let new_config = AppConfig::load()?;
    let mut config_guard = self.config.write().unwrap();
    *config_guard = new_config;
    Ok(())
}

Rust, of course, is uniquely well suited to enforce the safety rules on RwLock data. The single-writer-or-multiple-reader concept is the core of Rust’s borrow system. self.config.read() returns a guard that provides non-mut (shared) access to the AppConfig; self.config.write() returns a different type of guard that provides mut (exclusive) access.

Condition Variables (Condvar)

Often a thread needs to wait until a certain condition becomes true:

During server shutdown, the main thread may need to wait until all other threads are finished exiting.

When a worker thread has nothing to do, it needs to wait until there is some data to process.

A thread implementing a distributed consensus protocol may need to wait until a quorum of peers have responded.

Sometimes, there’s a convenient blocking API for the exact condition we want to wait on, like JoinHandle::join for the server shutdown example. In other cases, there is no built-in blocking API. Programs can use condition variables to build their own. In Rust, the std::sync::Condvar type implements condition variables. A Condvar has methods .wait() and .notify_all(); .wait() blocks until some other thread calls .notify_all().

There’s a bit more to it than that, since a condition variable is always about a particular true-or-false condition about some data protected by a particular Mutex. This Mutex and the Condvar are therefore related. A full explanation is more than we have room for here, but for the benefit of programmers who have used condition variables before, we’ll show the two key bits of code.

When the desired condition becomes true, we call Condvar::notify_all (or notify_one) to wake up any waiting threads:

self.has_data_condvar.notify_all();

To go to sleep and wait for a condition to become true, we use Condvar::wait():

while !guard.has_data() {
    guard = self.has_data_condvar.wait(guard).unwrap();
}

This while loop is a standard idiom for condition variables. However, the signature of Condvar::wait is unusual. It takes a MutexGuard object by value, consumes it, and returns a new MutexGuard on success. This captures the intuition that the wait method releases the mutex, then reacquires it before returning. Passing the MutexGuard by value is a way of saying, “I bestow upon you, .wait() method, my exclusive authority to release the mutex.”

Atomics

The std::sync::atomic module contains atomic types for lock-free concurrent programming. These types are basically the same as Standard C++ atomics:

AtomicIsize and AtomicUsize are shared integer types corresponding to the single-threaded isize and usize types.

An AtomicBool is a shared bool value.

An AtomicPtr<T> is a shared value of the unsafe pointer type *mut T.

The proper use of atomic data is beyond the scope of this book. Suffice it to say that multiple threads can read and write an atomic value at once without causing data races.

Instead of the usual arithmetic and logical operators, atomic types expose methods that perform atomic operations, individual loads, stores, exchanges, and arithmetic operations that happen safely, as a unit, even if other threads are also performing atomic operations that touch the same memory location. Incrementing an AtomicIsize named atom looks like this:

use std::sync::atomic::Ordering;

atom.fetch_add(1, Ordering::SeqCst);

These methods may compile to specialized machine language instructions. On the x86-64 architecture, this .fetch_add() call compiles to a lock incq instruction, where an ordinary n += 1 might compile to a plain incq instruction or any number of variations on that theme. The Rust compiler also has to forgo some optimizations around the atomic operation, since—unlike a normal load or store—it’s legitimately observable by other threads right away.

The argument Ordering::SeqCst is a memory ordering. Memory orderings are something like transaction isolation levels in a database. They tell the system how much you care about such philosophical notions as causes preceding effects and time not having loops, as opposed to performance. Memory orderings are crucial to program correctness, and they are tricky to understand and reason about. Happily, the performance penalty for choosing sequential consistency, the strictest memory ordering, is often quite low—unlike the performance penalty for putting a SQL database into SERIALIZABLE mode. So when in doubt, use Ordering::SeqCst. Rust inherits several other memory orderings from Standard C++ atomics, with various weaker guarantees about being and time. We won’t discuss them here.

One simple use of atomics is for cancellation. Suppose we have a thread that’s doing some long-running computation, such as rendering a video, and we would like to be able to cancel it asynchronously. The problem is to communicate to the thread that we want it to shut down. We can do this via a shared AtomicBool:

use std::sync::atomic::{AtomicBool, Ordering};

let cancel_flag = Arc::new(AtomicBool::new(false));
let worker_cancel_flag = cancel_flag.clone();

This code creates two Arc<AtomicBool> smart pointers that point to the same heap-allocated AtomicBool, whose initial value is false. The first, named cancel_flag, will stay in the main thread. The second, worker_cancel_flag, will be moved to the worker thread.

Here is the code for the worker:

let worker_handle = spawn(move || {
    for pixel in animation.pixels_mut() {
        render(pixel); // ray-tracing - this takes a few microseconds
        if worker_cancel_flag.load(Ordering::SeqCst) {
            return None;
        }
    }
    Some(animation)
});

After rendering each pixel, the thread checks the value of the flag by calling its .load() method:

worker_cancel_flag.load(Ordering::SeqCst)

If in the main thread we decide to cancel the worker thread, we store true in the AtomicBool, then wait for the thread to exit:

// Cancel rendering.
cancel_flag.store(true, Ordering::SeqCst);

// Discard the result, which is probably `None`.
worker_handle.join().unwrap();

Of course, there are other ways to implement this. The AtomicBool here could be replaced with a Mutex<bool> or a channel. The main difference is that atomics have minimal overhead. Atomic operations never use system calls. A load or store often compiles to a single CPU instruction.

Atomics are a form of interior mutability, like Mutex or RwLock, so their methods take self by shared (non-mut) reference. This makes them useful as simple global variables.

Global Variables

Suppose we are writing networking code. We would like to have a global variable, a counter that we increment every time we serve a packet:

/// Number of packets the server has successfully handled.
static PACKETS_SERVED: usize = 0;

This compiles fine. There’s just one problem. PACKETS_SERVED is not mutable, so we can never change it.

Rust does everything it reasonably can to discourage global mutable state. Constants declared with const are, of course, immutable. Static variables are also immutable by default, so there is no way to get a mut reference to one. A static can be declared mut, but then accessing it is unsafe. Rust’s insistence on thread safety is a major reason for all of these rules.

Global mutable state also has unfortunate software engineering consequences: it tends to make the various parts of a program more tightly coupled, harder to test, and harder to change later. Still, in some cases there’s just no reasonable alternative, so we had better find a safe way to declare mutable static variables.

The simplest way to support incrementing PACKETS_SERVED, while keeping it thread-safe, is to make it an atomic integer:

use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};

static PACKETS_SERVED: AtomicUsize = ATOMIC_USIZE_INIT;

The constant ATOMIC_USIZE_INIT is an AtomicUsize with the value 0. We use this constant instead of the expression AtomicUsize::new(0) because the initial value of a static must be a constant; as of Rust 1.17, method calls are not allowed. Similarly, ATOMIC_ISIZE_INIT is an AtomicIsize zero, and ATOMIC_BOOL_INIT is an AtomicBool with the value false.

Once this static is declared, incrementing the packet count is straightforward:

PACKETS_SERVED.fetch_add(1, Ordering::SeqCst);

Atomic globals are limited to simple integers and booleans. Still, creating a global variable of any other type amounts to solving the same two problems, both easy:

The variable must be made thread-safe somehow, because otherwise it can’t be global: for safety, static variables must be both Sync and non-mut.

Fortunately, we’ve already seen the solution for this problem. Rust has types for safely sharing values that change: Mutex, RwLock, and the atomic types. These types can be modified even when declared as non-mut. It’s what they do. (See “mut and Mutex”.)

As mentioned above, static initializers can’t call functions. This means that the obvious way to declare a static Mutex doesn’t work:

static HOSTNAME: Mutex<String> =
    Mutex::new(String::new());  // error: function call in static

We can use the lazy_static crate to get around this problem.

We introduced the lazy_static crate in “Building Regex Values Lazily”. Defining a variable with the lazy_static! macro lets you use any expression you like to initialize it; it runs the first time the variable is dereferenced, and the value is saved for all subsequent uses.

We can declare a global Mutex with lazy_static like this:

#[macro_use] extern crate lazy_static;

use std::sync::Mutex;

lazy_static! {
    static ref HOSTNAME: Mutex<String> = Mutex::new(String::new());
}

The same technique works for RwLock and AtomicPtr variables.

Using lazy_static! imposes a tiny performance cost on each access to the static data. The implementation uses std::sync::Once, a low-level synchronization primitive designed for one-time initialization. Behind the scenes, each time a lazy static is accessed, the program executes an atomic load instruction to check that initialization has already occurred. (Once is rather special-purpose, so we will not cover it in detail here. It is usually more convenient to use lazy_static! instead. However, it is handy for initializing non-Rust libraries; for an example, see “A Safe Interface to libgit2”.)

What Hacking Concurrent Code in Rust Is Like

We’ve shown three techniques for using threads in Rust: fork-join parallelism, channels, and shared mutable state with locks. Our aim has been to provide a good introduction to the pieces Rust provides, with a focus on how they can fit together into real programs.

Rust insists on safety, so from the moment you decide to write a multithreaded program, the focus is on building safe, structured communication. Keeping threads mostly isolated is a good way to convince Rust that what you’re doing is safe. It happens that isolation is also a good way to make sure what you’re doing is correct and maintainable. Again, Rust guides you toward good programs.

More important, Rust lets you combine techniques and experiment. You can iterate fast: arguing with the compiler gets you up and running correctly a lot faster than debugging data races.

Chapter 20. Macros

A cento (pronounced “cento,” from the Latin for “patchwork”) is a poem made up entirely of lines quoted from another poet.

Matt Madden

Your quote here.

Bjarne Stroustrup

Rust supports macros, a way to extend the language in ways that go beyond what you can do with functions alone. For example, we’ve seen the assert_eq! macro, which is handy for tests:

assert_eq!(gcd(6, 10), 2);

This could have been written as a generic function, but the assert_eq! macro does several things that functions can’t do. One is that when an assertion fails, assert_eq! generates an error message containing the filename and line number of the assertion. Functions have no way of getting that information. Macros can, because the way they work is completely different.

Macros are a kind of shorthand. During compilation, before types are checked and long before any machine code is generated, each macro call is expanded—that is, it’s replaced with some Rust code. The preceding macro call expands to this:

match (&gcd(6, 10), &2) {
    (left_val, right_val) => {
        if !(*left_val == *right_val) {
            panic!("assertion failed: `(left == right)`, \
                    (left: `{:?}`, right: `{:?}`)", left_val, right_val);
        }
    }
}

panic! is also a macro, so it then expands to some more Rust code. That code uses two other macros, file!() and line!(). Once every macro call in the crate is fully expanded, Rust moves on to the next phase of compilation.

At run time, an assertion failure would look like this (and would indicate a bug in the gcd() function, since 2 is the correct answer):

thread 'main' panicked at 'assertion failed: `(left == right)`, (left: `17`,
right: `2`)', gcd.rs:7

If you’re coming from C++, you may have had some bad experiences with macros. Rust macros take a different approach, similar to Scheme’s syntax-rules. Compared to C++ macros, Rust macros are better integrated with the rest of the language and therefore less error prone. Macro calls are always marked with an exclamation point, so they stand out when you’re reading code, and they can’t be called accidentally when you meant to call a function. Rust macros never insert unmatched brackets or parentheses. And Rust macros come with pattern matching, making it easier to write macros that are both maintainable and appealing to use.

In this chapter, we’ll show how to write macros using several examples. Then we’ll dig into how macros work, because like much of Rust, the tool rewards deep understanding. Lastly, we’ll see what we can do when simple pattern matching is not enough.

Macro Basics

Figure 20-1 shows part of the source code for the assert_eq! macro.

Figure 20-1. The assert_eq! macro

macro_rules! is the main way to define macros in Rust. Note that there is no ! after assert_eq in this macro definition: the ! is only included when calling a macro, not when defining it.

Not all macros are defined this way: a few, like file!, line!, and macro_rules! itself, are built into the compiler, and we’ll talk about another approach, called procedural macros, at the end of this chapter. But for the most part, we’ll focus on macro_rules!, which is (so far) the easiest way to write your own.

A macro defined with macro_rules! works entirely by pattern matching. The body of a macro is just a series of rules:

( pattern1 ) => ( template1 );

( pattern2 ) => ( template2 );

...

The version of assert_eq! in Figure 20-1 has just one pattern and one template.

Incidentally, you can use square brackets or curly braces instead of parentheses around the pattern or the template; it makes no difference to Rust. Likewise, when you call a macro, these are all equivalent:

assert_eq!(gcd(6, 10), 2);
assert_eq![gcd(6, 10), 2];
assert_eq!{gcd(6, 10), 2}

The only difference is that semicolons are usually optional after curly braces. By convention, we use parentheses when calling assert_eq!, square brackets for vec!, and curly braces for macro_rules!; but it’s just a convention.

Basics of Macro Expansion

Rust expands macros very early during compilation. The compiler reads your source code from beginning to end, defining and expanding macros as it goes. You can’t call a macro before it is defined, because Rust expands each macro call before it even looks at the rest of the program. (By contrast, functions and other items don’t have to be in any particular order. It’s OK to call a function that won’t be defined until later in the crate.)

When Rust expands an assert_eq! macro call, what happens is a lot like evaluating a match expression. Rust first matches the arguments against the pattern, as shown in Figure 20-2.

Figure 20-2. Expanding a macro, part 1: pattern matching on the arguments

Macro patterns are a mini-language within Rust. They’re essentially regular expressions for matching code. But where regular expressions operate on characters, patterns operate on tokens—the numbers, names, punctuation marks, and so forth that are the building blocks of Rust programs. This means you can use comments and whitespace freely in macro patterns to make them as readable as possible. Comments and whitespace aren’t tokens, so they don’t affect matching.

Another important difference between regular expressions and macro patterns is that parentheses, brackets, and braces always occur in matched pairs in Rust. This is checked before macros are expanded, not only in macro patterns but throughout the language.

In this example, our pattern contains $left:expr, which tells Rust to match an expression (in this case, gcd(6, 10)) and assign it the name $left. Rust then matches the comma in the pattern with the comma following gcd’s arguments. Just like regular expressions, patterns have only a few special characters that trigger interesting matching behavior; everything else, like this comma, has to match verbatim or else matching fails. Lastly, Rust matches the expression 2 and gives it the name $right.

Both code fragments in this pattern are of type expr: they expect expressions. We’ll see other types of code fragments in “Fragment Types”.

Since this pattern matched all of the arguments, Rust expands the corresponding template (Figure 20-3).

Figure 20-3. Expanding a macro, part 2: filling in the template

Rust replaces $left and $right with the code fragments it found during matching.

It’s a common mistake to include the fragment type in the output template: writing $left:expr rather than just $left. Rust does not immediately detect this kind of error. It sees $left as a substitution, and then it treats :expr just like everything else in the template: tokens to be included in the macro’s output. So the errors won’t happen until you call the macro; then it will generate bogus output that won’t compile. If you get error messages like expected type, found `:` when using a new macro, check it for this mistake. (“Debugging Macros” offers more general advice for situations like this.)

Macro templates aren’t much different from any of a dozen template languages commonly used in web programming. The only difference—and it’s a significant one—is that the output is Rust code.

Unintended Consequences

Plugging fragments of code into templates is subtly different from regular code that works with values. These differences aren’t always obvious at first. The macro we’ve been looking at, assert_eq!, contains some slightly strange bits of code for reasons that say a lot about macro programming. Let’s look at two funny bits in particular.

First, why does this macro create the variables left_val and right_val? Is there some reason we can’t simplify the template to look like this?

if !($left == $right) {
    panic!("assertion failed: `(left == right)` \
            (left: `{:?}`, right: `{:?}`)", $left, $right)
}

To answer this question, try mentally expanding the macro call assert_eq!(letters.pop(), Some('z')). What would the output be? Naturally, Rust would plug the matched expressions into the template in multiple places. It seems like a bad idea to evaluate the expressions all over again when building the error message, though, and not just because it would take twice as long: since letters.pop() removes a value from a vector, it’ll produce a different value the second time we call it! That’s why the real macro computes $left and $right only once and stores their values.

Moving on to the second question: why does this macro borrow references to the values of $left and $right? Why not just store the values in variables, like this?

macro_rules! bad_assert_eq {
    ($left:expr, $right:expr) => ({
        match ($left, $right) {
            (left_val, right_val) => {
                if !(left_val == right_val) {
                    panic!("assertion failed" /* ... */);
                }
            }
        }
    });
}

For the particular case we’ve been considering, where the macro arguments are integers, this would work fine. But if the caller passed, say, a String variable as $left or $right, this code would move the value out of the variable!

fn main() {
    let s = "a rose".to_string();
    bad_assert_eq!(s, "a rose");
    println!("confirmed: {} is a rose", s);  // error: use of moved value "s"
}

Since we don’t want assertions to move values, the macro borrows references instead.

(You may have wondered why the macro uses match rather than let to define the variables. We wondered too. It turns out there’s no particular reason for this. let would have been equivalent.)

In short, macros can do surprising things. If strange things happen around a macro you’ve written, it’s a good bet that the macro is to blame.

One bug that you won’t see is this classic C++ macro bug:

// buggy C++ macro to add 1 to a number
#define ADD_ONE(n)  n + 1

For reasons familiar to most C++ programmers, and not worth explaining fully here, unremarkable code like ADD_ONE(1) * 10 or ADD_ONE(1 << 4) produces very surprising results with this macro. To fix it, you’d add more parentheses to the macro definition. This isn’t necessary in Rust, because Rust macros are better integrated with the language. Rust knows when it’s handling expressions, so it effectively adds parentheses whenever it pastes one expression into another.

Repetition

The standard vec! macro comes in two forms:

// Repeat a value N times
let buffer = vec![0_u8; 1000];

// A list of values, separated by commas
let numbers = vec!["udon", "ramen", "soba"];

It can be implemented like this:

macro_rules! vec {
    ($elem:expr ; $n:expr) => {
        ::std::vec::from_elem($elem, $n)
    };
    ( $( $x:expr ),* ) => {
        <[_]>::into_vec(Box::new([ $( $x ),* ]))
    };
    ( $( $x:expr ),+ ,) => {
        vec![ $( $x ),* ]
    };
}

There are three rules here. We’ll explain how multiple rules work and then look at each rule in turn.

When Rust expands a macro call like vec![1, 2, 3], it starts by trying to match the arguments 1, 2, 3 with the pattern for the first rule, in this case $elem:expr ; $n:expr. This fails to match: 1 is an expression, but the pattern requires a semicolon after that, and we don’t have one. So Rust then moves on to the second rule, and so on. If no rules match, it’s an error.

The first rule handles uses like vec![0u8; 1000]. It happens that there is a standard function, std::vec::from_elem, that does exactly what’s needed here, so this rule is straightforward.

The second rule handles vec!["udon", "ramen", "soba"]. The pattern, $( $x:expr ),*, uses a feature we haven’t seen before: repetition. It matches 0 or more expressions, separated by commas. More generally, the syntax $( PATTERN ),* is used to match any comma-separated list, where each item in the list matches PATTERN.

The * here has the same meaning as in regular expressions (“0 or more”) although admittedly regexps do not have a special ,* repeater. You can also use + to require at least one match. There is no ? syntax. The following table gives the full suite of repetition patterns:

Pattern | Meaning
$( ... )* | Match 0 or more times with no separator
$( ... ),* | Match 0 or more times, separated by commas
$( ... );* | Match 0 or more times, separated by semicolons
$( ... )+ | Match 1 or more times with no separator
$( ... ),+ | Match 1 or more times, separated by commas
$( ... );+ | Match 1 or more times, separated by semicolons

The code fragment $x is not just a single expression but a list of expressions. The template for this rule uses repetition syntax too:

<[_]>::into_vec(Box::new([ $( $x ),* ]))

Again, there are standard methods that do exactly what we need. This code creates a boxed array, and then uses the [T]::into_vec method to convert the boxed array to a vector.

The first bit, <[_]>, is an unusual way to write the type “slice of something”, while expecting Rust to infer the element type. Types whose names are plain identifiers can be used in expressions without any fuss, but types like fn(), &str, or [_] must be wrapped in angle brackets.

Repetition comes in at the end of the template, where we have $($x),*. This $(...),* is the same syntax we saw in the pattern. It iterates over the list of expressions that we matched for $x and inserts them all into the template, separated by commas.

In this case, the repeated output looks just like the input. But that doesn’t have to be the case. We could have written the rule like this:

( $( $x:expr ),* ) => {
    {
        let mut v = Vec::new();
        $( v.push($x); )*
        v
    }
};

Here, the part of the template that reads $( v.push($x); )* inserts a call to v.push() for each expression in $x.

Unlike the rest of Rust, patterns using $( ... ),* do not automatically support an optional trailing comma. However, there’s a standard trick for supporting trailing commas by adding an extra rule. That is what the third rule of our vec! macro does:

( $( $x:expr ),+ ,) => {  // if trailing comma is present,
    vec![ $( $x ),* ]     // retry without it
};

We use $( ... ),+ , to match a list with an extra comma. Then, in the template, we call vec! recursively, leaving the extra comma out. This time the second rule will match.

Built-In Macros

The Rust compiler supplies several macros that are helpful when you’re defining your own macros. None of these could be implemented using macro_rules! alone. They’re hardcoded in rustc:

file!() expands to a string literal: the current filename. line!() and column!() expand to u32 literals giving the current line (counting from 1) and column (counting from 0).

If one macro calls another, which calls another, all in different files, and the last macro calls file!(), line!(), or column!(), it will expand to indicate the location of the first macro call.

stringify!(...tokens...) expands to a string literal containing the given tokens. The assert! macro uses this to generate an error message that includes the code of the assertion.

Macro calls in the argument are not expanded: stringify!(line!()) expands to the string "line!()".

Rust constructs the string from the tokens, so there are no line breaks or comments in the string.

concat!(str0, str1, ...) expands to a single string literal made by concatenating its arguments.

Rust also defines these macros for querying the build environment:

cfg!(...) expands to a Boolean constant, true if the current build configuration matches the condition in parentheses. For example, cfg!(debug_assertions) is true if you’re compiling with debug assertions enabled.

This macro supports exactly the same syntax as the #[cfg(...)] attribute described in “Attributes” but instead of conditional compilation, you get a true or false answer.

env!("VAR_NAME") expands to a string: the value of the specified environment variable at compile time. If the variable doesn’t exist, it’s a compilation error.

This would be fairly worthless except that Cargo sets several interesting environment variables when it compiles a crate. For example, to get your crate’s current version string, you can write:

let version = env!("CARGO_PKG_VERSION");

A full list of these environment variables is included in the Cargo documentation.

option_env!("VAR_NAME") is the same as env! except that it returns an Option<&'static str> that is None if the specified variable is not set.

Three more built-in macros let you bring in code or data from another file.

include!("file.rs") expands to the contents of the specified file, which must be valid Rust code—either an expression or a sequence of items.

include_str!("file.txt") expands to a &'static str containing the text of the specified file. You can use it like this:

const COMPOSITOR_SHADER: &str =
    include_str!("../resources/compositor.glsl");

If the file doesn’t exist, or is not valid UTF-8, you’ll get a compilation error.

include_bytes!("file.dat") is the same except the file is treated as binary data, not UTF-8 text. The result is a &'static [u8].

Like all macros, these are processed at compile time. If the file doesn’t exist or can’t be read, compilation fails. They can’t fail at run time. In all cases, if the filename is a relative path, it’s resolved relative to the directory that contains the current file.

Debugging Macros

Debugging a wayward macro can be challenging. The biggest problem is the lack of visibility into the process of macro expansion. Rust will often expand all macros, find some kind of error, and then print an error message that does not show the fully expanded code that contains the error!

Here are three tools to help troubleshoot macros. (These features are all unstable, but since they’re really designed to be used during development, not in code that you’d check in, that isn’t a big problem in practice.)

First and simplest, you can ask rustc to show what your code looks like after expanding all macros. Use cargo build --verbose to see how Cargo is invoking rustc. Copy the rustc command line and add -Z unstable-options --pretty expanded as options. The fully expanded code is dumped to your terminal. Unfortunately, this only works if your code is free of syntax errors.

Second, Rust provides a log_syntax!() macro that simply prints its arguments to the terminal at compile time. You can use this for println!-style debugging. This macro requires the #![feature(log_syntax)] feature flag.

Third, you can ask the Rust compiler to log all macro calls to the terminal. Insert trace_macros!(true); somewhere in your code. From that point on, each time Rust expands a macro, it will print the macro name and arguments. For example, this program:

#![feature(trace_macros)]

fn main() {
    trace_macros!(true);
    let numbers = vec![1, 2, 3];
    trace_macros!(false);
    println!("total: {}", numbers.iter().sum::<u64>());
}

produces this output:

$ rustup override set nightly
...
$ rustc trace_example.rs
note: trace_macro
 --> trace_example.rs:5:19
  |
5 |     let numbers = vec![1, 2, 3];
  |                   ^^^^^^^^^^^^^
  |
  = note: expanding `vec! { 1 , 2 , 3 }`
  = note: to `< [ _ ] > :: into_vec ( box [ 1 , 2 , 3 ] )`

The compiler shows the code of each macro call, both before and after expansion. The line trace_macros!(false); turns tracing off again, so the call to println!() is not traced.

The json! Macro

We’ve now discussed the core features of macro_rules!. In this section, we’ll incrementally develop a macro for building JSON data. We’ll use this example to show what it’s like to develop a macro, present the few remaining pieces of macro_rules!, and offer some advice on how to make sure your macros behave as desired.

Back in Chapter 10, we presented this enum for representing JSON data:

#[derive(Clone, PartialEq, Debug)]
enum Json {
    Null,
    Boolean(bool),
    Number(f64),
    String(String),
    Array(Vec<Json>),
    Object(Box<HashMap<String, Json>>)
}

The syntax for writing out Json values is unfortunately rather verbose:

let students = Json::Array(vec![
    Json::Object(Box::new(vec![
        ("name".to_string(), Json::String("Jim Blandy".to_string())),
        ("class_of".to_string(), Json::Number(1926.0)),
        ("major".to_string(), Json::String("Tibetan throat singing".to_string()))
    ].into_iter().collect())),
    Json::Object(Box::new(vec![
        ("name".to_string(), Json::String("Jason Orendorff".to_string())),
        ("class_of".to_string(), Json::Number(1702.0)),
        ("major".to_string(), Json::String("Knots".to_string()))
    ].into_iter().collect()))
]);

We would like to be able to write this using a more JSON-like syntax:

let students = json!([
    {
        "name": "Jim Blandy",
        "class_of": 1926,
        "major": "Tibetan throat singing"
    },
    {
        "name": "Jason Orendorff",
        "class_of": 1702,
        "major": "Knots"
    }
]);

What we want is a json! macro that takes a JSON value as an argument and expands to a Rust expression like the one in the previous example.

Fragment Types

The first job in writing any complex macro is figuring out how to match, or parse, the desired input.

We can already see that the macro will have several rules, because there are several different sorts of things in JSON data: objects, arrays, numbers, and so forth. In fact, we might guess that we’ll have one rule for each JSON type:

macro_rules! json {
    (null)    => { Json::Null };
    ([ ... ]) => { Json::Array(...) };
    ({ ... }) => { Json::Object(...) };
    (???)     => { Json::Boolean(...) };
    (???)     => { Json::Number(...) };
    (???)     => { Json::String(...) };
}

This is not quite correct, as macro patterns offer no way to tease apart the last three cases; but we’ll see how to deal with that later on. The first three cases, at least, clearly begin with different tokens, so let’s start with those.

The first rule already works:

macro_rules! json {
    (null) => {
        Json::Null
    }
}

#[test]
fn json_null() {
    assert_eq!(json!(null), Json::Null);  // passes!
}

To add support for JSON arrays, we might try matching the elements as exprs:

macro_rules! json {
    (null) => {
        Json::Null
    };
    ([ $( $element:expr ),* ]) => {
        Json::Array(vec![ $( $element ),* ])
    };
}

Unfortunately, this does not match all JSON arrays. Here’s a test that illustrates the problem:

#[test]
fn json_array_with_json_element() {
    let macro_generated_value = json!(
        [
            // valid JSON that doesn't match `$element:expr`
            {
                "pitch": 440.0
            }
        ]
    );
    let hand_coded_value =
        Json::Array(vec![
            Json::Object(Box::new(vec![
                ("pitch".to_string(), Json::Number(440.0))
            ].collect()))
        ]);
    assert_eq!(macro_generated_value, hand_coded_value);
}

The pattern $( $element:expr ),* means “a comma-separated list of Rust expressions.” But many JSON values, particularly objects, aren’t valid Rust expressions. They won’t match.

Since not every bit of code you want to match is an expression, Rust supports several other fragment types, listed in Table 20-1.

Table 20-1. Fragment types supported by macro_rules!

Fragment type
Matches (with examples)
Can be followed by...

expr

An expression:

2 + 2, "udon", x.len()

=> , ;

stmt

An expression or declaration, not including any trailing semicolon

(hard to use; try expr or block instead)

=> , ;

ty

A type:

String, Vec<u8>, (&str, bool)

=> , ; = | { [ : > as where

path

A path (discussed):

ferns, ::std::sync::mpsc

=> , ; = | { [ : > as where

pat

A pattern (discussed):

_, Some(ref x)

=> , = | if in

item

An item (discussed):

struct Point { x: f64, y: f64 }, mod ferns;

Anything

block

A block (discussed):

{ s += "ok\n"; true }

Anything

meta

The body of an attribute (discussed):

inline, derive(Copy, Clone), doc="3D models."

Anything

ident

An identifier:

std, Json, longish_variable_name

Anything

tt

A token tree (see text):

;, >=, {}, [0 1 (+ 0 1)]

Anything

Most of the options in this table strictly enforce Rust syntax. The expr type matches only Rust expressions (not JSON values), ty matches Rust types, and so on. They’re not extensible: there’s no way to define new arithmetic operators or new keywords that expr would recognize. We won’t be able to make any of these match arbitrary JSON data.

The last two, ident and tt, support matching macro arguments that don’t look like Rust code. ident matches any identifier. tt matches a single token tree: either a properly matched pair of brackets, (...) [...] or {...}, and everything in between, including nested token trees; or a single token that isn’t a bracket, like 1926 or "Knots".

Token trees are exactly what we need for our json! macro. Every JSON value is a single token tree: numbers, strings, Boolean values, and null are all single tokens; objects and arrays are bracketed. So we can write the patterns like this:

macro_rules! json {
    (null) => {
        Json::Null
    };
    ([ $( $element:tt ),* ]) => {
        Json::Array(...)
    };
    ({ $( $key:tt : $value:tt ),* }) => {
        Json::Object(...)
    };
    ($other:tt) => {
        ... // TODO: Return Number, String, or Boolean
    };
}

This version of the json! macro can match all JSON data. Now we just need to produce correct Rust code.

To make sure Rust can gain new syntactic features in the future without breaking any macros you write today, Rust restricts tokens that appear in patterns right after a fragment. The “Can be followed by...” column of Table 20-1 shows which tokens are allowed. For example, the pattern $x:expr ~ $y:expr is an error, because ~ isn’t allowed after an expr. The pattern $vars:pat : $t:ty is OK, because $vars:pat is followed by a colon, one of the allowed tokens for a pat; and $t:ty is followed by nothing, which is always allowed.

Recursion in Macros

You’ve already seen one trivial case of a macro calling itself: our implementation of vec! uses recursion to support trailing commas. Here we can show a more significant example: json! needs to call itself recursively.

We might try supporting JSON arrays without using recursion, like this:

([ $( $element:tt ),* ]) => {
    Json::Array(vec![ $( $element ),* ])
};

But this wouldn’t work. We’d be pasting JSON data (the $element token trees) right into a Rust expression. They’re two different languages.

We need to convert each element of the array from JSON form to Rust. Fortunately, there’s a macro that does this: the one we’re writing!

([ $($element:tt),* ]) => {
    Json::Array(vec![ $( json!($element) ),* ])
};

Objects can be supported in the same way:

({ $($key:tt : $value:tt),* }) => {
    Json::Object(Box::new(vec![
        $( ($key.to_string(), json!($value)) ),*
    ].into_iter().collect()))
};

The compiler imposes a recursion limit on macros: 64 calls, by default. That’s more than enough for normal uses of json!, but complex recursive macros sometimes hit the limit. You can adjust it by adding this attribute at the top of the crate where the macro is used:

#![recursion_limit = "256"]

Our json! macro is nearly complete. All that remains is to support Boolean, number, and string values.

Using Traits with Macros

Writing complex macros always poses puzzles. It’s important to remember that macros themselves are not the only puzzle-solving tool at your disposal.

Here, we need to support json!(true), json!(1.0), and json!("yes"), converting the value, whatever it may be, to the appropriate kind of Json value. But macros are not good at distinguishing types. We can imagine writing:

macro_rules! json {
    (true) => {
        Json::Boolean(true)
    };
    (false) => {
        Json::Boolean(false)
    };
    ...
}

This approach breaks down right away. There are only two Boolean values, but rather more numbers than that, and even more strings.

Fortunately, there is a standard way to convert values of various types to one specified type: the From trait, covered . We simply need to implement this trait for a few types:

impl From<bool> for Json {
    fn from(b: bool) -> Json {
        Json::Boolean(b)
    }
}

impl From<i32> for Json {
    fn from(i: i32) -> Json {
        Json::Number(i as f64)
    }
}

impl From<String> for Json {
    fn from(s: String) -> Json {
        Json::String(s)
    }
}

impl<'a> From<&'a str> for Json {
    fn from(s: &'a str) -> Json {
        Json::String(s.to_string())
    }
}
...

In fact, all 12 numeric types should have very similar implementations, so it might make sense to write a macro, just to avoid the copy-and-paste:

macro_rules! impl_from_num_for_json {
    ( $( $t:ident )* ) => {
        $(
            impl From<$t> for Json {
                fn from(n: $t) -> Json {
                    Json::Number(n as f64)
                }
            }
        )*
    };
}

impl_from_num_for_json!(u8 i8 u16 i16 u32 i32 u64 i64 usize isize f32 f64);

Now we can use Json::from(value) to convert a value of any supported type to Json. In our macro, it’ll look like this:

($other:tt) => {
    Json::from($other)  // Handle Boolean/number/string
};

Adding this rule to our json! macro makes it pass all the tests we’ve written so far. Putting together all the pieces, it currently looks like this:

macro_rules! json {
    (null) => {
        Json::Null
    };
    ([ $($element:tt),* ]) => {
        Json::Array(vec![ $( json!($element) ),* ])
    };
    ({ $($key:tt : $value:tt),* }) => {
        Json::Object(Box::new(vec![
            $( ($key.to_string(), json!($value)) ),*
        ].into_iter().collect()))
    };
    ($other:tt) => {
        Json::from($other)  // Handle Boolean/number/string
    };
}

As it turns out, the macro unexpectedly supports the use of variables and even arbitrary Rust expressions inside the JSON data, a handy extra feature:

let width = 4.0;
let desc =
    json!({
        "width": width,
        "height": (width * 9.0 / 4.0)
    });

Because (width * 9.0 / 4.0) is parenthesized, it’s a single token tree, so the macro successfully matches it with $value:tt when parsing the object.

Scoping and Hygiene

A surprisingly tricky aspect of writing macros is that they involve pasting code from different scopes together. So the next few pages cover the two ways Rust handles scoping: one way for local variables and arguments, and another way for everything else.

To show why this matters, let’s rewrite our rule for parsing JSON objects (the third rule in the json! macro shown previously) to eliminate the temporary vector. We can write it like this:

({ $($key:tt : $value:tt),* }) => {
    {
        let mut fields = Box::new(HashMap::new());
        $( fields.insert($key.to_string(), json!($value)); )*
        Json::Object(fields)
    }
};

Now we’re populating the HashMap not by using collect() but by repeatedly calling the .insert() method. This means we need to store the map in a temporary variable, which we’ve called fields.

But then what happens if the code that calls json! happens to use a variable of its own, also named fields?

let fields = "Fields, W.C.";
let role = json!({
    "name": "Larson E. Whipsnade",
    "actor": fields
});

Expanding the macro would paste together two bits of code, both using the name fields for different things!

let fields = "Fields, W.C.";
let role = {
    let mut fields = Box::new(HashMap::new());
    fields.insert("name".to_string(), Json::from("Larson E. Whipsnade"));
    fields.insert("actor".to_string(), Json::from(fields));
    Json::Object(fields)
};

This may seem like an unavoidable pitfall whenever macros use temporary variables, and you may already be thinking through the possible fixes. Perhaps we should rename the variable that the json! macro defines to something that its callers aren’t likely to pass in: instead of fields, we could call it __json$fields.

The surprise here is that the macro works as is. Rust renames the variable for you! This feature, first implemented in Scheme macros, is called hygiene, and so Rust is said to have hygienic macros.

The easiest way to understand macro hygiene is to imagine that every time a macro is expanded, the parts of the expansion that come from the macro itself are painted a different color.

Variables of different colors, then, are treated as if they had different names:

let fields = "Fields, W.C.";
let role = {
    let mut fields = Box::new(HashMap::new());
    fields.insert("name".to_string(), Json::from("Larson E. Whipsnade"));
    fields.insert("actor".to_string(), Json::from(fields));
    Json::Object(fields)
};

Note that bits of code that were passed in by the macro caller and pasted into the output, such as "name" and "actor", keep their original color (black). Only tokens that originate from the macro template are painted.

Now there’s one variable named fields (declared in the caller) and a separate variable named fields (introduced by the macro). Since the names are different colors, the two variables don’t get confused.

If a macro really does need to refer to a variable in the caller’s scope, the caller has to pass the name of the variable to the macro.

(The paint metaphor isn’t meant to be an exact description of how hygiene works. The real mechanism is even a little smarter than that, recognizing two identifiers as the same, regardless of “paint,” if they refer to a common variable that’s in scope for both the macro and its caller. But cases like this are rare in Rust. If you understand the preceding example, you know enough to use hygienic macros.)

You may have noticed that many other identifiers were painted one or more colors as the macros were expanded: Box, HashMap, and Json, for example. Despite the paint, Rust had no trouble recognizing these type names. That’s because hygiene in Rust is limited to local variables and arguments. When it comes to constants, types, methods, modules, and macro names, Rust is “colorblind.”

This means that if our json! macro is used in a module where Box, HashMap, or Json is not in scope, the macro won’t work. We’ll show how to avoid this problem in the next section.

First, we’ll consider a case where Rust’s strict hygiene gets in the way, and we need to work around it. Suppose we have many functions that contain this line of code:

let req = ServerRequest::new(server_socket.session());

Copying and pasting that line is a pain. Can we use a macro instead?

macro_rules! setup_req {
    () => {
        let req = ServerRequest::new(server_socket.session());
    }
}

fn handle_http_request(server_socket: &ServerSocket) {
    setup_req!();  // declares `req`, uses `server_socket`
    ... // code that uses `req`
}

As written, this doesn’t work. It would require the name server_socket in the macro to refer to the local server_socket declared in the function, and vice versa for the variable req. But hygiene prevents names in macros from “colliding” with names in other scopes—even in cases like this, where that’s what you want.

The solution is to pass the macro any identifiers you plan on using both inside and outside the macro code:

macro_rules! setup_req {
    ($req:ident, $server_socket:ident) => {
        let $req = ServerRequest::new($server_socket.session());
    }
}

fn handle_http_request(server_socket: &ServerSocket) {
    setup_req!(req, server_socket);
    ... // code that uses `req`
}

Since req and server_socket are now provided by the function, they’re the right “color” for that scope.

Hygiene makes this macro a little wordier to use, but that’s a feature, not a bug: it’s easier to reason about hygienic macros knowing that they can’t mess with local variables behind your back. If you search for an identifier like server_socket in a function, you’ll find all the places where it’s used, including macro calls.

Importing and Exporting Macros

Since macros are expanded early in compilation, before Rust knows the full module structure of your project, they aren’t imported and exported in the usual way.

Within a single crate:

Macros that are visible in one module are automatically visible in its child modules.

To export macros from a module “upward” to its parent module, use the #[macro_use] attribute. For example, suppose our lib.rs looks like this:

#[macro_use] mod macros;
mod client;
mod server;

All macros defined in the macros module are imported into lib.rs and therefore visible throughout the rest of the crate, including in client and server.

When working with multiple crates:

To import macros from another crate, use #[macro_use] on the extern crate declaration.

To export macros from your crate, mark each public macro with #[macro_export].

Of course, actually doing any of these things means your macro may be called in other modules. An exported macro therefore shouldn’t rely on anything being in scope—there’s no telling what will be in scope where it’s used. Even features of the standard prelude can be shadowed.

Instead, the macro should use absolute paths to any names it uses. macro_rules! provides the special fragment $crate to help with this. It acts like an absolute path to the root module of the crate where the macro was defined. Instead of saying Json, we can write $crate::Json, which works even if Json was not imported. HashMap can be changed to either ::std::collections::HashMap or $crate::macros::HashMap. In the latter case, we’ll have to re-export HashMap, because $crate can’t be used to access private features of a crate. It really just expands to something like ::jsonlib, an ordinary path. Visibility rules are unaffected.

After moving the macro to its own module macros and modifying it to use $crate, it looks like this. This is the final version.

// macros.rs
pub use std::collections::HashMap;
pub use std::boxed::Box;
pub use std::string::ToString;

#[macro_export]
macro_rules! json {
    (null) => {
        $crate::Json::Null
    };
    ([ $( $element:tt ),* ]) => {
        $crate::Json::Array(vec![ $( json!($element) ),* ])
    };
    ({ $( $key:tt : $value:tt ),* }) => {
        {
            let mut fields = $crate::macros::Box::new(
                $crate::macros::HashMap::new());
            $( fields.insert($crate::ToString::to_string($key), json!($value)); )*
            $crate::Json::Object(fields)
        }
    };
    ($other:tt) => {
        $crate::Json::from($other)
    };
}

Since the .to_string() method is part of the standard ToString trait, we use $crate to refer to that as well, using syntax we introduced in “Fully Qualified Method Calls”: $crate::ToString::to_string($key). In our case, this isn’t strictly necessary to make the macro work, because ToString is in the standard prelude. But if you’re calling methods of a trait that may not be in scope at the point where the macro is called, a fully qualified method call is the best way to do it.

Avoiding Syntax Errors During Matching

The following macro seems reasonable, but it gives Rust some trouble:

macro_rules! complain {
    ($msg:expr) => {
        println!("Complaint filed: {}", $msg);
    };
    (user : $userid:tt , $msg:expr) => {
        println!("Complaint from user {}: {}", $userid, $msg);
    };
}

Suppose we call it like this:

complain!(user: "jimb", "the AI lab's chatbots keep picking on me");

To human eyes, this obviously matches the second pattern. But Rust tries the first rule first, attempting to match all of the input with $msg:expr. This is where things start to go badly for us. user: "jimb" is not an expression, of course, so we get a syntax error. Rust refuses to sweep a syntax error under the rug—macros are already hard enough to debug. Instead, it’s reported immediately and compilation halts.

If any other token in a pattern fails to match, Rust moves on the next rule. Only syntax errors are fatal, and they only happen when trying to match fragments.

The problem here is not so hard to understand: we’re attempting to match a fragment, $msg:expr, in the wrong rule. It’s not going to match because we’re not even supposed to be here. The caller wanted the other rule. There are two easy ways to avoid this.

First, avoid confusable rules. We could, for example, change the macro so that every pattern starts with a different identifier:

macro_rules! complain {
    (msg : $msg:expr) => {
        println!("Complaint filed: {}", $msg);
    };
    (user : $userid:tt , msg : $msg:expr) => {
        println!("Complaint from user {}: {}", $userid, $msg);
    };
}

When the macro arguments start with msg, we’ll get rule 1. When they start with user, we’ll get rule 2. Either way, we know we’ve got the right rule before we try to match a fragment.

The other way to avoid spurious syntax errors is by putting more specific rules first. Putting the user: rule first fixes the problem with complain!, because the rule that causes the syntax error is never reached.

Beyond macro_rules!

Macro patterns can parse input that’s even more intricate than JSON, but we’ve found that the complexity quickly gets out of hand.

The Little Book of Rust Macros, by Daniel Keep et al., is an excellent handbook of advanced macro_rules! programming. The book is clear and smart, and it describes every aspect of macro expansion in more detail than we have here. It also presents several very clever techniques for pressing macro_rules! patterns into service as a sort of esoteric programming language, to parse complex input. This we’re less enthusiastic about. Use with care.

Rust 1.15 introduced a separate mechanism called procedural macros. This feature supports extending the #[derive] attribute to handle custom traits, as shown in Figure 20-4.

Figure 20-4. Invoking a hypothetical IntoJson procedural macro via a #[derive] attribute

There is no IntoJson trait, but it doesn’t matter: a procedural macro can use this hook to insert whatever code it wants (in this case, probably impl From<Money> for Json { ... }).

What makes a procedural macro “procedural” is that it’s implemented as a Rust function, not a declarative rule-set. As of this writing, procedural macros are still new and expected to continue evolving, so we refer you to the online documentation.

Perhaps, having read all this, you’ve decided that you hate macros. What then? An alternative is to generate Rust code using a build script. The Cargo documentation shows how to do it step by step. It involves writing a program that generates the Rust code you want, adding a line to Cargo.toml to run that program as part of the build process, and using include! to get the generated code into your crate.

Chapter 21. Unsafe Code

Let no one think of me that I am humble or weak or passive;
Let them understand I am of a different kind:
dangerous to my enemies, loyal to my friends.
To such a life glory belongs.

Euripides, Medea

The secret joy of systems programming is that, underneath every single safe language and carefully designed abstraction is a swirling maelstrom of wildly unsafe machine language and bit-fiddling. You can write that in Rust, too.

The language we’ve presented up to this point in the book ensures your programs are free of memory errors and data races entirely automatically, through types, lifetimes, bounds checks, and so on. But this sort of automated reasoning has its limits; there are many valuable techniques that Rust cannot recognize as safe.

Unsafe code lets you tell Rust, “In this case, just trust me.” By marking off a block or function as unsafe, you acquire the ability to call unsafe functions in the standard library, dereference unsafe pointers, and call functions written in other languages like C and C++, among other powers. All of Rust’s usual safety checks still apply: type checks, lifetime checks, and bounds checks on indices all occur normally. Unsafe code just enables a small set of additional features.

This ability to step outside the boundaries of safe Rust is what makes it possible to implement many of Rust’s most fundamental features in Rust itself, as is commonly done in C and C++ systems. Unsafe code is what allows the Vec type to manage its buffer efficiently; the std::io module to talk to the operating system; and the std::thread and std::sync modules to provide concurrency primitives.

This chapter covers the essentials of working with unsafe features:

Rust’s unsafe blocks establish the boundary between ordinary, safe Rust code and code that uses unsafe features.

You can mark functions as unsafe, alerting callers to the presence of extra contracts they must follow to avoid undefined behavior.

Raw pointers and their methods allow unconstrained access to memory, and let you build data structures Rust’s type system would otherwise forbid.

Understanding the definition of undefined behavior will help you appreciate why it can have consequences far more serious than just getting incorrect results.

Rust’s foreign function interface lets you use libraries written in other languages.

Unsafe traits, analogous to unsafe functions, impose a contract that each implementation (rather than each caller) must follow.

Unsafe from What?

At the start of this book, we showed a C program that crashes in a surprising way because it fails to follow one of the rules prescribed by the C standard. You can do the same in Rust:

$ cat crash.rs
fn main() {
    let mut a: usize = 0;
    let ptr = &mut a as *mut usize;
    unsafe {
        *ptr.offset(3) = 0x7ffff72f484c;
    }
}
$ cargo build
   Compiling unsafe-samples v0.1.0
    Finished debug [unoptimized + debuginfo] target(s) in 0.44 secs
$ ../../target/debug/crash
crash: Error: .netrc file is readable by others.
crash: Remove password or make file unreadable by others.
Segmentation fault (core dumped)
$

This program borrows a mutable reference to the local variable a, casts it to a raw pointer of type *mut usize, and then uses the offset method to produce a pointer three words further along in memory. This happens to be where main’s return address is stored. The program overwrites the return address with a constant, such that returning from main behaves in a surprising way. What makes this crash possible is the program’s incorrect use of unsafe features—in this case, the ability to dereference raw pointers.

An unsafe feature is one that imposes a contract: rules that Rust cannot enforce automatically, but which you must nonetheless follow to avoid undefined behavior.

A contract goes beyond the usual type checks and lifetime checks, imposing further rules specific to that unsafe feature. Typically, Rust itself doesn’t know about the contract at all; it’s just explained in the feature’s documentation. For example, the raw pointer type has a contract forbidding you to dereference a pointer that has been advanced beyond the end of its original referent. The expression *ptr.offset(3) = ... in this example breaks this contract. But, as the transcript shows, Rust compiles the program without complaint: its safety checks do not detect this violation. When you use unsafe features, you, as the programmer, bear the responsibility for checking that your code adheres to their contracts.

Lots of features have rules you should follow to use them correctly, but such rules are not contracts in the sense we mean here unless the possible consequences include undefined behavior. Undefined behavior is behavior Rust firmly assumes your code could never exhibit. For example, Rust assumes you will not overwrite a function call’s return address with something else. Code that passes Rust’s usual safety checks and complies with the contracts of the unsafe features it uses cannot possibly do such a thing. Since the program violates the raw pointer contract, its behavior is undefined, and it goes off the rails.

If your code exhibits undefined behavior, you have broken your half of your bargain with Rust, and Rust declines to predict the consequences. Dredging up irrelevant error messages from the depths of system libraries and crashing is one possible consequence; handing control of your computer over to an attacker is another. The effects could vary from one release of Rust to the next, without warning. Sometimes, however, undefined behavior has no visible consequences. For example, if the main function never returns (perhaps it calls std::process::exit to terminate the program early), then the corrupted return address probably won’t matter.

You may only use unsafe features within an unsafe block or an unsafe function; we’ll explain both in the sections that follow. This makes it harder to use unsafe features unknowingly: by forcing you to write an unsafe block or function, Rust makes sure you have acknowledged that your code may have additional rules to follow.

Unsafe Blocks

An unsafe block looks just like an ordinary Rust block preceded by the unsafe keyword, with the difference that you can use unsafe features in the block:

unsafe {
    String::from_utf8_unchecked(ascii)
}

Without the unsafe keyword in front of the block, Rust would object to the use of from_utf8_unchecked, which is an unsafe function. With the unsafe block around it, you can use this code anywhere.

Like an ordinary Rust block, the value of an unsafe block is that of its final expression, or () if it doesn’t have one. The call to String::from_utf8_unchecked shown earlier provides the value of the block.

An unsafe block unlocks four additional options for you:

You can call unsafe functions. Each unsafe function must specify its own contract, depending on its purpose.

You can dereference raw pointers. Safe code can pass raw pointers around, compare them, and create them by conversion from references (or even from integers), but only unsafe code can actually use them to access memory. We’ll cover raw pointers in detail and explain how to use them safely in “Raw Pointers”.

You can access mutable static variables. As explained in “Global Variables”, Rust can’t be sure when threads are using mutable static variables, so their contract requires you to ensure all access is properly synchronized.

You can access functions and variables declared through Rust’s foreign function interface. These are considered unsafe even when immutable, since they are visible to code written in other languages that may not respect Rust’s safety rules.

Restricting unsafe features to unsafe blocks doesn’t really prevent you from doing whatever you want. It’s perfectly possible to just stick an unsafe block into your code and move on. The benefit of the rule lies mainly in drawing human attention to code whose safety Rust can’t guarantee:

You won’t accidentally use unsafe features, and then discover you were responsible for contracts you didn’t even know existed.

An unsafe block attracts more attention from reviewers. Some projects even have automation to ensure this, flagging code changes that affect unsafe blocks for special attention.

When you’re considering writing an unsafe block, you can take a moment to ask yourself whether your task really requires such measures. If it’s for performance, do you have measurements to show that this is actually a bottleneck? Perhaps there is a good way to accomplish the same thing in safe Rust.

Example: An Efficient ASCII String Type

Here’s the definition of Ascii, a string type that ensures its contents are always valid ASCII. This type uses an unsafe feature to provide zero-cost conversion into String:

mod my_ascii {
    use std::ascii::AsciiExt; // for u8::is_ascii

    /// An ASCII-encoded string.
    #[derive(Debug, Eq, PartialEq)]
    pub struct Ascii(
        // This must hold only well-formed ASCII text:
        // bytes from `0` to `0x7f`.
        Vec<u8>
    );

    impl Ascii {
        /// Create an `Ascii` from the ASCII text in `bytes`. Return a
        /// `NotAsciiError` error if `bytes` contains any non-ASCII
        /// characters.
        pub fn from_bytes(bytes: Vec<u8>) -> Result<Ascii, NotAsciiError> {
            if bytes.iter().any(|&byte| !byte.is_ascii()) {
                return Err(NotAsciiError(bytes));
            }
            Ok(Ascii(bytes))
        }
    }

    // When conversion fails, we give back the vector we couldn't convert.
    // This should implement `std::error::Error`; omitted for brevity.
    #[derive(Debug, Eq, PartialEq)]
    pub struct NotAsciiError(pub Vec<u8>);

    // Safe, efficient conversion, implemented using unsafe code.
    impl From<Ascii> for String {
        fn from(ascii: Ascii) -> String {
            // If this module has no bugs, this is safe, because
            // well-formed ASCII text is also well-formed UTF-8.
            unsafe { String::from_utf8_unchecked(ascii.0) }
        }
    }
    ...
}

The key to this module is the definition of the Ascii type. The type itself is marked pub, to make it visible outside the my_ascii module. But the type’s Vec<u8> element is not public, so only the my_ascii module can construct an Ascii value or refer to its element. This leaves the module’s code in complete control over what may or may not appear there. As long as the public constructors and methods ensure that freshly created Ascii values are well-formed and remain so throughout their lives, then the rest of the program cannot violate that rule. And indeed, the public constructor Ascii::from_bytes carefully checks the vector it’s given before agreeing to construct an Ascii from it. For brevity’s sake, we don’t show any methods, but you can imagine a set of text-handling methods that ensure Ascii values always contain proper ASCII text, just as a String’s methods ensure that its contents remain well-formed UTF-8.

This arrangement lets us implement From<Ascii> for String very efficiently. The unsafe function String::from_utf8_unchecked takes a byte vector and builds a String from it without checking whether its contents are well-formed UTF-8 text; the function’s contract holds its caller responsible for that. Fortunately, the rules enforced by the Ascii type are exactly what we need to satisfy from_utf8_unchecked’s contract. As we explained in “UTF-8”, any block of ASCII text is also well-formed UTF-8, so an Ascii’s underlying Vec<u8> is immediately ready to serve as a String’s buffer.

With these definitions in place, you can write:

use my_ascii::Ascii;

let bytes: Vec<u8> = b"ASCII and ye shall receive".to_vec();

// This call entails no allocation or text copies, just a scan.
let ascii: Ascii = Ascii::from_bytes(bytes)
    .unwrap(); // We know these chosen bytes are ok.

// This call is zero-cost: no allocation, copies, or scans.
let string = String::from(ascii);

assert_eq!(string, "ASCII and ye shall receive");

No unsafe blocks are required to use Ascii. We have implemented a safe interface using unsafe operations, and arranged to meet their contracts depending only on the module’s own code, not on its users’ behavior.

An Ascii is nothing more than a wrapper around a Vec<u8>, hidden inside a module that enforces extra rules about its contents. A type of this sort is called a newtype, a common pattern in Rust. Rust’s own String type is defined in exactly the same way, except that its contents are restricted to be UTF-8, not ASCII. In fact, here’s the definition of String from the standard library:

pub struct String {
    vec: Vec<u8>,
}

At the machine level, with Rust’s types out of the picture, a newtype and its element have identical representations in memory, so constructing a newtype doesn’t require any machine instructions at all. In Ascii::from_bytes, the expression Ascii(bytes) simply deems the Vec<u8>’s representation to now hold an Ascii value. Similarly, String::from_utf8_unchecked probably requires no machine instructions when inlined: the Vec<u8> is now considered to be a String.

Unsafe Functions

An unsafe function definition looks like an ordinary function definition preceded by the unsafe keyword. The body of an unsafe function is automatically considered an unsafe block.

You may call unsafe functions only within unsafe blocks. This means that marking a function unsafe warns its callers that the function has a contract they must satisfy to avoid undefined behavior.

For example, here’s a new constructor for the Ascii type we introduced before that builds an Ascii from a byte vector without checking if its contents are valid ASCII:

// This must be placed inside the `my_ascii` module.
impl Ascii {
    /// Construct an `Ascii` value from `bytes`, without checking
    /// whether `bytes` actually contains well-formed ASCII.
    ///
    /// This constructor is infallible, and returns an `Ascii` directly,
    /// rather than a `Result<Ascii, NotAsciiError>` as the `from_bytes`
    /// constructor does.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `bytes` contains only ASCII
    /// characters: bytes no greater than 0x7f. Otherwise, the effect is
    /// undefined.
    pub unsafe fn from_bytes_unchecked(bytes: Vec<u8>) -> Ascii {
        Ascii(bytes)
    }
}

Presumably, code calling Ascii::from_bytes_unchecked already knows somehow that the vector in hand contains only ASCII characters, so the check that Ascii::from_bytes insists on carrying out would be a waste of time, and the caller would have to write code to handle Err results that it knows will never occur. Ascii::from_bytes_unchecked lets such a caller sidestep the checks and the error handling.

But the comment above the definition of the Ascii type says, “Nothing in this module permits the introduction of non-ASCII bytes into an Ascii value.” Isn’t that exactly what this new from_bytes_unchecked constructor does?

Not quite: from_bytes_unchecked meets its obligations by passing them on to its caller via its contract. The presence of this contract is what makes it correct to mark this function unsafe: despite the fact that the function itself carries out no unsafe operations, its callers must follow rules Rust cannot enforce automatically to avoid undefined behavior.

Can you really cause undefined behavior by breaking the contract of Ascii::from_bytes_unchecked? Yes. You can construct a String holding ill-formed UTF-8 as follows:

// Imagine that this vector is the result of some complicated process
// that we expected to produce ASCII. Something went wrong!
let bytes = vec![0xf7, 0xbf, 0xbf, 0xbf];

let ascii = unsafe {
    // This unsafe function's contract is violated
    // when `bytes` holds non-ASCII bytes.
    Ascii::from_bytes_unchecked(bytes)
};

let bogus: String = ascii.into();

// `bogus` now holds ill-formed UTF-8. Parsing its first character
// produces a `char` that is not a valid Unicode code point.
assert_eq!(bogus.chars().next().unwrap() as u32, 0x1fffff);

This illustrates two critical facts about bugs and unsafe code:

Bugs that occur before the unsafe block can break contracts. Whether an unsafe block causes undefined behavior can depend not just on the code in the block itself, but also on the code that supplies the values it operates on. Everything that your unsafe code relies on to satisfy contracts is safety-critical. The conversion from Ascii to String based on String::from_utf8_unchecked is well-defined only if the rest of the module properly maintains Ascii’s invariants.

The consequences of breaking a contract may appear after you leave the unsafe block. The undefined behavior courted by failing to comply with an unsafe feature’s contract often does not occur within the unsafe block itself. Constructing a bogus String as shown before may not cause problems until much later in the program’s execution.

Essentially, Rust’s type checker, borrow checker, and other static checks are inspecting your program and trying to construct a proof that it cannot exhibit undefined behavior. When Rust compiles your program successfully, that means it succeeded in proving your code sound. An unsafe block is a gap in this proof: “This code,” you are saying to Rust, “is fine, trust me.” Whether your claim is true could depend on any part of the program that influences what happens in the unsafe block, and the consequences of being wrong could appear anywhere influenced by the unsafe block. Writing the unsafe keyword amounts to a reminder that you are not getting the full benefit of the language’s safety checks.

Given the choice, you should naturally prefer to create safe interfaces, without contracts. These are much easier to work with, since users can count on Rust’s safety checks to ensure their code is free of undefined behavior. Even if your implementation uses unsafe features, it’s best to use Rust’s types, lifetimes, and module system to meet their contracts while using only what you can guarantee yourself, rather than passing responsibilities on to your callers.

Unfortunately, it’s not unusual to come across unsafe functions in the wild whose documentation does not bother to explain their contracts. You are expected to infer the rules yourself, based on your experience and knowledge of how the code behaves. If you’ve ever uneasily wondered whether what you’re doing with a C or C++ API is OK, then you know what that’s like.

Unsafe Block or Unsafe Function?

You may find yourself wondering whether to use an unsafe block or just mark the whole function unsafe. The approach we recommend is to first make a decision about the function:

If it’s possible to misuse the function in a way that compiles fine but still causes undefined behavior, you must mark it as unsafe. The rules for using the function correctly are its contract; the existence of a contract is what makes the function unsafe.

Otherwise, the function is safe: no well-typed call to it can cause undefined behavior. It should not be marked unsafe.

Whether the function uses unsafe features in its body is irrelevant; what matters is the presence of a contract. Before, we showed an unsafe function that uses no unsafe features, and a safe function that does use unsafe features.

Don’t mark a safe function unsafe just because you use unsafe features in its body. This makes the function harder to use, and confuses readers who will (correctly) expect to find a contract explained somewhere. Instead, use an unsafe block, even if it’s the function’s entire body.

Undefined Behavior

In the introduction, we said that the term undefined behavior means “behavior that Rust firmly assumes your code could never exhibit.” This is a strange turn of phrase, especially since we know from our experience with other languages that these behaviors do occur by accident with some frequency. Why is this concept helpful in setting out the obligations of unsafe code?

A compiler is a translator from one programming language to another. The Rust compiler takes a Rust program and translates it into an equivalent machine language program. But what does it mean to say that two programs in such completely different languages are equivalent?

Fortunately, this question is easier for programmers than it is for linguists. We usually say that two programs are equivalent if they will always have the same visible behavior when executed: they make the same system calls, interact with foreign libraries in equivalent ways, and so on. It’s a bit like a Turing test for programs: if you can’t tell whether you’re interacting with the original or the translation, then they’re equivalent.

Now consider the following code:

let i = 10;
very_trustworthy(&i);
println!("{}", i * 100);

Even knowing nothing about the definition of very_trustworthy, we can see that it receives only a shared reference to i, so the call cannot change i’s value. Since the value passed to println! will always be 1000, Rust can translate this code into machine language as if we had written:

very_trustworthy(&10);
println!("{}", 1000);

This transformed version has the same visible behavior as the original, and it’s probably a bit faster. But it makes sense to consider the performance of this version only if we agree it has the same meaning as the original. What if very_trustworthy were defined as follows?

fn very_trustworthy(shared: &i32) {
    unsafe {
        // Turn the shared reference into a mutable pointer.
        // This is undefined behavior.
        let mutable = shared as *const i32 as *mut i32;
        *mutable = 20;
    }
}

This code breaks the rules for shared references: it changes the value of i to 20, even though it should be frozen because i is borrowed for sharing. As a result, the transformation we made to the caller now has a very visible effect: if Rust transforms the code, the program prints 1000; if it leaves the code alone and uses the new value of i, it prints 2000. Breaking the rules for shared references in very_trustworthy means that shared references won’t behave as expected in its callers.

This sort of problem arises with almost every kind of transformation Rust might attempt. Even inlining a function into its call site assumes, among other things, that when the callee finishes, control flow returns to the call site. But we opened the chapter with an example of ill-behaved code that violates even that assumption.

It’s basically impossible for Rust (or any other language) to assess whether a transformation to a program preserves its meaning unless it can trust the fundamental features of the language to behave as designed. And whether they do or not can depend not just on the code at hand, but on other, potentially distant, parts of the program. In order to do anything at all with your code, Rust must assume that the rest of your program is well-behaved.

Here, then, are Rust’s rules for well-behaved programs:

The program must not read uninitialized memory.

The program must not create invalid primitive values:

References or boxes that are null

bool values that are not either a 0 or 1

enum values with invalid discriminant values

char values that are not valid, nonsurrogate Unicode code points

str values that are not well-formed UTF-8

The rules for references explained in Chapter 5 must be followed. No reference may outlive its referent; shared access is read-only access; and mutable access is exclusive access.

The program must not dereference null, incorrectly aligned, or dangling pointers.

The program must not use a pointer to access memory outside the allocation with which the pointer is associated. We will explain this rule in detail in “Dereferencing Raw Pointers Safely”.

The program must be free of data races. A data race occurs when two threads access the same memory location without synchronization, and at least one of the accesses is a write.

The program must not unwind across a call made from another language, via the foreign function interface, as explained in “Unwinding”.

The program must comply with the contracts of standard library functions.

These rules are all that Rust assumes in the process of optimizing your program and translating it into machine language. Undefined behavior is, simply, any violation of these rules. This is why we say that Rust assumes your program will not exhibit undefined behavior: this assumption is necessary if we hope to conclude that the compiled program is a faithful translation of the source code.

Rust code that does not use unsafe features is guaranteed to follow all of the preceding rules, once it compiles. Only when you use unsafe features do these rules become your responsibility. In C and C++, the fact that your program compiles without errors or warnings means much less; as we mentioned in the introduction to this book, even the best C and C++ programs written by well-respected projects that hold their code to high standards exhibit undefined behavior in practice.

Unsafe Traits

An unsafe trait is a trait that has a contract Rust cannot check or enforce that implementers must satisfy to avoid undefined behavior. To implement an unsafe trait, you must mark the implementation as unsafe. It is up to you to understand the trait’s contract, and make sure your type satisfies it.

A function that bounds its type variables with an unsafe trait is typically one that uses unsafe features itself, and satisfies their contracts only by depending on the unsafe trait’s contract. An incorrect implementation of the trait could cause such a function to exhibit undefined behavior.

The classic examples of unsafe traits are std::marker::Send and std::marker::Sync. These traits don’t define any methods, so they’re trivial to implement for any type you like. But they do have contracts: Send requires implementers to be safe to move to another thread, and Sync requires them to be safe to share among threads via shared references. Implementing Send for an inappropriate type, for example, would make std::sync::Mutex no longer safe from data races.

As a simple example, the Rust library includes an unsafe trait, core::nonzero::Zeroable, for types that can be safely initialized by setting all their bytes to zero. Clearly, zeroing a usize is fine, but zeroing a &T gives you a null reference, which will cause a crash if dereferenced. For types that are zeroable, some optimizations are possible: you can initialize an array of them quickly with std::mem::write_bytes (Rust’s equivalent of memset), or use operating system calls that allocate zeroed pages. (As of Rust 1.17, Zeroable is experimental, so it may be changed or removed in future versions of Rust, but it’s a good, simple, real-world example.)

Zeroable is a typical marker trait, lacking methods or associated types:

pub unsafe trait Zeroable {}

The implementations for appropriate types are similarly straightforward:

unsafe impl Zeroable for u8 {}
unsafe impl Zeroable for i32 {}
unsafe impl Zeroable for usize {}
// and so on for all the integer types

With these definitions, we can write a function that quickly allocates a vector of a given length containing a Zeroable type:

#![feature(nonzero)]  // permits `Zeroable`

extern crate core;
use core::nonzero::Zeroable;

fn zeroed_vector<T>(len: usize) -> Vec<T>
    where T: Zeroable
{
    let mut vec = Vec::with_capacity(len);
    unsafe {
        std::ptr::write_bytes(vec.as_mut_ptr(), 0, len);
        vec.set_len(len);
    }
    vec
}

This function starts by creating an empty Vec with the required capacity, and then calls write_bytes to fill the unoccupied buffer with zeros. (The write_byte function treats len as a number of T elements, not a number of bytes, so this call does fill the entire buffer.) A vector’s set_len method changes its length without doing anything to the buffer; this is unsafe, because you must ensure that the newly enclosed buffer space actually contains properly initialized values of type T. But this is exactly what the T: Zeroable bound establishes: a block of zero bytes represent a valid T value. Our use of set_len is safe.

Here, we put it to use:

let v: Vec<usize> = zeroed_vector(100_000);
assert!(v.iter().all(|&u| u == 0));

Clearly, Zeroable must be an unsafe trait, since an implementation that doesn’t respect its contract can lead to undefined behavior:

struct HoldsRef<'a>(&'a mut i32);

unsafe impl<'a> Zeroable for HoldsRef<'a> { }

let mut v: Vec<HoldsRef> = zeroed_vector(1);
*v[0].0 = 1;   // crashes: dereferences null pointer

Rust compiles this without complaint: it has no idea what Zeroable is meant to signify, so it can’t tell when it’s being implemented for an inappropriate type. As with any other unsafe feature, it’s up to you to understand and adhere to an unsafe trait’s contract.

Note that unsafe code must not depend on ordinary, safe traits being implemented correctly. For example, suppose there were an implementation of the std::hash::Hasher trait that simply returned a random hash value, with no relation to the values being hashed. The trait requires that hashing the same bits twice must produce the same hash value, but this implementation doesn’t meet that requirement; it’s simply incorrect. But because Hasher is not an unsafe trait, unsafe code must not exhibit undefined behavior when it uses this hasher. The std::collections::​HashMap type is carefully written to respect the contracts of the unsafe features it uses regardless of how the hasher behaves. Certainly, the table won’t function correctly: lookups will fail, and entries will appear and disappear at random. But the table will not exhibit undefined behavior.

Raw Pointers

A raw pointer in Rust is an unconstrained pointer. You can use raw pointers to form all sorts of structures that Rust’s checked pointer types cannot, like doubly linked lists or arbitrary graphs of objects. But because raw pointers are so flexible, Rust cannot tell whether you are using them safely or not, so you can dereference them only in an unsafe block.

Raw pointers are essentially equivalent to C or C++ pointers, so they’re also useful for interacting with code written in those languages.

There are two kinds of raw pointers:

A *mut T is a raw pointer to a T that permits modifying its referent.

A *const T is a raw pointer to a T that only permits reading its referent.

(There is no plain *T type; you must always specify either const or mut.)

You can create a raw pointer by conversion from a reference, and dereference it with the * operator:

let mut x = 10;
let ptr_x = &mut x as *mut i32;

let y = Box::new(20);
let ptr_y = &*y as *const i32;

unsafe {
    *ptr_x += *ptr_y;
}
assert_eq!(x, 30);

Unlike boxes and references, raw pointers can be null, like NULL in C or nullptr in C++:

fn option_to_raw<T>(opt: Option<&T>) -> *const T {
    match opt {
        None => std::ptr::null(),
        Some(r) => r as *const T
    }
}

assert!(!option_to_raw(Some(&("pea", "pod"))).is_null());
assert_eq!(option_to_raw::<i32>(None), std::ptr::null());

This example has no unsafe blocks: creating raw pointers, passing them around, and comparing them are all safe. Only dereferencing a raw pointer is unsafe.

A raw pointer to an unsized type is a fat pointer, just as the corresponding reference or Box type would be. A *const [u8] pointer includes a length along with the address, and a trait object like *mut std::io::Write pointer carries a vtable.

Although Rust implicitly dereferences safe pointer types in various situations, raw pointer dereferences must be explicit:

The . operator will not implicitly dereference a raw pointer; you must write (*raw).field or (*raw).method(...).

Raw pointers do not implement Deref, so deref coercions do not apply to them.

Operators like == and < compare raw pointers as addresses: two raw pointers are equal if they point to the same location in memory. Similarly, hashing a raw pointer hashes the address it points to, not the value of its referent.

Formatting traits like std::fmt::Display follow references automatically, but don’t handle raw pointers at all. The exceptions are std::fmt::Debug and std::fmt::Pointer, which show raw pointers as hexadecimal addresses, without dereferencing them.

Unlike the + operator in C and C++, Rust’s + does not handle raw pointers, but you can perform pointer arithmetic via their offset and wrapping_offset methods. There is no standard operation for finding the distance between two pointers, as the - operator does in C and C++, but you can write one yourself:

fn distance<T>(left: *const T, right: *const T) -> isize {
    (left as isize - right as isize) / std::mem::size_of::<T>() as isize
}

let trucks = vec!["garbage truck", "dump truck", "moonstruck"];
let first = &trucks[0];
let last = &trucks[2];
assert_eq!(distance(last, first), 2);
assert_eq!(distance(first, last), -2);

Even though distance’s parameters are raw pointers, we can pass it references: Rust implicitly coerces references to raw pointers (but not the other way around, of course).

The as operator permits almost every plausible conversion from references to raw pointers or between two raw pointer types. However, you may need to break up a complex conversion into a series of simpler steps. For example:

&vec![42_u8] as *const String  // error: invalid conversion
&vec![42_u8] as *const Vec<u8> as *const String;  // permitted

Note that as will not convert raw pointers to references. Such conversions would be unsafe, and as should remain a safe operation. Instead, you must dereference the raw pointer (in an unsafe block), and then borrow the resulting value.

Be very careful when you do this: a reference produced this way has an unconstrained lifetime: there’s no limit on how long it can live, since the raw pointer gives Rust nothing to base such a decision on. In “A Safe Interface to libgit2” later in this chapter, we show several examples of how to properly constrain lifetimes.

Many types have as_ptr and as_mut_ptr methods that return a raw pointer to their contents. For example, array slices and strings return pointers to their first elements, and some iterators return a pointer to the next element they will produce. Owning pointer types like Box, Rc, and Arc have into_raw and from_raw functions that convert to and from raw pointers. Some of these methods’ contracts impose surprising requirements, so check their documentation before using them.

You can also construct raw pointers by conversion from integers, although the only integers you can trust for this are generally those you got from a pointer in the first place. “Example: RefWithFlag” uses raw pointers this way.

Unlike references, raw pointers are neither Send nor Sync. As a result, any type that includes raw pointers does not implement these traits by default. There is nothing inherently unsafe about sending or sharing raw pointers between threads; after all, wherever they go, you still need an unsafe block to dereference them. But given the roles raw pointers typically play, the language designers considered this behavior to be the more helpful default. We already discussed how to implement Send and Sync yourself in “Unsafe Traits”.

Dereferencing Raw Pointers Safely

Here are some common-sense guidelines for using raw pointers safely:

Dereferencing null pointers or dangling pointers is undefined behavior, as is referring to uninitialized memory, or values that have gone out of scope.

Dereferencing pointers that are not properly aligned for their referent type is undefined behavior.

You may borrow values out of a dereferenced raw pointer only if doing so obeys the rules for reference safety explained in Chapter 5: No reference may outlive its referent; shared access is read-only access; and mutable access is exclusive access. (This rule is easy to violate by accident, since raw pointers are often used to create data structures with nonstandard sharing or ownership.)

You may use a raw pointer’s referent only if it is a well-formed value of its type. For example, you must ensure that dereferencing a *const char yields a proper, nonsurrogate Unicode code point.

You may use the offset and wrapping_offset methods on raw pointers only to point to bytes within the variable or heap-allocated block of memory that the original pointer referred to, or to the first byte beyond such a region.

If you do pointer arithmetic by converting the pointer to an integer, doing arithmetic on the integer, and then converting it back to a pointer, the result must be a pointer that the rules for the offset method would have allowed you to produce.

If you assign to a raw pointer’s referent, you must not violate the invariants of any type of which the referent is a part. For example, if you have a *mut u8 pointing to a byte of a String, you may only store values in that u8 that leave the String holding well-formed UTF-8.

The borrowing rule aside, these are essentially the same rules you must follow when using pointers in C or C++.

The reason for not violating types’ invariants should be clear. Many of Rust’s standard types use unsafe code in their implementation, but still provide safe interfaces on the assumption that Rust’s safety checks, module system, and visibility rules will be respected. Using raw pointers to circumvent these protective measures can lead to undefined behavior.

The complete, exact contract for raw pointers is not easily stated, and may change as the language evolves. But the principles outlined here should keep you in safe territory.

Example: RefWithFlag

Here’s an example of how to take a classic¹ bit-level hack made possible by raw pointers, and wrap it up as a completely safe Rust type. This module defines a type, RefWithFlag<'a, T>, that holds both a &'a T and a bool, like the tuple (&'a T, bool), and yet still manages to occupy only one machine word instead of two. This sort of technique is used regularly in garbage collectors and virtual machines, where certain types—say, the type representing an object—are so numerous that adding even a single word to each value would drastically increase memory use:

mod ref_with_flag {
    use std::marker::PhantomData;
    use std::mem::align_of;

    /// A `&T` and a `bool`, wrapped up in a single word.
    /// The type `T` must require at least two-byte alignment.
    ///
    /// If you're the kind of programmer who's never met a pointer whose
    /// 2⁰-bit you didn't want to steal, well, now you can do it safely!
    /// ("But it's not nearly as exciting this way...")
    pub struct RefWithFlag<'a, T: 'a> {
        ptr_and_bit: usize,
        behaves_like: PhantomData<&'a T> // occupies no space
    }

    impl<'a, T: 'a> RefWithFlag<'a, T> {
        pub fn new(ptr: &'a T, flag: bool) -> RefWithFlag<T> {
            assert!(align_of::<T>() % 2 == 0);
            RefWithFlag {
                ptr_and_bit: ptr as *const T as usize | flag as usize,
                behaves_like: PhantomData
            }
        }

        pub fn get_ref(&self) -> &'a T {
            unsafe {
                let ptr = (self.ptr_and_bit & !1) as *const T;
                &*ptr
            }
        }

        pub fn get_flag(&self) -> bool {
            self.ptr_and_bit & 1 != 0
        }
    }
}

This code takes advantage of the fact that many types must be placed at even addresses in memory: since an even address’s least significant bit is always zero, we can store something else there, and then reliably reconstruct the original address just by masking off the bottom bit. Not all types qualify; for example, the types u8 and (bool, [i8; 2]) can be placed at any address. But we can check the type’s alignment on construction and refuse types that won’t work.

You can use RefWithFlag like this:

use ref_with_flag::RefWithFlag;

let vec = vec![10, 20, 30];
let flagged = RefWithFlag::new(&vec, true);
assert_eq!(flagged.get_ref()[1], 20);
assert_eq!(flagged.get_flag(), true);

The constructor RefWithFlag::new takes a reference and a bool value, asserts that the reference’s type is suitable, and then converts the reference to a raw pointer, and then a usize. The usize type is defined to be large enough to hold a pointer on whatever processor we’re compiling for, so converting a raw pointer to a usize and back is well-defined. Once we have a usize, we know it must be even, so we can use the | bitwise-or operator to combine it with the bool, which we’ve converted to an integer 0 or 1.

The get_flag method extracts the bool component of a RefWithFlag. It’s simple: just mask off the bottom bit and check if it’s nonzero.

The get_ref method extracts the reference from a RefWithFlag. First, it masks off the usize’s bottom bit and converts it to a raw pointer. The as operator will not convert raw pointers to references, but we can dereference the raw pointer (in an unsafe block, naturally) and borrow that. Borrowing a raw pointer’s referent gives you a reference with an unbounded lifetime: Rust will accord the reference whatever lifetime would make the code around it check, if there is one. Usually, though, there is some specific lifetime which is more accurate, and would thus catch more mistakes. In this case, since get_ref’s return type is &'a T, Rust infers that the reference’s lifetime must be the RefWithFlag’s argument, which is just what we want: that’s the lifetime of the reference we started with.

In memory, a RefWithFlag looks just like a usize: since PhantomData is a zero-sized type, the behaves_like field takes up no space in the structure. But the PhantomData is necessary for Rust to know how to treat lifetimes in code that uses RefWithFlag. Imagine what the type would look like without the behaves_like field:

// This won't compile.
pub struct RefWithFlag<'a, T: 'a> {
    ptr_and_bit: usize
}

In Chapter 5, we pointed out that any structure containing references must not outlive the values they borrow, lest the references become dangling pointers. The structure must abide by the restrictions that apply to its fields. This certainly applies to RefWithFlag: in the example code we just looked at, flagged must not outlive vec, since flagged.get_ref() returns a reference to it. But our reduced RefWithFlag type contains no references at all, and never uses its lifetime parameter 'a. It’s just a usize. How should Rust know that any restrictions apply to pab’s lifetime? Including a PhantomData<&'a T> field tells Rust to treat RefWithFlag<'a, T> as if it contained a &'a T, without actually affecting the struct’s representation.

Although Rust doesn’t really know what’s going on (that’s what makes RefWithFlag unsafe), it will do its best to help you out with this. If you omit the _marker field, Rust will complain that the parameters 'a and T are unused, and suggest using a PhantomData.

RefWithFlag uses the same tactics as the Ascii type we presented earlier to avoid undefined behavior in its unsafe block. The type itself is pub, but its fields are not, meaning that only code within the pointer_and_bool module can create or look inside a RefWithFlag value. You don’t have to inspect much code to have confidence that the ptr_and_bit field is well constructed.

Nullable Pointers

A null raw pointer in Rust is a zero address, just as in C and C++. For any type T, the std::ptr::null<T> function returns a *const T null pointer, and std::ptr::null_mut<T> returns a *mut T null pointer.

There are a few ways to check whether a raw pointer is null. The simplest is the is_null method, but the as_ref method may be more convenient: it takes a *const T pointer and returns an Option<&'a T>, turning a null pointer into a None. Similarly, the as_mut method converts *mut T pointers into Option<&'a mut T> values.

Type Sizes and Alignments

A value of any Sized type occupies a constant number of bytes in memory, and must be placed at an address that is a multiple of some alignment value, determined by the machine architecture. For example, an (i32, i32) tuple occupies eight bytes, and most processors prefer it to be placed at an address that is a multiple of four.

The call std::mem::size_of::<T>() returns the size of a value of type T, in bytes, and std::mem::align_of::<T>() returns its required alignment. For example:

assert_eq!(std::mem::size_of::<i64>(), 8);
assert_eq!(std::mem::align_of::<(i32, i32)>(), 4);

Any type’s alignment is always a power of two.

A type’s size is always rounded up to a multiple of its alignment, even if it technically could fit in less space. For example, even though a tuple like (f32, u8) requires only five bytes, size_of::<(f32, u8)>() is 8, because align_of::<(f32, u8)>() is 4. This ensures that if you have an array, the size of the element type always reflects the spacing between one element and the next.

For unsized types, the size and alignment depend on the value at hand. Given a reference to an unsized value, the std::mem::size_of_val and std::mem::align_of_val functions return the value’s size and alignment. These functions can operate on references to both Sized and unsized types.

// Fat pointers to slices carry their referent's length.
let slice: &[i32] = &[1, 3, 9, 27, 81];
assert_eq!(std::mem::size_of_val(slice), 20);

let text: &str = "alligator";
assert_eq!(std::mem::size_of_val(text), 9);

use std::fmt::Display;
let unremarkable: &Display = &193_u8;
let remarkable: &Display = &0.0072973525664;

// These return the size/alignment of the value the
// trait object points to, not those of the trait object
// itself. This information comes from the vtable the
// trait object refers to.
assert_eq!(std::mem::size_of_val(unremarkable), 1);
assert_eq!(std::mem::align_of_val(remarkable), 8);

Pointer Arithmetic

Rust lays out the elements of an array, slice, or vector as a single contiguous block of memory, as shown in Figure 21-1. Elements are regularly spaced, so that if each element occupies size bytes, then the i’th element starts with the i * size’th byte.

Figure 21-1. An array in memory

One nice consequence of this is that if you have two raw pointers to elements of an array, comparing the pointers gives the same results as comparing the elements’ indices: if i < j, then a raw pointer to the i’th element is less than a raw pointer to the j’th element. This makes raw pointers useful as bounds on array traversals. In fact, the standard library’s simple iterator over a slice is defined like this:

struct Iter<'a, T: 'a> {
    ptr: *const T,
    end: *const T,
    ...
}

The ptr field points to the next element iteration should produce, and the end field serves as the limit: when ptr == end, the iteration is complete.

Another nice consequence of array layout: if element_ptr is a *const T or *mut T raw pointer to the i’th element of some array, then element_ptr.offset(o) is a raw pointer to the (i + o)’th element. Its definition is equivalent to this:

fn offset(self: *const T, count: isize) -> *const T
    where T: Sized
{
    let bytes_per_element = std::mem::size_of::<T>() as isize;
    let byte_offset = count * bytes_per_element;
    (self as isize).checked_add(byte_offset).unwrap() as *const T
}

The std::mem::size_of::<T> function returns the size of the type T in bytes. Since isize is, by definition, large enough to hold an address, you can convert the base pointer to an isize, do arithmetic on that value, and then convert the result back to a pointer.

It’s fine to produce a pointer to the first byte after the end of an array. You cannot dereference such a pointer, but it can be useful to represent the limit of a loop, or for bounds checks.

However, it is undefined behavior to use offset to produce a pointer beyond that point, or before the start of the array, even if you never dereference it. For the sake of optimization, Rust would like to assume that ptr.offset(i) > ptr when i is positive, and that ptr.offset(i) < ptr when i is negative. This assumption seems safe, but it may not hold if the arithmetic in offset overflows an isize value. If i is constrained to stay within the same array as ptr, no overflow can occur: after all, the array itself does not overflow the bounds of the address space. (To make pointers to the first byte after the end safe, Rust never places values at the upper end of the address space.)

If you do need to offset pointers beyond the limits of the array they are associated with, you can use the wrapping_offset method. This is equivalent to offset, but Rust makes no assumptions about the relative ordering of ptr.wrapping_offset(i) and ptr itself. Of course, you still can’t dereference such pointers unless they fall within the array.

Moving into and out of Memory

If you are implementing a type that manages its own memory, you will need to track which parts of your memory hold live values and which are uninitialized, just as Rust does with local variables. Consider this code:

let pot = "pasta".to_string();
let plate;

plate = pot;

After this code has run, the situation looks like Figure 21-2.

Figure 21-2. Moving a string from one local variable to another

After the assignment, pot is uninitialized, and plate is the owner of the string.

At the machine level, it’s not specified what a move does to the source, but in practice it usually does nothing at all. The assignment probably leaves pot still holding a pointer, capacity, and length for the string. Naturally, it would be disastrous to treat this as a live value, and Rust ensures that you don’t.

The same considerations apply to data structures that manage their own memory. Suppose you run this code:

let mut noodles = vec!["udon".to_string()];
let soba = "soba".to_string();
let last;

In memory, the state looks like Figure 21-3.

Figure 21-3. A vector with uninitialized, spare capacity

The vector has the spare capacity to hold one more element, but its contents are junk, probably whatever that memory held previously. Suppose you then run this code:

noodles.push(soba);

Pushing the string onto the vector transforms that uninitialized memory into a new element, as illustrated in Figure 21-4.

Figure 21-4. After pushing soba’s value onto the vector

The vector has initialized its empty space to own the string, and incremented its length to mark this as a new, live element. The vector is now the owner of the string; you can refer to its second element, and dropping the vector would free both strings. And soba is now uninitialized.

Finally, consider what happens when we pop a value from the vector:

last = noodles.pop().unwrap();

In memory, things now look like Figure 21-5.

Figure 21-5. After popping an element from the vector into last

The variable last has taken ownership of the string. The vector has decremented its length to indicate that the space that used to hold the string is now uninitialized.

Just as with pot and pasta earlier, all three of soba, last, and the vector’s free space probably hold identical bit patterns. But only last is considered to own the value. Treating either of the other two locations as live would be a mistake.

The true definition of an initialized value is one that is treated as live. Writing to a value’s bytes is usually a necessary part of initialization, but only because doing so prepares the value to be treated as live.

Rust tracks local variables at compile time. Types like Vec, HashMap, Box, and so on track their buffers dynamically. If you implement a type that manages its own memory, you will need to do the same.

Rust provides two essential operations for implementing such types:

std::ptr::read(src) moves a value out of the location src points to, transferring ownership to the caller. After calling read, you must treat *src as uninitialized memory. The src argument should be a *const T raw pointer, where T is a sized type.

This is the operation behind Vec::pop. Popping a value calls read to move the value out of the buffer, and then decrements the length to mark that space as uninitialized capacity.

std::ptr::write(dest, value) moves value into the location dest points to, which must be uninitialized memory before the call. The referent now owns the value. Here, dest must be a *mut T raw pointer and value a T value, where T is a sized type.

This is the operation behind Vec::push. Pushing a value calls write to move the value into the next available space, and then increments the length to mark that space as a valid element.

Both are free functions, not methods on the raw pointer types.

Note that you cannot do these things with any of Rust’s safe pointer types. They all require their referents to be initialized at all times, so transforming uninitialized memory into a value, or vice versa, is outside their reach. Raw pointers fit the bill.

The standard library also provides functions for moving arrays of values from one block of memory to another:

std::ptr::copy(src, dst, count) moves the array of count values in memory starting at src to the memory at dst, just as if you had written a loop of read and write calls to move them one at a time. The destination memory must be uninitialized before the call, and afterward the source memory is left uninitialized. The src and dest arguments must be *const T and *mut T raw pointers, and count must be a usize.

std::ptr::copy_nonoverlapping(src, dst, count) is like the corresponding call to copy, except that its contract further requires that the source and destination blocks of memory must not overlap. This may be slightly faster than calling copy.

There are two other families of read and write functions, also in the std::ptr module:

The read_unaligned and write_unaligned functions are like read and write, except that the pointer need not be aligned as normally required for the referent type. These functions may be slower than the plain read and write functions.

The read_volatile and write_volatile functions are the equivalent of volatile reads and writes in C or C++.

Example: GapBuffer

Here’s an example that puts the raw pointer functions just described to use.

Suppose you’re writing a text editor, and you’re looking for a type to represent the text. You could choose String, and use the insert and remove methods to insert and delete characters as the user types. But if they’re editing text at the beginning of a large file, those methods can be expensive: inserting a new character involves shifting the entire rest of the string to the right in memory, and deletion shifts it all back to the left. You’d like such common operations to be cheaper.

The Emacs text editor uses a simple data structure called a gap buffer which can insert and delete characters in constant time. Whereas a String keeps all its spare capacity at the end of the text, which makes push and pop cheap, a gap buffer keeps its spare capacity in the midst of the text, at the point where editing is taking place. This spare capacity is called the gap. Inserting or deleting elements at the gap is cheap: you simply shrink or enlarge the gap as needed. You can move the gap to any location you like by shifting text from one side of the gap to the other. When the gap is empty, you migrate to a larger buffer.

While insertion and deletion in a gap buffer are fast, changing the position at which they take place entails moving the gap to the new position. Shifting the elements requires time proportional to the distance being moved. Fortunately, typical editing activity involves making a bunch of changes in one neighborhood of the buffer before going off and fiddling with text someplace else.

In this section we’ll implement a gap buffer in Rust. To avoid being distracted by UTF-8, we’ll make our buffer store char values directly, but the principles of operation would be the same if we stored the text in some other form.

First, we’ll show a gap buffer in action. This code creates a GapBuffer, inserts some text in it, and then moves the insertion point to sit just before the last word:

use gap::GapBuffer;

let mut buf = GapBuffer::new();
buf.insert_iter("Lord of the Rings".chars());
buf.set_position(12);

After running this code, the buffer looks as shown in Figure 21-6.

Figure 21-6. A gap buffer containing some text

Insertion is a matter of filling in the gap with new text. This code adds a word and ruins the film:

buf.insert_iter("Onion ".chars());

This results in the state shown in Figure 21-7.

Figure 21-7. A gap buffer containing some more text

Here’s our GapBuffer type:

mod gap {
    use std;
    use std::ops::Range;

    pub struct GapBuffer<T> {
        // Storage for elements. This has the capacity we need, but its length
        // always remains zero. GapBuffer puts its elements and the gap in this
        // `Vec`'s "unused" capacity.
        storage: Vec<T>,

        // Range of uninitialized elements in the middle of `storage`.
        // Elements before and after this range are always initialized.
        gap: Range<usize>
    }

    ...
}

GapBuffer uses its storage field in a strange way.² It never actually stores any elements in the vector—or not quite. It simply calls Vec::with_capacity(n) to get a block of memory large enough to hold n values, obtains raw pointers to that memory via the vector’s as_ptr and as_mut_ptr methods, and then uses the buffer directly for its own purposes. The vector’s length always remains zero. When the Vec gets dropped, the Vec doesn’t try to free its elements, because it doesn’t know it has any, but it does free the block of memory. This is what GapBuffer wants; it has its own Drop implementation that knows where the live elements are and drops them correctly.

GapBuffer’s simplest methods are what you’d expect:

impl<T> GapBuffer<T> {
    pub fn new() -> GapBuffer<T> {
        GapBuffer { storage: Vec::new(), gap: 0..0 }
    }

    /// Return the number of elements this GapBuffer could hold without
    /// reallocation.
    pub fn capacity(&self) -> usize {
        self.storage.capacity()
    }

    /// Return the number of elements this GapBuffer currently holds.
    pub fn len(&self) -> usize {
        self.capacity() - self.gap.len()
    }

    /// Return the current insertion position.
    pub fn position(&self) -> usize {
        self.gap.start
    }

    ...
}

It cleans up many of the following functions to have a utility method that returns a raw pointer to the buffer element at a given index. This being Rust, we end up needing one method for mut pointers and one for const. Unlike the preceding methods, these are not public. Continuing this impl block:

/// Return a pointer to the `index`'th element of the underlying storage,
/// regardless of the gap.
///
/// Safety: `index` must be a valid index into `self.storage`.
unsafe fn space(&self, index: usize) -> *const T {
    self.storage.as_ptr().offset(index as isize)
}

/// Return a mutable pointer to the `index`'th element of the underlying
/// storage, regardless of the gap.
///
/// Safety: `index` must be a valid index into `self.storage`.
unsafe fn space_mut(&mut self, index: usize) -> *mut T {
    self.storage.as_mut_ptr().offset(index as isize)
}

To find the element at a given index, you must consider whether the index falls before or after the gap, and adjust appropriately:

/// Return the offset in the buffer of the `index`'th element, taking
/// the gap into account. This does not check whether index is in range,
/// but it never returns an index in the gap.
fn index_to_raw(&self, index: usize) -> usize {
    if index < self.gap.start {
        index
    } else {
        index + self.gap.len()
    }
}

/// Return a reference to the `index`'th element,
/// or `None` if `index` is out of bounds.
pub fn get(&self, index: usize) -> Option<&T> {
    let raw = self.index_to_raw(index);
    if raw < self.capacity() {
        unsafe {
            // We just checked `raw` against self.capacity(),
            // and index_to_raw skips the gap, so this is safe.
            Some(&*self.space(raw))
        }
    } else {
        None
    }
}

When we start making insertions and deletions in a different part of the buffer, we need to move the gap to the new location. Moving the gap to the right entails shifting elements to the left, and vice versa, just as the bubble in a spirit level moves in one direction when the fluid flows in the other:

/// Set the current insertion position to `pos`.
/// If `pos` is out of bounds, panic.
pub fn set_position(&mut self, pos: usize) {
    if pos > self.len() {
        panic!("index {} out of range for GapBuffer", pos);
    }

    unsafe {
        let gap = self.gap.clone();
        if pos > gap.start {
            // `pos` falls after the gap. Move the gap right
            // by shifting elements after the gap to before it.
            let distance = pos - gap.start;
            std::ptr::copy(self.space(gap.end),
                           self.space_mut(gap.start),
                           distance);
        } else if pos < gap.start {
            // `pos` falls before the gap. Move the gap left
            // by shifting elements before the gap to after it.
            let distance = gap.start - pos;
            std::ptr::copy(self.space(pos),
                           self.space_mut(gap.end - distance),
                           distance);
        }

        self.gap = pos .. pos + gap.len();
    }
}

This function uses the std::ptr::copy method to shift the elements; copy requires that the destination be uninitialized, and leaves the source uninitialized. The source and destination ranges may overlap, but copy handles that case correctly. Since the gap is uninitialized memory before the call, and the function adjusts the gap’s position to cover space vacated by the copy, the copy function’s contract is satisfied.

Element insertion and removal are relatively simple. Insertion takes over one space from the gap for the new element, whereas removal moves one value out, and enlarges the gap to cover the space it used to occupy:

/// Insert `elt` at the current insertion position,
/// and leave the insertion position after it.
pub fn insert(&mut self, elt: T) {
    if self.gap.len() == 0 {
        self.enlarge_gap();
    }

    unsafe {
        let index = self.gap.start;
        std::ptr::write(self.space_mut(index), elt);
    }
    self.gap.start += 1;
}

/// Insert the elements produced by `iter` at the current insertion
/// position, and leave the insertion position after them.
pub fn insert_iter<I>(&mut self, iterable: I)
    where I: IntoIterator<Item=T>
{
    for item in iterable {
        self.insert(item)
    }
}

/// Remove the element just after the insertion position
/// and return it, or return `None` if the insertion position
/// is at the end of the GapBuffer.
pub fn remove(&mut self) -> Option<T> {
    if self.gap.end == self.capacity() {
        return None;
    }

    let element = unsafe {
        std::ptr::read(self.space(self.gap.end))
    };
    self.gap.end += 1;
    Some(element)
}

Similar to the way Vec uses std::ptr::write for push and std::ptr::read for pop, GapBuffer uses write for insert, and read for remove. And just as Vec must adjust its length to maintain the boundary between initialized elements and spare capacity, GapBuffer adjusts its gap.

When the gap has been filled in, the insert method must grow the buffer to acquire more free space. The enlarge_gap method (the last in the impl block) handles this:

/// Double the capacity of `self.storage`.
fn enlarge_gap(&mut self) {
    let mut new_capacity = self.capacity() * 2;
    if new_capacity == 0 {
        // The existing vector is empty.
        // Choose a reasonable starting capacity.
        new_capacity = 4;
    }

    // We have no idea what resizing a Vec does with its "unused"
    // capacity. So just create a new vector and move over the elements.
    let mut new = Vec::with_capacity(new_capacity);
    let after_gap = self.capacity() - self.gap.end;
    let new_gap = self.gap.start .. new.capacity() - after_gap;

    unsafe {
        // Move the elements that fall before the gap.
        std::ptr::copy_nonoverlapping(self.space(0),
                                      new.as_mut_ptr(),
                                      self.gap.start);

        // Move the elements that fall after the gap.
        let new_gap_end = new.as_mut_ptr().offset(new_gap.end as isize);
        std::ptr::copy_nonoverlapping(self.space(self.gap.end),
                                      new_gap_end,
                                      after_gap);
    }

    // This frees the old Vec, but drops no elements,
    // because the Vec's length is zero.
    self.storage = new;
    self.gap = new_gap;
}

Whereas set_position must use copy to move elements back and forth in the gap, enlarge_gap can use copy_nonoverlapping, since it is moving elements to an entirely new buffer.

Moving the new vector into self.storage drops the old vector. Since its length is zero, the old vector believes it has no elements to drop, and simply frees its buffer. Neatly, copy_nonoverlapping leaves its source uninitialized, so the old vector is correct in this belief: all the elements are now owned by the new vector.

Finally, we need to make sure that dropping a GapBuffer drops all its elements:

impl<T> Drop for GapBuffer<T> {
    fn drop(&mut self) {
        unsafe {
            for i in 0 .. self.gap.start {
                std::ptr::drop_in_place(self.space_mut(i));
            }
            for i in self.gap.end .. self.capacity() {
                std::ptr::drop_in_place(self.space_mut(i));
            }
        }
    }
}

The elements lie before and after the gap, so we iterate over each region and use the std::ptr::drop_in_place function to drop each one. The drop_in_place function is a utility that behaves like drop(std::ptr::read(ptr)), but doesn’t bother moving the value to its caller (and hence works on unsized types). And just as in enlarge_gap, by the time the vector self.storage is dropped, its buffer really is uninitialized.

Like the other types we’ve shown in this chapter, GapBuffer ensures that its own invariants are sufficient to ensure that the contract of every unsafe feature it uses is followed, so none of its public methods need be marked unsafe. GapBuffer implements a safe interface for a feature that cannot be written efficiently in safe code.

Panic Safety in Unsafe Code

In Rust, panics can’t usually cause undefined behavior; the panic! macro is not an unsafe feature. But when you decide to work with unsafe code, panic safety becomes part of your job.

Consider the GapBuffer::remove method from the previous section:

pub fn remove(&mut self) -> Option<T> {
    if self.gap.end == self.capacity() {
        return None;
    }

    let element = unsafe {
        std::ptr::read(self.space(self.gap.end))
    };
    self.gap.end += 1;
    Some(element)
}

The call to read moves the element immediately after the gap out of the buffer, leaving behind uninitialized space. Fortunately, the very next statement enlarges the gap to cover that space, so by the time we return, everything is as it should be: all elements outside the gap are initialized, and all elements inside the gap are uninitialized.

But consider what would happen if, after the call to read but before the adjustment to self.gap.end, this code tried to use a feature that might panic—say, indexing a slice. Exiting the method abruptly anywhere between those two actions would leave the GapBuffer with an uninitialized element outside the gap. The next call to remove could try to read it again; and even simply dropping the GapBuffer would try to drop it. Both are undefined behavior, because they access uninitialized memory.

It’s all but unavoidable for a type’s methods to momentarily relax the type’s invariants while they do their job, and then put everything back to rights before they return. A panic mid-method could cut that cleanup process short, leaving the type in an inconsistent state.

If the type uses only safe code, then this inconsistency may make the type misbehave, but it can’t introduce undefined behavior. But code using unsafe features is usually counting on its invariants to meet the contracts of those features. Broken invariants lead to broken contracts, which lead to undefined behavior.

When working with unsafe features, you must take special care to identify these sensitive regions, and ensure that they do nothing that might panic.

Foreign Functions: Calling C and C++ from Rust

Rust’s foreign function interface lets Rust code call functions written in C or C++.

In this section, we’ll write a program that links with libgit2, a C library for working with the Git version control system. First, we’ll show what it’s like to use C functions directly from Rust. Then, we’ll show how to construct a safe interface to libgit2, taking inspiration from the open source git2-rs crate, which does exactly that.

We’ll assume that you’re familiar with C and the mechanics of compiling and linking C programs. Working with C++ is similar. We’ll also assume that you’re somewhat familiar with the Git version control system.

Finding Common Data Representations

The common denominator of Rust and C is machine language, so in order to anticipate what Rust values look like to C code, or vice versa, you need to consider their machine-level representations. Throughout the book, we’ve made a point of showing how values are actually represented in memory, so you’ve probably noticed that the data worlds of C and Rust have a lot in common: a Rust usize and a C size_t are identical, for example, and structs are fundamentally the same idea in both languages. To establish a correspondence between Rust and C types, we’ll start with primitives and then work our way up to more complicated types.

Given its primary use as a systems programming language, C has always been surprisingly loose about its types’ representations: an int is typically 32 bits long, but could be longer, or as short as 16 bits; a C char may be signed or unsigned; and so on. To cope with this variability, Rust’s std::os::raw module defines a set of Rust types that are guaranteed to have the same representation as certain C types. These cover the primitive integer and character types:

C type | Corresponding std::os::raw type
short | c_short
int | c_int
long | c_long
long long | c_longlong
unsigned short | c_ushort
unsigned, unsigned int | c_uint
unsigned long | c_ulong
unsigned long long | c_ulonglong
char | c_char
signed char | c_schar
unsigned char | c_uchar
float | c_float
double | c_double
void *, const void * | *mut c_void, *const c_void

Some notes about the table:

Except for c_void, all the Rust types here are aliases for some primitive Rust type: c_char, for example, is either i8 or u8.

There is no endorsed Rust type corresponding to C’s bool. At the moment, a Rust bool is always either a zero or a one byte, the same representation used by all major C and C++ implementations. However, the Rust language team has not committed to keep this representation in the future, since doing so may close opportunities for optimization.

Rust’s 32-bit char type is not the analogue of wchar_t, whose width and encoding vary from one implementation to another. C’s char32_t type is closer, but its encoding is still not guaranteed to be Unicode.

Rust’s primitive usize and isize types have the same representations as C’s size_t and ptrdiff_t.

C and C++ pointers and C++ references correspond to Rust’s raw pointer types, *mut T and *const T.

Technically, the C standard permits implementations to use representations for which Rust has no corresponding type: 36-bit integers, sign-and-magnitude representations for signed values, and so on. In practice, on every platform Rust has been ported to, every common C integer type has a match in Rust, bool aside.

For defining Rust struct types compatible with C structs, you can use the #[repr(C)] attribute. Placing #[repr(C)] above a struct definition asks Rust to lay out the struct’s fields in memory the same way a C compiler would lay out the analogous C struct type. For example, libgit2’s git2/errors.h header file defines the following C struct to provide details about a previously reported error:

typedef struct {
    char *message;
    int klass;
} git_error;

You can define a Rust type with an identical representation as follows:

#[repr(C)]
pub struct git_error {
    pub message: *const c_char,
    pub klass: c_int
}

The #[repr(C)] attribute affects only the layout of the struct itself, not the representations of its individual fields, so to match the C struct, each field must use the C-like type as well: *const c_char for char *, and c_int for int, and so on.

In this particular case, the #[repr(C)] attribute probably doesn’t change the layout of git_error. There really aren’t too many interesting ways to lay out a pointer and an integer. But whereas C and C++ guarantee that a structure’s members appear in memory in the order they’re declared, each at a distinct address, Rust reorders fields to minimize the overall size of the struct, and zero-sized types take up no space. The #[repr(C)] attribute tells Rust to follow C’s rules for the given type.

You can also use #[repr(C)] to control the representation of C-style enums:

#[repr(C)]
enum git_error_code {
    GIT_OK         =  0,
    GIT_ERROR      = -1,
    GIT_ENOTFOUND  = -3,
    GIT_EEXISTS    = -4,
     ...
}

Normally, Rust plays all sorts of games when choosing how to represent enums. For example, we mentioned the trick Rust uses to store Option<&T> in a single word (if T is sized). Without #[repr(C)], Rust would use a single byte to represent the git_error_code enum; with #[repr(C)], Rust uses a value the size of a C int, just as C would.

You can also ask Rust to give an enum the same representation as some integer type. Starting the preceding definition with #[repr(i16)] would give you a 16-bit type with the same representation as the following C++ enum:

#include <stdint.h>

enum git_error_code: int16_t {
    GIT_OK         =  0,
    GIT_ERROR      = -1,
    GIT_ENOTFOUND  = -3,
    GIT_EEXISTS    = -4,
    ...
};

Passing strings between Rust and C is a little harder. C represents a string as a pointer to an array of characters, terminated by a null character. Rust, on the other hand, stores the length of a string explicitly, either as a field of a String, or as the second word of a fat reference &str. Rust strings are not null-terminated; in fact, they may include null characters in their contents, like any other character.

This means that you can’t borrow a Rust string as a C string: if you pass C code a pointer into a Rust string, it could mistake an embedded null character for the end of the string, or run off the end looking for a terminating null that isn’t there. Going the other direction, you may be able to borrow a C string as a Rust &str, as long as its contents are well-formed UTF-8.

This situation effectively forces Rust to treat C strings as types entirely distinct from String and &str. In the std::ffi module, the CString and CStr types represent owned and borrowed null-terminated arrays of bytes. Compared to String and str, the methods on CString and CStr are quite limited, restricted to construction and conversion to other types. We’ll show these types in action in the next section.

Declaring Foreign Functions and Variables

An extern block declares functions or variables defined in some other library that the final Rust executable will be linked with. For example, every Rust program is linked against the standard C library, so we can tell Rust about the C library’s strlen function like this:

use std::os::raw::c_char;

extern {
    fn strlen(s: *const c_char) -> usize;
}

This gives Rust the function’s name and type, while leaving the definition to be linked in later.

Rust assumes that functions declared inside extern blocks use C conventions for passing arguments and accepting return values. They are defined as unsafe functions. These are the right choices for strlen: it is indeed a C function; and its specification in C requires that you pass it a valid pointer to a properly terminated string, which is a contract that Rust cannot enforce. (Almost any function that takes a raw pointer must be unsafe: safe Rust can construct raw pointers from arbitrary integers, and dereferencing such a pointer would be undefined behavior.)

With this extern block, we can call strlen like any other Rust function, although its type gives it away as a tourist:

use std::ffi::CString;

let rust_str = "I'll be back";
let null_terminated = CString::new(rust_str).unwrap();
unsafe {
    assert_eq!(strlen(null_terminated.as_ptr()), 12);
}

The CString::new function builds a null-terminated C string. It first checks its argument for embedded null characters, since those cannot be represented in a C string, and returns an error if it finds any (hence the need to unwrap the result). Otherwise, it adds a null byte to the end, and returns a CString owning the resulting characters.

The cost of CString::new depends on what type you pass it. It accepts anything that implements Into<Vec<u8>>. Passing a &str entails an allocation and a copy, as the conversion to Vec<u8> builds a heap-allocated copy of the string for the vector to own. But passing a String by value simply consumes the string and takes over its buffer, so unless appending the null character forces the buffer to be resized, the conversion requires no copying of text or allocation at all.

CString dereferences to CStr, whose as_ptr method returns a *const c_char pointing at the start of the string. This is the type that strlen expects. In the example, strlen runs down the string, finds the null character that CString::new placed there, and returns the length, as a byte count.

You can also declare global variables in extern blocks. POSIX systems have a global variable named environ that holds the values of the process’s environment variables. In C, it’s declared:

extern char **environ;

In Rust, you would say:

use std::ffi::CStr;
use std::os::raw::c_char;

extern {
    static environ: *mut *mut c_char;
}

To print the environment’s first element, you could write:

unsafe {
    if !environ.is_null() && !(*environ).is_null() {
        let var = CStr::from_ptr(*environ);
        println!("first environment variable: {}",
                 var.to_string_lossy())
    }
}

After making sure environ has a first element, the code calls CStr::from_ptr to build a CStr that borrows it. The to_string_lossy method returns a Cow<str>: if the C string contains well-formed UTF-8, the Cow borrows its content as a &str, not including the terminating null byte. Otherwise, to_string_lossy makes a copy of the text in the heap, replaces the ill-formed UTF-8 sequences with the official Unicode replacement character, '�', and builds an owning Cow from that. Either way, the result implements Display, so you can print it with the {} format parameter.

Using Functions from Libraries

To use functions provided by a particular library, you can place a #[link] attribute atop the extern block that names the library Rust should link the executable with. For example, here’s a program that calls libgit2’s initialization and shutdown methods, but does nothing else:

use std::os::raw::c_int;

#[link(name = "git2")]
extern {
    pub fn git_libgit2_init() -> c_int;
    pub fn git_libgit2_shutdown() -> c_int;
}

fn main() {
    unsafe {
        git_libgit2_init();
        git_libgit2_shutdown();
    }
}

The extern block declares the extern functions as before. The #[link(name = "git2")] attribute leaves a note in the crate to the effect that, when Rust creates the final executable or shared library, it should link against the git2 library. Rust uses the system linker to build executables; on Unix, this passes the argument -lgit2 on the linker command line; on Windows, it passes git2.LIB.

#[link] attributes work in library crates, too. When you build a program that depends on other crates, Cargo gathers together the link notes from the entire dependency graph, and includes them all in the final link.

In this example, if you would like to follow along on your own machine, you’ll need to build libgit2 for yourself. We used libgit2 version 0.25.1, available from https://libgit2.github.com. To compile libgit2, you will need to install the CMake build tool and the Python language; we used CMake version 3.8.0 and Python version 2.7.13, downloaded from https://cmake.org and https://www.python.org.

The full instructions for building libgit2 are available on its website, but they’re simple enough that we’ll show the essentials here. On Linux, assume you’ve already unzipped the library’s source into the directory /home/jimb/libgit2-0.25.1:

$ cd /home/jimb/libgit2-0.25.1
$ mkdir build
$ cd build
$ cmake ..
$ cmake --build .

On Linux, this produces a shared library /home/jimb/libgit2-0.25.1/build/libgit2.so.0.25.1 with the usual nest of symlinks pointing to it, including one named libgit2.so. On macOS, the results are similar, but the library is named libgit2.dylib.

On Windows, things are also straightforward. Assume you’ve unzipped the source into the directory C:\Users\JimB\libgit2-0.25.1. In a Visual Studio command prompt:

> cd C:\Users\JimB\libgit2-0.25.1
> mkdir build
> cd build
> cmake -A x64 ..
> cmake --build .

These are the same commands as used on Linux, except that you must request a 64-bit build when you run CMake the first time, to match your Rust compiler. (If you have installed the 32-bit Rust toolchain, then you should omit the -A x64 flag to the first cmake command.) This produces an import library git2.LIB and a dynamic-link library git2.DLL, both in the directory C:\Users\JimB\libgit2-0.25.1\build\Debug. (The remaining instructions are shown for Unix, except where Windows is substantially different.)

Create the Rust program in a separate directory:

$ cd /home/jimb
$ cargo new --bin git-toy

Put the code above in src/main.rs. Naturally, if you try to build this, Rust has no idea where to find the libgit2 you built:

$ cd git-toy
$ cargo run
   Compiling git-toy v0.1.0 (file:///home/jimb/git-toy)
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" ... "-l" "git2" ...
  = note: /usr/bin/ld: cannot find -lgit2
          collect2: error: ld returned 1 exit status

error: aborting due to previous error

error: Could not compile `git-toy`.

To learn more, run the command again with --verbose.
$

You can tell Rust where to search for libraries by writing a build script, Rust code that Cargo compiles and runs at build time. Build scripts can do all sorts of things: generate code dynamically, compile C code to be included in the crate, and so on. In this case, all you need is to add a library search path to the executable’s link command. When Cargo runs the build script, it parses the build script’s output for information of this sort, so the build script simply needs to print the right magic to its standard output.

To create your build script, add a file named build.rs in the same directory as the Cargo.toml file, with the following contents:

fn main() {
    println!(r"cargo:rustc-link-search=native=/home/jimb/libgit2-0.25.1/build");
}

This is the right path for Linux; on Windows, you would change the path following the text native= to C:\Users\JimB\libgit2-0.25.1\build\Debug. (We’re cutting some corners to keep this example simple; in a real application, you should avoid using absolute paths in your build script. We cite documentation that shows how to do it right at the end of this section.)

Next, tell Cargo that this is your build script by adding the line build = "build.rs" to the [package] section of your Cargo.toml file. The entire file should now read:

[package]
name = "git-toy"
version = "0.1.0"
authors = ["You <you@example.com>"]
build = "build.rs"

[dependencies]

Now you can almost run the program. On macOS it may work immediately; on a Linux system you will probably see something like the following:

$ cargo run
   Compiling git-toy v0.1.0 (file:///home/jimb/git-toy)
    Finished dev [unoptimized + debuginfo] target(s) in 0.64 secs
     Running `target/debug/git-toy`
target/debug/git-toy: error while loading shared libraries:
libgit2.so.25: cannot open shared object file: No such file or directory
$

This means that, although Cargo succeeded in linking the executable against the library, it doesn’t know where to find the shared library at run time. Windows reports this failure by popping up a dialog box. On Linux, you must set the LD_LIBRARY_PATH environment variable:

$ export LD_LIBRARY_PATH=/home/jimb/libgit2-0.25.1/build:$LD_LIBRARY_PATH
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/git-toy`
$

On macOS, you may need to set DYLD_LIBRARY_PATH instead.

On Windows, you must set the PATH environment variable:

> set PATH=C:\Users\JimB\libgit2-0.25.1\build\Debug;%PATH%
> cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/git-toy`
>

Naturally, in a deployed application you’d want to avoid having to set environment variables just to find your library’s code. One alternative is to statically link the C library into your crate. This copies the library’s object files into the crate’s .rlib file, alongside the object files and metadata for the crate’s Rust code. The entire collection then participates in the final link.

It is a Cargo convention that a crate that provides access to a C library should be named LIB-sys, where LIB is the name of the C library. A -sys crate should contain nothing but the statically linked library and Rust modules containing extern blocks and type definitions. Higher-level interfaces then belong in crates that depend on the -sys crate. This allows multiple upstream crates to depend on the same -sys crate, assuming there is a single version of the -sys crate that meets everyone’s needs.

For the full details on Cargo’s support for build scripts and linking with system libraries, see the online Cargo documentation. It shows how to avoid absolute paths in build scripts, control compilation flags, use tools like pkg-config, and so on. The git2-rs crate also provides good examples to emulate; its build script handles some complex situations.

A Raw Interface to libgit2

Figuring out how to use libgit2 properly breaks down into two questions:

What does it take to use libgit2 functions in Rust?

How can we build a safe Rust interface around them?

We’ll take these questions one at a time. In this section, we’ll write a program that’s essentially a single giant unsafe block filled with nonidiomatic Rust code, reflecting the clash of type systems and conventions that is inherent in mixing languages. We’ll call this the raw interface. The code will be messy, but it will make plain all the steps that must occur for Rust code to use libgit2.

Then, in the next section, we’ll build a safe interface to libgit2 that puts Rust’s types to use enforcing the rules libgit2 imposes on its users. Fortunately, libgit2 is an exceptionally well-designed C library, so the questions that Rust’s safety requirements force us to ask all have pretty good answers, and we can construct an idiomatic Rust interface with no unsafe functions.

The program we’ll write is very simple: it takes a path as a command-line argument, opens the Git repository there, and prints out the head commit. But this is enough to illustrate the key strategies for building safe and idiomatic Rust interfaces.

For the raw interface, the program will end up needing a somewhat larger collection of functions and types from libgit2 than we used before, so it makes sense to move the extern block into its own module. We’ll create a file named raw.rs in git-toy/src whose contents are as follows:

#![allow(non_camel_case_types)]

use std::os::raw::{c_int, c_char, c_uchar};

#[link(name = "git2")]
extern {
    pub fn git_libgit2_init() -> c_int;
    pub fn git_libgit2_shutdown() -> c_int;
    pub fn giterr_last() -> *const git_error;

    pub fn git_repository_open(out: *mut *mut git_repository,
                               path: *const c_char) -> c_int;
    pub fn git_repository_free(repo: *mut git_repository);

    pub fn git_reference_name_to_id(out: *mut git_oid,
                                    repo: *mut git_repository,
                                    reference: *const c_char) -> c_int;

    pub fn git_commit_lookup(out: *mut *mut git_commit,
                             repo: *mut git_repository,
                             id: *const git_oid) -> c_int;

    pub fn git_commit_author(commit: *const git_commit) -> *const git_signature;
    pub fn git_commit_message(commit: *const git_commit) -> *const c_char;
    pub fn git_commit_free(commit: *mut git_commit);
}

pub enum git_repository {}
pub enum git_commit {}

#[repr(C)]
pub struct git_error {
    pub message: *const c_char,
    pub klass: c_int
}

#[repr(C)]
pub struct git_oid {
    pub id: [c_uchar; 20]
}

pub type git_time_t = i64;

#[repr(C)]
pub struct git_time {
    pub time: git_time_t,
    pub offset: c_int
}

#[repr(C)]
pub struct git_signature {
    pub name: *const c_char,
    pub email: *const c_char,
    pub when: git_time
}

Each item here is modeled on a declaration from libgit2’s own header files. For example, libgit2-0.25.1/include/git2/repository.h includes this declaration:

extern int git_repository_open(git_repository **out, const char *path);

This function tries to open the Git repository at path. If all goes well, it creates a git_repository object and stores a pointer to it in the location pointed to by out. The equivalent Rust declaration is the following:

pub fn git_repository_open(out: *mut *mut git_repository,
                           path: *const c_char) -> c_int;

The libgit2 public header files define the git_repository type as a typedef for an incomplete struct type:

typedef struct git_repository git_repository;

Since the details of this type are private to the library, the public headers never define struct git_repository, ensuring that the library’s users can never build an instance of this type themselves. One possible analogue to an incomplete struct type in Rust is this:

pub enum git_repository {}

This is an enum type with no variants. There is no way in Rust to make a value of such a type. This is an oddity, but it’s perfect as the reflection of a C type that only libgit2 should ever construct, and which is manipulated solely through raw pointers.

Writing large extern blocks by hand can be a chore. If you are creating a Rust interface to a complex C library, you may want to try using the bindgen crate, which has functions you can use from your build script to parse C header files and generate the corresponding Rust declarations automatically. We don’t have space to show bindgen in action here, but bindgen’s page on crates.io includes links to its documentation.

Next we’ll rewrite main.rs completely. First, we need to declare the raw module:

mod raw;

According to libgit2’s conventions, fallible functions return an integer code that is positive or zero on success, and negative on failure. If an error occurs, the giterr_last function will return a pointer to a git_error structure providing more details about what went wrong. libgit2 owns this structure, so we don’t need to free it ourselves, but it could be overwritten by the next library call we make. A proper Rust interface would use Result, but in the raw version, we want to use the libgit2 functions just as they are, so we’ll have to roll our own function for handling errors:

use std::ffi::CStr;
use std::os::raw::c_int;

fn check(activity: &'static str, status: c_int) -> c_int {
    if status < 0 {
        unsafe {
            let error = &*raw::giterr_last();
            println!("error while {}: {} ({})",
                     activity,
                     CStr::from_ptr(error.message).to_string_lossy(),
                     error.klass);
            std::process::exit(1);
        }
    }

    status
}

We’ll use this function to check the results of libgit2 calls like this:

check("initializing library", raw::git_libgit2_init());

This uses the same CStr methods used earlier: from_ptr to construct the CStr from a C string, and to_string_lossy to turn that into something Rust can print.

Next, we need a function to print out a commit:

unsafe fn show_commit(commit: *const raw::git_commit) {
    let author = raw::git_commit_author(commit);

    let name = CStr::from_ptr((*author).name).to_string_lossy();
    let email = CStr::from_ptr((*author).email).to_string_lossy();
    println!("{} <{}>\n", name, email);

    let message = raw::git_commit_message(commit);
    println!("{}", CStr::from_ptr(message).to_string_lossy());
}

Given a pointer to a git_commit, show_commit calls git_commit_author and git_commit_message to retrieve the information it needs. These two functions follow a convention that the libgit2 documentation explains as follows:

If a function returns an object as a return value, that function is a getter and the object’s lifetime is tied to the parent object.

In Rust terms, author and message are borrowed from commit: show_commit doesn’t need to free them itself, but it must not hold on to them after commit is freed. Since this API uses raw pointers, Rust won’t check their lifetimes for us: if we do accidentally create dangling pointers, we probably won’t find out about it until the program crashes.

The preceding code assumes these fields hold UTF-8 text, which is not always correct. Git permits other encodings as well. Interpreting these strings properly would probably entail using the encoding crate. For brevity’s sake, we’ll gloss over those issues here.

Our program’s main function reads as follows:

use std::ffi::CString;
use std::mem;
use std::ptr;
use std::os::raw::c_char;

fn main() {
    let path = std::env::args().skip(1).next()
        .expect("usage: git-toy PATH");
    let path = CString::new(path)
        .expect("path contains null characters");

    unsafe {
        check("initializing library", raw::git_libgit2_init());

        let mut repo = ptr::null_mut();
        check("opening repository",
              raw::git_repository_open(&mut repo, path.as_ptr()));

        let c_name = b"HEAD\0".as_ptr() as *const c_char;
        let mut oid = mem::uninitialized();
        check("looking up HEAD",
              raw::git_reference_name_to_id(&mut oid, repo, c_name));

        let mut commit = ptr::null_mut();
        check("looking up commit",
              raw::git_commit_lookup(&mut commit, repo, &oid));

        show_commit(commit);

        raw::git_commit_free(commit);

        raw::git_repository_free(repo);

        check("shutting down library", raw::git_libgit2_shutdown());
    }
}

This starts with code to handle the path argument and initialize the library, all of which we’ve seen before. The first novel code is this:

let mut repo = ptr::null_mut();
check("opening repository",
      raw::git_repository_open(&mut repo, path.as_ptr()));

The call to git_repository_open tries to open the Git repository at the given path. If it succeeds, it allocates a new git_repository object for it, and sets repo to point to that. Rust implicitly coerces references into raw pointers, so passing &mut repo here provides the *mut *mut git_repository the call expects.

This shows another libgit2 convention in use. Again, from the libgit2 documentation:

Objects which are returned via the first argument as a pointer-to-pointer are owned by the caller and it is responsible for freeing them.

In Rust terms, functions like git_repository_open pass ownership of the new value to the caller.

Next, consider the code that looks up the object hash of the repository’s current head commit:

let mut oid = mem::uninitialized();
check("looking up HEAD",
      raw::git_reference_name_to_id(&mut oid, repo, c_name));

The git_oid type stores an object identifier—a 160-bit hash code that Git uses internally (and throughout its delightful user interface) to identify commits, individual versions of files, and so on. This call to git_reference_name_to_id looks up the object identifier of the current "HEAD" commit.

In C it’s perfectly normal to initialize a variable by passing a pointer to it to some function that fills in its value; this is how git_reference_name_to_id expects to treat its first argument. But Rust won’t let us borrow a reference to an uninitialized variable. We could initialize oid with zeros, but this is a waste: any value stored there will simply be overwritten.

Initializing oid to uninitialized() gets around this problem. The std::mem::uninitialized function returns a value of any type you like, except that the value consists entirely of uninitialized bits, and no machine code is actually spent producing the value. Rust, however, considers oid to have been assigned some value, so it lets us borrow the reference to it. As you can imagine, in the general case, this is very unsafe. Reading an uninitialized value is undefined behavior, and if any part of the value implements Drop, even dropping it is undefined behavior as well. There are only a few safe things you can do:

You can overwrite it with std::ptr::write, which requires its destination to be uninitialized.

You can pass it to std::mem::forget, which takes ownership of its argument and makes it disappear without dropping it (applying this to an initialized value might be a leak).

You can pass it to a foreign function designed to initialize it, like git_reference_name_to_id.

If the call succeeds, then oid becomes truly initialized, and all is well. If the call fails, the function doesn’t use oid, and its type doesn’t need to be dropped, so the code is safe in that case too.

We could use uninitialized for the repo and commit variables as well, but since these are just single words and uninitialized is so dicey to use, we just go ahead and initialize them to null:

let mut commit = ptr::null_mut();
check("looking up commit",
      raw::git_commit_lookup(&mut commit, repo, &oid));

This takes the commit’s object identifier and looks up the actual commit, storing a git_commit pointer in commit on success.

The remainder of the main function should be self-explanatory. It calls the show_commit function defined earlier, frees the commit and repository objects, and shuts down the library.

Now we can try out the program on any Git repository ready at hand:

$ cargo run /home/jimb/rbattle
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/git-toy /home/jimb/rbattle`
Jim Blandy <jimb@red-bean.com>

Animate goop a bit.

$

A Safe Interface to libgit2

The raw interface to libgit2 is a perfect example of an unsafe feature: it certainly can be used correctly (as we do here, so far as we know), but Rust can’t enforce the rules you must follow. Designing a safe API for a library like this is a matter of identifying all these rules, and then finding ways to turn any violation of them into a type or borrow-checking error.

Here, then, are libgit2’s rules for the features the program uses:

You must call git_libgit2_init before using any other library function. You must not use any library function after calling git_libgit2_shutdown.

All values passed to libgit2 functions must be fully initialized, except for output parameters.

When a call fails, output parameters passed to hold the results of the call are left uninitialized, and you must not use their values.

A git_commit object refers to the git_repository object it is derived from, so the former must not outlive the latter. (This isn’t spelled out in the libgit2 documentation; we inferred it from the presence of certain functions in the interface, and then verified it by reading the source code.)

Similarly, a git_signature is always borrowed from a given git_commit, and the former must not outlive the latter. (The documentation does cover this case.)

The message associated with a commit and the name and email address of the author are all borrowed from the commit, and must not be used after the commit is freed.

Once a libgit2 object has been freed, it must never be used again.

As it turns out, you can build a Rust interface to libgit2 that enforces all of these rules, either through Rust’s type system, or by managing details internally.

Before we get started, let’s restructure the project a little bit. We’d like to have a git module that exports the safe interface, of which the raw interface from the previous program is a private submodule.

The whole source tree will look like this:

git-toy/
├── Cargo.toml
├── build.rs
└── src/
    ├── main.rs
    └── git/
        ├── mod.rs
        └── raw.rs

Following the rules we explained in “Modules in Separate Files”, the source for the git module appears in git/mod.rs, and the source for its git::raw submodule goes in git/raw.rs.

Once again, we’re going to rewrite main.rs completely. It should start with a declaration of the git module:

mod git;

Then, we’ll need to create the git subdirectory, and move raw.rs into it:

$ cd /home/jimb/git-toy
$ mkdir src/git
$ mv src/raw.rs src/git/raw.rs

The git module needs to declare its raw submodule. The file src/git/mod.rs must say:

mod raw;

Since it’s not pub, this submodule is not visible to the main program.

In a bit we’ll need to use some functions from the libc crate, so we must add a dependency in Cargo.toml. The full file now reads:

[package]
name = "git-toy"
version = "0.1.0"
authors = ["Jim Blandy <jimb@red-bean.com>"]
build = "build.rs"

[dependencies]
libc = "0.2.23"

The corresponding extern crate item must appear in src/main.rs:

extern crate libc;

Now that we’ve restructured our modules, let’s consider error handling. Even libgit2’s initialization function can return an error code, so we’ll need to have this sorted out before we can get started. An idiomatic Rust interface needs its own Error type that captures the libgit2 failure code as well as the error message and class from giterr_last. A proper error type must implement the usual Error, Debug, and Display traits. Then, it needs its own Result type that uses this Error type. Here are the necessary definitions in src/git/mod.rs:

use std::error;
use std::fmt;
use std::result;

#[derive(Debug)]
pub struct Error {
    code: i32,
    message: String,
    class: i32
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> result::Result<(), fmt::Error> {
        // Displaying an `Error` simply displays the message from libgit2.
        self.message.fmt(f)
    }
}

impl error::Error for Error {
    fn description(&self) -> &str { &self.message }
}

pub type Result<T> = result::Result<T, Error>;

To check the result from raw library calls, the module needs a function that turns a libgit2 return code into a Result:

use std::os::raw::c_int;
use std::ffi::CStr;

fn check(code: c_int) -> Result<c_int> {
    if code >= 0 {
        return Ok(code);
    }

    unsafe {
        let error = raw::giterr_last();

        // libgit2 ensures that (*error).message is always non-null and null
        // terminated, so this call is safe.
        let message = CStr::from_ptr((*error).message)
            .to_string_lossy()
            .into_owned();

        Err(Error {
            code: code as i32,
            message,
            class: (*error).klass as i32
        })
    }
}

The main difference between this and the check function from the raw version is that this constructs an Error value instead of printing an error message and exiting immediately.

Now we’re ready to tackle libgit2 initialization. The safe interface will provide a Repository type that represents an open Git repository, with methods for resolving references, looking up commits, and so on. Continuing in git/mod.rs, here’s the definition of Repository:

/// A Git repository.
pub struct Repository {
    // This must always be a pointer to a live `git_repository` structure.
    // No other `Repository` may point to it.
    raw: *mut raw::git_repository
}

A Repository’s raw field is not public. Since only code in this module can access the raw::git_repository pointer, getting this module right should ensure the pointer is always used correctly.

If the only way to create a Repository is to successfully open a fresh Git repository, that will ensure that each Repository points to a distinct git_repository object:

use std::path::Path;

impl Repository {
    pub fn open<P: AsRef<Path>>(path: P) -> Result<Repository> {
        ensure_initialized();

        let path = path_to_cstring(path.as_ref())?;
        let mut repo = null_mut();
        unsafe {
            check(raw::git_repository_open(&mut repo, path.as_ptr()))?;
        }
        Ok(Repository { raw: repo })
    }
}

Since the only way to do anything with the safe interface is to start with a Repository value, and Repository::open starts with a call to ensure_initialized, we can be confident that ensure_initialized will be called before any libgit2 functions. Its definition is as follows:

use std;
use libc;

fn ensure_initialized() {
    static ONCE: std::sync::Once = std::sync::ONCE_INIT;
    ONCE.call_once(|| {
        unsafe {
            check(raw::git_libgit2_init())
                .expect("initializing libgit2 failed");
            assert_eq!(libc::atexit(shutdown), 0);
        }
    });
}

use std::io::Write;

extern fn shutdown() {
    unsafe {
        if let Err(e) = check(raw::git_libgit2_shutdown()) {
            let _ = writeln!(std::io::stderr(),
                             "shutting down libgit2 failed: {}",
                             e);
            std::process::abort();
        }
    }
}

The std::sync::Once type helps run initialization code in a thread-safe way. Only the first thread to call ONCE.call_once runs the given closure. Any subsequent calls, by this thread or any other, block until the first has completed and then return immediately, without running the closure again. Once the closure has finished, calling ONCE.call_once is cheap, requiring nothing more than an atomic load of a flag stored in ONCE.

In the preceding code, the initialization closure calls git_libgit2_init and checks the result. It punts a bit and just uses expect to make sure initialization succeeded, instead of trying to propagate errors back to the caller.

To make sure the program calls git_libgit2_shutdown, the initialization closure uses the C library’s atexit function, which takes a pointer to a function to invoke before the process exits. Rust closures cannot serve as C function pointers: a closure is a value of some anonymous type carrying the values of whatever variables it captures, or references to them; a C function pointer is just a pointer. However, Rust fn types work fine, as long as you declare them extern so that Rust knows to use the C calling conventions. The local function shutdown fits the bill, and ensures libgit2 gets shut down properly.

In “Unwinding”, we mentioned that it is undefined behavior for a panic to cross language boundaries. The call from atexit to shutdown is such a boundary, so it is essential that shutdown not panic. This is why shutdown can’t simply use .expect to handle errors reported from raw::git_libgit2_shutdown. Instead, it must report the error and terminate the process itself. POSIX forbids calling exit within an atexit handler, so shutdown calls std::process::abort to terminate the program abruptly.

It might be possible to arrange to call git_libgit2_shutdown sooner—say, when the last Repository value is dropped. But no matter how we arrange things, calling git_libgit2_shutdown must be the safe API’s responsibility. The moment it is called, any extant libgit2 objects become unsafe to use, so a safe API must not expose this function directly.

A Repository’s raw pointer must always point to a live git_repository object. This implies that the only way to close a repository is to drop the Repository value that owns it:

impl Drop for Repository {
    fn drop(&mut self) {
        unsafe {
            raw::git_repository_free(self.raw);
        }
    }
}

By calling git_repository_free only when the sole pointer to the raw::git_repository is about to go away, the Repository type also ensures the pointer will never be used after it’s freed.

The Repository::open method uses a private function called path_to_cstring, which has two definitions—one for Unix-like systems, and one for Windows:

use std::ffi::CString;

#[cfg(unix)]
fn path_to_cstring(path: &Path) -> Result<CString> {
    // The `as_bytes` method exists only on Unix-like systems.
    use std::os::unix::ffi::OsStrExt;

    Ok(CString::new(path.as_os_str().as_bytes())?)
}

#[cfg(windows)]
fn path_to_cstring(path: &Path) -> Result<CString> {
    // Try to convert to UTF-8. If this fails, libgit2 can't handle the path
    // anyway.
    match path.to_str() {
        Some(s) => Ok(CString::new(s)?),
        None => {
            let message = format!("Couldn't convert path '{}' to UTF-8",
                                  path.display());
            Err(message.into())
        }
    }
}

The libgit2 interface makes this code a little tricky. On all platforms, libgit2 accepts paths as null-terminated C strings. On Windows, libgit2 assumes these C strings hold well-formed UTF-8 and converts them internally to the 16-bit paths Windows actually requires. This usually works, but it’s not ideal. Windows permits filenames that are not well-formed Unicode, and thus cannot be represented in UTF-8. If you have such a file, it’s impossible to pass its name to libgit2.

In Rust, the proper representation of a filesystem path is a std::path::Path, carefully designed to handle any path that can appear on Windows or POSIX. This means that there are Path values on Windows that one cannot pass to libgit2, because they are not well-formed UTF-8. So although path_to_cstring’s behavior is less than ideal, it’s actually the best we can do given libgit2’s interface.

The two path_to_cstring definitions just shown rely on conversions to our Error type: the ? operator attempts such conversions, and the Windows version explicitly calls .into(). These conversions are unremarkable:

impl From<String> for Error {
    fn from(message: String) -> Error {
        Error { code: -1, message, class: 0 }
    }
}

// NulError is what `CString::new` returns if a string
// has embedded zero bytes.
impl From<std::ffi::NulError> for Error {
    fn from(e: std::ffi::NulError) -> Error {
        Error { code: -1, message: e.to_string(), class: 0 }
    }
}

Next, let’s figure out how to resolve a Git reference to an object identifier. Since an object identifier is just a 20 byte hash value, it’s perfectly fine to expose it in the safe API:

/// The identifier of some sort of object stored in the Git object
/// database: a commit, tree, blob, tag, etc. This is a wide hash of the
/// object's contents.
pub struct Oid {
    pub raw: raw::git_oid
}

We’ll add a method to Repository to perform the lookup:

use std::mem::uninitialized;
use std::os::raw::c_char;

impl Repository {
    pub fn reference_name_to_id(&self, name: &str) -> Result<Oid> {
        let name = CString::new(name)?;
        unsafe {
            let mut oid = uninitialized();
            check(raw::git_reference_name_to_id(&mut oid, self.raw,
                                                name.as_ptr() as *const c_char))?;
            Ok(Oid { raw: oid })
        }
    }
}

Although oid is left uninitialized when the lookup fails, this function guarantees that its caller can never see the uninitialized value simply by following Rust’s Result idiom: either the caller gets an Ok carrying a properly initialized Oid value, or it gets an Err.

Next, the module needs a way to retrieve commits from the repository. We’ll define a Commit type as follows:

use std::marker::PhantomData;

pub struct Commit<'repo> {
    // This must always be a pointer to a usable `git_commit` structure.
    raw: *mut raw::git_commit,
    _marker: PhantomData<&'repo Repository>
}

As we mentioned earlier, a git_commit object must never outlive the git_repository object it was retrieved from. Rust’s lifetimes let the code capture this rule precisely.

The RefWithFlag example earlier in this chapter used a PhantomData field to tell Rust to treat a type as if it contained a reference with a given lifetime, even though the type apparently contained no such reference. The Commit type needs to do something similar. In this case, the _marker field’s type is PhantomData<&'repo Repository>, indicating that Rust should treat Commit<'repo> as if it held a reference with lifetime 'repo to some Repository.

The method for looking up a commit is as follows:

use std::ptr::null_mut;

impl Repository {
    pub fn find_commit(&self, oid: &Oid) -> Result<Commit> {
        let mut commit = null_mut();
        unsafe {
            check(raw::git_commit_lookup(&mut commit, self.raw, &oid.raw))?;
        }
        Ok(Commit { raw: commit, _marker: PhantomData })
    }
}

How does this relate the Commit’s lifetime to the Repository’s? The signature of find_commit omits the lifetimes of the references involved according to the rules outlined in “Omitting Lifetime Parameters”. If we were to write the lifetimes out, the full signature would read:

fn find_commit<'repo, 'id>(&'repo self, oid: &'id Oid)
    -> Result<Commit<'repo>>

This is exactly what we want: Rust treats the returned Commit as if it borrows something from self, which is the Repository.

When a Commit is dropped, it must free its raw::git_commit:

impl<'repo> Drop for Commit<'repo> {
    fn drop(&mut self) {
        unsafe {
            raw::git_commit_free(self.raw);
        }
    }
}

From a Commit, you can borrow a Signature (a name and email address) and the text of the commit message:

impl<'repo> Commit<'repo> {
    pub fn author(&self) -> Signature {
        unsafe {
            Signature {
                raw: raw::git_commit_author(self.raw),
                _marker: PhantomData
            }
        }
    }

    pub fn message(&self) -> Option<&str> {
        unsafe {
            let message = raw::git_commit_message(self.raw);
            char_ptr_to_str(self, message)
        }
    }
}

Here’s the Signature type:

pub struct Signature<'text> {
    raw: *const raw::git_signature,
    _marker: PhantomData<&'text str>
}

A git_signature object always borrows its text from elsewhere; in particular, signatures returned by git_commit_author borrow their text from the git_commit. So our safe Signature type includes a PhantomData<&'text str> to tell Rust to behave as if it contained a &str with a lifetime of 'text. Just as before, Commit::author properly connects this 'text lifetime of the Signature it returns to that of the Commit without us needing to write a thing. The Commit::message method does the same with the Option<&str> holding the commit message.

A Signature includes methods for retrieving the author’s name and email address:

impl<'text> Signature<'text> {
    /// Return the author's name as a `&str`,
    /// or `None` if it is not well-formed UTF-8.
    pub fn name(&self) -> Option<&str> {
        unsafe {
            char_ptr_to_str(self, (*self.raw).name)
        }
    }

    /// Return the author's email as a `&str`,
    /// or `None` if it is not well-formed UTF-8.
    pub fn email(&self) -> Option<&str> {
        unsafe {
            char_ptr_to_str(self, (*self.raw).email)
        }
    }
}

The preceding methods depend on a private utility function char_ptr_to_str:

/// Try to borrow a `&str` from `ptr`, given that `ptr` may be null or
/// refer to ill-formed UTF-8. Give the result a lifetime as if it were
/// borrowed from `_owner`.
///
/// Safety: if `ptr` is non-null, it must point to a null-terminated C
/// string that is safe to access.
unsafe fn char_ptr_to_str<T>(_owner: &T, ptr: *const c_char) -> Option<&str> {
    if ptr.is_null() {
        return None;
    } else {
        CStr::from_ptr(ptr).to_str().ok()
    }
}

The _owner parameter’s value is never used, but its lifetime is. Making the lifetimes in this function’s signature explicit gives us:

fn char_ptr_to_str<'o, T: 'o>(_owner: &'o T, ptr: *const c_char)
    -> Option<&'o str>

The CStr::from_ptr function returns a &CStr whose lifetime is completely unbounded, since it was borrowed from a dereferenced raw pointer. Unbounded lifetimes are almost always inaccurate, so it’s good to constrain them as soon as possible. Including the _owner parameter causes Rust to attribute its lifetime to the return value’s type, so callers can receive a more accurately bounded reference.

It is not clear from the libgit2 documentation whether a git_signature’s email and author pointers can be null, and this despite the documentation for libgit2 being quite good. Your authors dug around in the source code for some time without being able to persuade themselves one way or the other, and finally decided that char_ptr_to_str had better be prepared for null pointers just in case. In Rust, this sort of question is answered immediately by the type: if it’s &str, you can count on the string to be there; if it’s Option<&str>, it’s optional.

Finally, we’ve provided safe interfaces for all the functionality we need. The new main function in src/main.rs is slimmed down quite a bit, and looks like real Rust code:

fn main() {
    let path = std::env::args_os().skip(1).next()
        .expect("usage: git-toy PATH");

    let repo = git::Repository::open(&path)
        .expect("opening repository");

    let commit_oid = repo.reference_name_to_id("HEAD")
        .expect("looking up 'HEAD' reference");

    let commit = repo.find_commit(&commit_oid)
        .expect("looking up commit");

    let author = commit.author();
    println!("{} <{}>\n",
             author.name().unwrap_or("(none)"),
             author.email().unwrap_or("none"));

    println!("{}", commit.message().unwrap_or("(none)"));
}

In this section, we’ve built a safe API on an unsafe API by arranging for any violation of the latter’s contract to be a Rust type error. The result is an interface that Rust can ensure you are using correctly. For the most part, the rules we’ve made Rust enforce are simply the sorts of rules that C and C++ programmers end up imposing on themselves anyway. What makes Rust feel so much stricter than C and C++ is not that the rules are so foreign, but that that enforcement is mechanical and comprehensive.

Conclusion

Rust is not a simple language. Its goal is to span two very different worlds. It’s a modern programming language, safe by design, with conveniences like closures and iterators; yet it aims to put you in control of the raw capabilities of the machine it runs on, with minimal runtime overhead.

The contours of the language are determined by these goals. Rust manages to bridge most of the gap with safe code. Its borrow checker and zero-cost abstractions put you as close to the bare metal as possible without risking undefined behavior. When that’s not enough, or when you want to leverage existing C code, unsafe code stands ready. But again, the language doesn’t just offer you these unsafe features and wish you luck. The goal is always to use unsafe features to build safe APIs. That’s what we did with libgit2. It’s also what the Rust team has done with Box, Vec, the other collections, channels, and more: the standard library is full of safe abstractions, implemented with some unsafe code behind the scenes.

A language with Rust’s ambitions was, perhaps, not destined to be the simplest of tools. But Rust is safe, fast, concurrent—and effective. Use it to build large, fast, secure, robust systems that take advantage of the full power of the hardware they run on. Use it to make software better.

¹ Well, it’s a classic where we come from.

² There are better ways to handle this using the RawVec type from the alloc crate, but that crate is still unstable.

Index

Symbols

! operator, Unary Operators

! type, Why Rust Has loop

!= operator, Equality Tests

#!, Attributes

#[cfg] attribute, Attributes, Platform-Specific Features

#[inline] attribute, Attributes

#[link] attribute, Using Functions from Libraries

$ (command prompt), Downloading and Installing Rust

& operator, Handling Command-Line Arguments, Rust References Versus C++ References

& pattern, Reference Patterns

&mut operator, Rust References Versus C++ References

&mut [T] type, Arrays, Vectors, and Slices

&str (string slice), Strings in Memory

&[T] type, Arrays, Vectors, and Slices

* operator, Handling Command-Line Arguments, Rust References Versus C++ References, Reference Operators, Generic Traits (or How Operator Overloading Works)

* wildcard, Versions

*const T, Raw Pointers

*mut T, Raw Pointers

+ operator, Binary Operators, Appending and Inserting Text

- operator, Arithmetic, Bitwise, Comparison, and Logical Operators, Unary Operators

. operator, Rust References Versus C++ References

/// (documentation comments), What the Mandelbrot Set Actually Is

<T>, Tuples

= operator, Assignment

== operator, Equality Tests

? operator, Propagating Errors

@ patterns, @ patterns

[T; N] type, Arrays, Vectors, and Slices

[t] slices, Slices

(see also slices)

| (vertical bar), Matching Multiple Possibilities

A

aborting, Aborting

absolute path, Paths and Imports

adapter methods

by_ref, by_ref

chain, chain

cloned, cloned

cycle, cycle

enumerate, enumerate

filter_map and flat_map adapters, filter_map and flat_map

fuse, fuse

inspect, inspect

iterators, Iterator Adapters

map and filter, map and filter

peekable, peekable

reversible iterators and rev, Reversible Iterators and rev

scan adapter, scan

skip and skip_while, skip and skip_while

take and take_while, take and take_while

zip, zip

ahead-of-time compilation, Basic Types

alignment, raw pointers, Type Sizes and Alignments

all method, any and all

any method, any and all

Arc pointer type, Rc and Arc: Shared Ownership

arguments

passing references as, Passing References as Arguments

referring to by index or name, Referring to Arguments by Index or Name

Arguments type, Using the Formatting Language in Your Own Code

arithmetic operators, Arithmetic, Bitwise, Comparison, and Logical Operators, Arithmetic and Bitwise Operators

binary operators, Binary Operators

compound assignment operators, Compound Assignment Operators

arrays

basics, Arrays

raw pointers, Pointer Arithmetic

slices and, Slices

tuples vs., Tuples

arrays of arrays, Joining

as operator, Integer Types, Raw Pointers

ASCII, ASCII, Latin-1, and Unicode

Ascii string type

unsafe code for conversion into String, Example: An Efficient ASCII String Type

unsafe functions, Unsafe Functions

AsMut trait, AsRef and AsMut

AsRef trait, AsRef and AsMut

assert! macro, A Simple Function

assert_eq! macro, Macros

assignment

expressions, Assignment

in Python, Moves

in Rust, Moves

moves and (see moves)

references, Assigning References

to a variable, More Operations That Move

associated functions, Defining Methods with impl

associated types, Associated Types (or How Iterators Work)

atomic types, Atomics

attributes, Writing and Running Unit Tests, Attributes

B

backpressure, Channel Features and Performance

binary input/output, Binary Data, Compression, and Serialization

binary operators, Arithmetic, Bitwise, Comparison, and Logical Operators, Binary Operators

binary trees, populating with patterns, Populating a Binary Tree

BinaryHeap<T> collection type

about, BinaryHeap<T>

defined, Overview

bitwise integer operators, Arithmetic, Bitwise, Comparison, and Logical Operators

bitwise operators

binary operators, Binary Operators

compound assignment operators, Compound Assignment Operators

blocks, Blocks and Semicolons

declarations in, Declarations

unsafe, Unsafe Blocks

Boolean type (bool), The bool Type, if and match

Borrow trait, Borrow and BorrowMut

borrowing

and iteration, Handling Command-Line Arguments

arbitrary expressions and, Borrowing References to Arbitrary Expressions

defined, Why Rust?, References

local variables and, Borrowing a Local Variable

BorrowMut trait, Borrow and BorrowMut

bounds, reverse-engineering, Reverse-Engineering Bounds

Box<T>, Ownership

boxes, Boxes

break expressions, Loops

BTreeMap<K, V> collection type

about, HashMap<K, V> and BTreeMap<K, V>

defined, Overview

entries, Entries

map iteration, Map Iteration

BTreeSet<T> collection type, HashSet<T> and BTreeSet<T>

defined, Overview

iteration, Set Iteration

methods when "equal" values are different, When Equal Values Are Different

whole-set operations, Whole-Set Operations

buddy traits, Buddy Traits (or How rand::random() Works)

buffered readers, Buffered Readers

BufRead trait, Input and Output

bugs, unsafe code and, Unsafe Functions

build script, Using Functions from Libraries

BuildHasher trait, Using a Custom Hashing Algorithm

byte literals, Integer Types

byte strings, Byte Strings

by_ref adapter, by_ref

C

C

calling from Rust (see foreign functions)

lack of type safety, Type Safety

passing strings between Rust and, Finding Common Data Representations

type representation, Finding Common Data Representations

C++

assignment in, Moves

calling from Rust (see foreign functions)

lack of type safety, Type Safety

macros, Macros, Unintended Consequences

mutexes, What Is a Mutex?

reference creation in, Rust References Versus C++ References

callbacks, closures and, Callbacks

cancellation, atomics and, Atomics

capacity of a vector, Growing and Shrinking Vectors

Cargo, Downloading and Installing Rust

build script, Using Functions from Libraries

creating new Rust package with, Downloading and Installing Rust

documentation, Documentation

src/bin directory, The src/bin Directory

versioning, Versions

cargo build, Crates

cargo doc, Documentation

cargo package, Publishing Crates to crates.io

cargo test, Tests and Documentation

Cargo.lock, Cargo.lock

case conversion

for characters, Case Conversion for Characters

for strings, Case Conversion for Strings

casts, Type Casts

Cell<T> struct, Interior Mutability

Cells, Interior Mutability

chain adapter, chain

channels, Channels

features and performance, Channel Features and Performance

multi-producer channels using mutex, Multi-producer Channels Using Mutexes

non-pipeline uses, Beyond Pipelines

optimization, Channel Features and Performance

piping iterator to, Piping Almost Any Iterator to a Channel

receiving values, Receiving Values

running pipeline, Running the Pipeline

sending values, Sending Values

thread safety with Send and Sync, Thread Safety: Send and Sync

char type, Characters, Characters (char)

character literals, Characters

characters (char), Characters (char)

case conversion, Case Conversion for Characters

classifying, Classifying Characters

digits, Handling Digits

integer conversion, Conversions to and from Integers

numeric types vs., Integer Types

child process, Other Reader and Writer Types

Clone trait, Clone

cloned adapter, cloned

closures, Closures, Closures

"that kill", Closures That Kill

borrowing references, Closures That Borrow

callbacks, Callbacks

capturing variables, Capturing Variables

defined, A Concurrent Mandelbrot Program

dropping values, Closures That Kill

effective use of, Using Closures Effectively

FnMut, FnMut

FnOnce, FnOnce

inspect adapter and, inspect

layout in memory, Closure Performance

moves with, Closures That Steal

performance, Closure Performance

safety, Closures and Safety

types, Function and Closure Types

code, unsafe (see unsafe code)

coherence rule, Traits and Other People’s Types

collections, Collections

BinaryHeap<T>, BinaryHeap<T>

BTreeMap<K, V>, HashMap<K, V> and BTreeMap<K, V>

BTreeSet<T>, HashSet<T> and BTreeSet<T>

hashing, Hashing

HashMap<K, V>, HashMap<K, V> and BTreeMap<K, V>

HashSet<T>, HashSet<T> and BTreeSet<T>

LinkedList<T>, LinkedList<T>

strings as generic collections, Strings as Generic Collections

summary of standard collections, Overview

Vec<T>, Vec<T>

VecDeque<T>, VecDeque<T>

command prompt ($), Downloading and Installing Rust

command-line arguments, Handling Command-Line Arguments, Parsing Pair Command-Line Arguments

community, Rust, More Nice Things

comparison operators, Arithmetic, Bitwise, Comparison, and Logical Operators

references and, Comparing References

with iterators, Comparing Item Sequences

with strings, Using Strings

compiler, Downloading and Installing Rust

complex numbers, Mapping from Pixels to Complex Numbers

compound assignment operators, Compound Assignment Operators

compression, Binary Data, Compression, and Serialization

concurrency/concurrent programming, Why Rust?, Concurrency

channels, Channels

fork-join parallelism, Fork-Join Parallelism

Mandelbrot set, A Concurrent Mandelbrot Program

max_by and min_by methods, max_by, min_by

Rust's support for, Concurrency, Ownership

Rust's type safety and, Type Safety

shared mutable state, Shared Mutable State

condition (with if statement), if and match

conditional variables (Condvar), Condition Variables (Condvar)

constants, Items, the Building Blocks of Rust

consuming iterators, Consuming Iterators

and all methods, any and all

collect method, Building Collections: collect and FromIterator

comparing item sequences, Comparing Item Sequences

count method, Simple Accumulation: count, sum, product

Extend trait, The Extend Trait

find method, find

fold method, fold

FromIterator trait, Building Collections: collect and FromIterator

last method, last

max_by_key and min_by_key methods, max_by_key, min_by_key

min and max methods, max, min

nth method, nth

partition method, partition

position, rposition, and ExactSizeIterator, position, rposition, and ExactSizeIterator

product method, Simple Accumulation: count, sum, product

simple accumulation, Simple Accumulation: count, sum, product

sum method, Simple Accumulation: count, sum, product

contracts

bugs and, Unsafe Functions

unsafe features and, Unsafe from What?

unsafe traits and, Unsafe Traits

Copy types, Copy Types: The Exception to Moves, Copy

count method, Simple Accumulation: count, sum, product

Cow (clone on write) type, Borrow and ToOwned at Work: The Humble Cow, Putting Off Allocation

crates, Crates

#[inline] attribute, Attributes

doc-tests, Doc-Tests

for libraries, Turning a Program into a Library

publishing to crates.io, Publishing Crates to crates.io

specifying dependencies, Specifying Dependencies

src/bin directory and, The src/bin Directory

workspaces, Workspaces

crates.io, Publishing Crates to crates.io

critical section, What Is a Mutex?

Cursor, Other Reader and Writer Types

cycle adapter, cycle

D

dangling pointer, Ownership

data races

mutexes and, What Is a Mutex?

Rust's ability to prevent, Why Rust?, Type Safety

Rust's mechanisms for avoiding, Concurrency

safety and, Safety Is Invisible

deadlock, Deadlock

Debug trait, Converting Other Types to Strings

debugging

formatting pointers for, Formatting Pointers for Debugging

formatting values for, Formatting Values for Debugging

macros, Debugging Macros

declarations, Declarations, Declaring Foreign Functions and Variables

default implementation, Default Methods

Default trait, Default

dependencies

Cargo.lock, Cargo.lock

in crate context, Crates

specifying, Specifying Dependencies

versions and, Versions

deque, VecDeque<T>

(see also VecDeque<T> collection type)

deref coercions, Type Casts

Deref trait, Deref and DerefMut

dereferencing, Handling Command-Line Arguments

* operator, Rust References Versus C++ References

raw pointers, Raw Pointers, Raw Pointers-Dereferencing Raw Pointers Safely

DerefMut trait, Deref and DerefMut

digits, handling, Handling Digits

directionality of text, Text Directionality

directories

modules and, Modules in Separate Files

reading, Reading Directories

src/bin, The src/bin Directory

Display trait, Converting Other Types to Strings

divergent function, Why Rust Has loop

doc comments, Documentation

doc-tests, Doc-Tests

documentation, Downloading and Installing Rust, Documentation

documentation comments, What the Mandelbrot Set Actually Is

drain method, drain Methods

drop trait, Drop

dropping values

and ownership, Ownership

FnOnce, FnOnce

in closures, Closures That Kill

in Rust, Ownership

E

elements, expressions and, Fields and Elements

empty statements, Blocks and Semicolons

entries, defined, HashMap<K, V> and BTreeMap<K, V>

Entry type, Entries

enumerate adapter

about, enumerate

zip and, zip

enums, Enums and Patterns, The Big Picture

C-style, Enums

defined, What the Mandelbrot Set Actually Is

generic, Generic Enums

hash implementation, Hashing

implementing rich data structures with, Rich Data Structures Using Enums

in memory, Enums in Memory

with data, Enums with Data

equality operators, Equality Tests

error handling, Error Handling

across threads, Error Handling Across Threads

catching errors, Catching Errors

channels and, Sending Values

declaring a custom error type, Declaring a Custom Error Type

errors that “can’t happen”, Dealing with Errors That “Can’t Happen”

ignoring errors, Ignoring Errors

in main(), Handling Errors in main()

panic, Panic

printing errors, Printing Errors

propagating errors, Propagating Errors

Result type, Result

Result type aliases, Result Type Aliases

Result vs. exceptions, Why Results?

unsafe code and, A Safe Interface to libgit2

with multiple error types, Working with Multiple Error Types

ExactSizeIterator trait, position, rposition, and ExactSizeIterator

exceptions, Result vs., Why Results?

exclusive (half-open) ranges, Fields and Elements, Matching Multiple Possibilities

expression language, Rust as, An Expression Language

expressions, Expressions

assignment, Assignment

blocks and semicolons, Blocks and Semicolons

closures, Closures

declarations, Declarations

fields and elements, Fields and Elements

function/method calls, Function and Method Calls

if and match, if and match

if let, if let

loops, Loops

precedence and associativity, Precedence and Associativity

reference operators, Reference Operators

return, return Expressions

Rust as expression language, An Expression Language

statements vs., An Expression Language

syntax, Precedence and Associativity

type casts, Type Casts

Extend trait, The Extend Trait

extension traits, Traits and Other People’s Types

extern blocks, Items, the Building Blocks of Rust, Declaring Foreign Functions and Variables

F

fat pointer, Slices, References to Slices and Trait Objects

fields, expressions and, Fields and Elements

files

opening for readers/writers, Files

Seek trait, Seeking

files and directories, Files and Directories

filesystem access functions, Filesystem Access Functions

OsStr and Path, OsStr and Path

Path and PathBuf methods, Path and PathBuf Methods

platform-specific features, Platform-Specific Features

reading directories, Reading Directories

filesystem access functions, Filesystem Access Functions

filter adapter, map and filter

filter_map adapter, filter_map and flat_map

find method, find

flate2 crate, Binary Data, Compression, and Serialization

flat_map adapter, filter_map and flat_map

floating-point literals, Floating-Point Types

floating-point types, Floating-Point Types

flow-sensitive analyses, Why Rust Has loop

Flux, Using Closures Effectively

fmt method, Formatting Your Own Types

fn keyword, A Simple Function, Declarations

Fn trait, FnMut

FnMut trait, FnMut

FnOnce trait, FnOnce

fold method, fold

for loop, Loops

foreign functions, Foreign Functions: Calling C and C++ from Rust

declaring foreign functions and variables, Declaring Foreign Functions and Variables

finding common data representations, Finding Common Data Representations

from libraries, Using Functions from Libraries

raw interface to libgit2, A Raw Interface to libgit2

safe interface to libgit2, A Safe Interface to libgit2

fork-join parallelism, Fork-Join Parallelism

advantages of, Fork-Join Parallelism

error handling across threads, Error Handling Across Threads

Mandelbrot set rendering, Revisiting the Mandelbrot Set

Rayon library, Rayon

sharing immutable data across threads, Sharing Immutable Data Across Threads

spawn and join, spawn and join

format parameters, Formatting Values

formatting values

dynamic widths and precisions, Dynamic Widths and Precisions

for debugging, Formatting Values for Debugging

implementing traits for, Formatting Your Own Types

numbers, Formatting Numbers

pointers for debugging, Formatting Pointers for Debugging

referring to arguments by index or name, Referring to Arguments by Index or Name

strings and text, Formatting Values

text values, Formatting Text Values

using formatting language in your own code, Using the Formatting Language in Your Own Code

various standard library types, Formatting Other Types

format_args! macro, Using the Formatting Language in Your Own Code

free functions, Defining Methods with impl

From trait, From and Into

fully qualified method calls, Fully Qualified Method Calls

functions

calling, Function and Method Calls

declaring, with foreign functions, Declaring Foreign Functions and Variables

syntax for, A Simple Function

types, Function and Closure Types

unsafe, Unsafe Functions

fuse adapter, fuse

G

GapBuffer, Example: GapBuffer

garbage collection

closures and, Capturing Variables

pointers and, Pointer Types, Ownership

generic code

IntoIterator and, IntoIterator Implementations

trait objects vs., Which to Use

generic collections, strings as, Strings as Generic Collections

generic enums, Generic Enums

generic functions, Parsing Pair Command-Line Arguments, Basic Types, Generic Functions

generic structs, What the Mandelbrot Set Actually Is, Generic Structs

generic swaps, Tuples

generic traits, Generic Traits (or How Operator Overloading Works)

generics

defined, Traits and Generics

reverse-engineering bounds, Reverse-Engineering Bounds

global variables, Global Variables

guards, Pattern Guards

H

half-open ranges, Fields and Elements, Matching Multiple Possibilities

hashing

collections and, Hashing

using a custom algorithm, Using a Custom Hashing Algorithm

HashMap<K, V> collection type

about, HashMap<K, V> and BTreeMap<K, V>

defined, Overview

entries, Entries

map iteration, Map Iteration

HashSet<T> collection type, HashSet<T> and BTreeSet<T>

defined, Overview

iteration, Set Iteration

methods when "equal" values are different, When Equal Values Are Different

whole-set operations, Whole-Set Operations

hexadecimals, Characters

hygiene, macros and, Scoping and Hygiene

I

if expressions, if and match

if let expressions, if let

image files, for Mandelbrot set, Writing Image Files

immutable references, References

impl block, Items, the Building Blocks of Rust, Defining Methods with impl

imports, Paths and Imports

Index trait, Index and IndexMut

indexed content, Moves and Indexed Content

IndexMut trait, Index and IndexMut

infinite loops, Loops

inlining, Closure Performance

input and output, Input and Output

files and directories, Files and Directories

(see also files and directories)

networking, Networking

readers and writers, Readers and Writers

(see also readers, writers)

inspect adapter, inspect

installation, Rust, Downloading and Installing Rust

integer literals, Integer Types, Literals, Variables, and Wildcards in Patterns

integer types, Integer Types

integers, converting characters to/from, Conversions to and from Integers

integration tests, Integration Tests

interior mutability

defined, Rc and Arc: Shared Ownership

structs, Interior Mutability

Into trait, From and Into

IntoIterator trait, The Iterator and IntoIterator Traits

implementations, IntoIterator Implementations

implementing for your own types, Implementing Your Own Iterators

invalidation errors, Rust Rules Out Invalidation Errors

invariants, What Is a Mutex?

inverted index, Sending Values

irrefutable patterns, Where Patterns Are Allowed

isize type, Integer Types

item declarations, Declarations

items, Items, the Building Blocks of Rust

attributes, Attributes

defined, Modules

iter method, iter and iter_mut Methods

iterable type, The Iterator and IntoIterator Traits

iterating

over a map, Map Iteration

over text, Iterating over Text

Iterator trait

implementing for your own types, Implementing Your Own Iterators

standard, Associated Types (or How Iterators Work)

iterators, Iterators

adapter methods, Iterator Adapters

and associated types, Associated Types (or How Iterators Work)

by_ref adapter, by_ref

chain adapter, chain

cloned adapter, cloned

consuming, Consuming Iterators

creating, Creating Iterators

cycle adapter, cycle

defined, Handling Command-Line Arguments

drain methods, drain Methods

enumerate adapter, enumerate

filter_map and flat_map adapters, filter_map and flat_map

fuse adapter, fuse

implementing for your own types, Implementing Your Own Iterators

in standard library, Other Iterator Sources

inspect adapter, inspect

IntoIterator implementations, IntoIterator Implementations

iter and iter_mut methods, iter and iter_mut Methods

map and filter methods, map and filter

peekable, peekable

reversible, Reversible Iterators and rev

scan adapter, scan

skip and skip_while adapters, skip and skip_while

take and take_while adapters, take and take_while

traits, The Iterator and IntoIterator Traits

zip adapter, zip

iter_mut method, iter and iter_mut Methods

J

Java, object-mutex relationship in, What Is a Mutex?

join function, spawn and join

join() method, spawn and join

json! macro, The json! Macro

fragment types, Fragment Types

importing and exporting, Importing and Exporting Macros

recursion in, Recursion in Macros

scoping and hygiene, Scoping and Hygiene

using traits with, Using Traits with Macros

L

last method, last

Latin-1 character set, ASCII, Latin-1, and Unicode

lazy_static crate, Building Regex Values Lazily

let declarations, Declarations

Li, Peng, Type Safety

libgit2, Foreign Functions: Calling C and C++ from Rust

raw interface to, A Raw Interface to libgit2

safe interface to, A Safe Interface to libgit2

libraries, Turning a Program into a Library

doc-tests, Doc-Tests

documentation, Documentation

foreign functions from, Using Functions from Libraries

src/bin directory, The src/bin Directory

third-party (see crates)

lifetime

defined, Borrowing a Local Variable

distinct parameters for references, Distinct Lifetime Parameters

omitting parameters from references, Omitting Lifetime Parameters

parameters for generic functions, Generic Functions

structs with, Structs with Lifetime Parameters

LinkedList<T> collection type

about, LinkedList<T>

defined, Overview

Linux

Rust package for, Downloading and Installing Rust

using functions from libraries, Using Functions from Libraries

literals, in patterns, Literals, Variables, and Wildcards in Patterns

lock (see mutex)

logging

channels for, Beyond Pipelines

formatting pointers for, Formatting Pointers for Debugging

formatting values for, Formatting Values for Debugging

logical operators, Arithmetic, Bitwise, Comparison, and Logical Operators

log_syntax!() macro, Debugging Macros

loop (for infinite loops), Loops

looping expressions, Loops

lvalues, Fields and Elements

M

machine language, Finding Common Data Representations

machine types, Machine Types

Boolean, The bool Type

characters, Characters

floating-point types, Floating-Point Types

integer types, Integer Types

macOS

Rust package for, Downloading and Installing Rust

using functions from libraries, Using Functions from Libraries

macros, Macros

avoiding syntax errors during matching, Avoiding Syntax Errors During Matching

basics, Macro Basics

built-in, Built-In Macros

debugging, Debugging Macros

expansion, Macros, Basics of Macro Expansion

fragment types, Fragment Types

importing and exporting, Importing and Exporting Macros

json!, The json! Macro

procedural, Beyond macro_rules!

recursion in, Recursion in Macros

repetition, Repetition

scoping and hygiene, Scoping and Hygiene

unintended consequences, Unintended Consequences

using traits with, Using Traits with Macros

macro_rules!, Macro Basics

about, Macro Basics

fragment types supported by, Fragment Types

main(), Handling Errors in main()

Mandelbrot set

basics of calculation, What the Mandelbrot Set Actually Is

concurrent implementation, Concurrency

concurrent program for, A Concurrent Mandelbrot Program

mapping from pixels to complex numbers, Mapping from Pixels to Complex Numbers

parsing pair command-line arguments, Parsing Pair Command-Line Arguments

plotting, Plotting the Set

rendering with fork-join parallelism, Revisiting the Mandelbrot Set

running the plotter, Running the Mandelbrot Plotter

writing image files, Writing Image Files

map adapter, map and filter

map types

BTreeMap<K, V>, HashMap<K, V> and BTreeMap<K, V>

HashMap<K, V>, HashMap<K, V> and BTreeMap<K, V>

map, defined, HashMap<K, V> and BTreeMap<K, V>

map.entry(key), Entries

mapping, Mapping from Pixels to Complex Numbers

match expressions, A Simple Web Server, if and match

Matsakis, Niko, Rayon

max method, max, min

max_by method, max_by, min_by

max_by_key method, max_by_key, min_by_key

memory

enums in, Enums in Memory

raw pointers and, Moving into and out of Memory

strings in, Strings in Memory

types for representing sequence of values in, Arrays, Vectors, and Slices

memory ordering, Atomics

method calls, fully qualified, Fully Qualified Method Calls

methods

calling, Function and Method Calls

defining with impl, Defining Methods with impl

fully qualified method calls, Fully Qualified Method Calls

integers and, Integer Types

min method, max, min

min_by method, max_by, min_by

min_by_key method, max_by_key, min_by_key

Model-View-Controller (MVC), Using Closures Effectively

modules, Modules

in separate files, Modules in Separate Files

items, Items, the Building Blocks of Rust

libraries and, Turning a Program into a Library

paths and imports, Paths and Imports

standard prelude, The Standard Prelude

Morris worm, Why Rust?, Type Safety

moves, Moves

and control flow, Moves and Control Flow

and indexed content, Moves and Indexed Content

assigning to a variable, More Operations That Move

closures and, Closures That Steal

constructing new values, More Operations That Move

Copy types as exception to, Copy Types: The Exception to Moves

defined, Why Rust?

passing values to a function, More Operations That Move

returning values to a function, More Operations That Move

Mozilla, Why Rust?

mpsc (multi-producer, single-consumer) communication, Channel Features and Performance

multiplication (mul), Generic Traits (or How Operator Overloading Works)

multithreaded programming, Why Rust?

(see also concurrency, concurrent programming)

type safety and, Type Safety

mut (exclusive access), A Simple Function, mut and Mutex

mut statics, Items, the Building Blocks of Rust

mutability, interior, Interior Mutability

mutable references (&mutT), References

FnMut, FnMut

IntoIterator implementation, IntoIterator Implementations

rules for, Sharing Versus Mutation

shared references vs., Sharing Versus Mutation

mutable statics, Receiving References as Parameters

mutexes

basics, What Is a Mutex?

creating, Mutex<T>

deadlocks and, Deadlock

in Rust, Mutex<T>

limitations, Why Mutexes Are Not Always a Good Idea

multi-producer channels using, Multi-producer Channels Using Mutexes

mut and, mut and Mutex

poisoned, Poisoned Mutexes

N

named-field structs, Named-Field Structs

namespaces (see modules)

NaN (not-a-number) values, Equality Tests

networking, Networking

newtypes, Tuple-Like Structs

normalization

forms, Normalization Forms

strings and text, Normalization

unicode-normalization crate, The unicode-normalization Crate

not-a-number (NaN) values, Equality Tests

nth method, nth

null pointers, References Are Never Null

null raw pointers, Nullable Pointers

null references, References

numbers, formatting, Formatting Numbers

numeric types, Machine Types

floating-point types, Floating-Point Types

integer types, Integer Types

O

offset method, Unsafe from What?, Raw Pointers, Dereferencing Raw Pointers Safely, Pointer Arithmetic

operator overloading, Operator Overloading

arithmetic/bitwise operators, Arithmetic and Bitwise Operators

binary operators, Binary Operators

compound assignment operators, Compound Assignment Operators

equality tests, Equality Tests

generic traits and, Generic Traits (or How Operator Overloading Works)

Index and IndexMut, Index and IndexMut

limitations on, Other Operators

ordered comparisons, Ordered Comparisons

unary operators, Unary Operators

operator precedence, Precedence and Associativity

operators

arithmetic, Arithmetic, Bitwise, Comparison, and Logical Operators

bitwise, Arithmetic, Bitwise, Comparison, and Logical Operators

comparison, Arithmetic, Bitwise, Comparison, and Logical Operators

logical, Arithmetic, Bitwise, Comparison, and Logical Operators

Option<&T>, References Are Never Null

ordered comparison operators, Ordered Comparisons

OsStr string type, OsStr and Path

ownership, Ownership

and iteration, Handling Command-Line Arguments

Arc, Rc and Arc: Shared Ownership

basics, Ownership

Cow, Borrow and ToOwned at Work: The Humble Cow

defined, Why Rust?

moves, Moves

Rc, Rc and Arc: Shared Ownership

shared, Rc and Arc: Shared Ownership

P

panic, Panic

aborting, Aborting

poisoned mutexes, Poisoned Mutexes

safety in unsafe code, Panic Safety in Unsafe Code

unwinding, Unwinding

panic!() macro, Panic

parameters

dynamic widths and precisions, Dynamic Widths and Precisions

receiving references as, Receiving References as Parameters

PartialEq trait, Equality Tests

PartialOrd trait, Ordered Comparisons

partition method, partition

passing by value/by reference, References, IntoIterator Implementations

Path, OsStr and Path

Path method, Path and PathBuf Methods

PathBuf method, Path and PathBuf Methods

paths, Paths and Imports

patterns, Patterns

@ patterns, @ patterns

about, Patterns

avoiding syntax errors during matching in macros, Avoiding Syntax Errors During Matching

for searching text, Patterns for Searching Text

guards, Pattern Guards

literals in, Literals, Variables, and Wildcards in Patterns

match expressions and, if and match

matching multiple possibilities with |, Matching Multiple Possibilities

populating a binary tree, Populating a Binary Tree

reference patterns, Reference Patterns

searching and replacing, Searching and Replacing

situations that allow, Where Patterns Are Allowed

struct patterns, Tuple and Struct Patterns

tuple patterns, Tuple and Struct Patterns

variables in, Literals, Variables, and Wildcards in Patterns

wildcards in, Literals, Variables, and Wildcards in Patterns

peekable iterator, peekable

pipeline

limitations, Running the Pipeline

running, Running the Pipeline

plotting

Mandelbrot set, Plotting the Set

running the Mandelbrot plotter, Running the Mandelbrot Plotter

pointer types, Pointer Types

boxes, Boxes

non-owning (see references)

raw pointers, Raw Pointers

references, References (see references)

pointers, Rust's restrictions on, Ownership

polymorphism, Traits and Generics

position method, position, rposition, and ExactSizeIterator

print!(), Writers

println!(), Printing Errors

procedural macros, Beyond macro_rules!

product method, Simple Accumulation: count, sum, product

profiler, Build Profiles

propagating errors, Propagating Errors

Python, Type Safety

R

race conditions, Why Mutexes Are Not Always a Good Idea

rand::random(), Buddy Traits (or How rand::random() Works)

ranges

half-open, Fields and Elements, Matching Multiple Possibilities

in patterns, Matching Multiple Possibilities

raw interface, A Raw Interface to libgit2

raw pointers, Raw Pointers, Raw Pointers

and unsafe code, Type Safety

dereferencing safely, Dereferencing Raw Pointers Safely

GapBuffer example, Example: GapBuffer

moving into/out of memory, Moving into and out of Memory

nullable, Nullable Pointers

panic safety in unsafe code, Panic Safety in Unsafe Code

pointer arithmetic, Pointer Arithmetic

RefWithFlag, Example: RefWithFlag

type sizes and alignments, Type Sizes and Alignments

raw strings, String Literals

Rayon library, Rayon

Rc pointer type, Rc and Arc: Shared Ownership

Read trait, Input and Output

read-only access, shared access as, Sharing Versus Mutation

read/write locks (RwLock), Read/Write Locks (RwLock<T>)

readers

basics, Readers

binary data, compression, serialization, Binary Data, Compression, and Serialization

buffered, Buffered Readers

collecting lines, Collecting Lines

defined, Readers and Writers

opening files, Files

reading lines, Reading Lines

Seek trait, Seeking

various types, Other Reader and Writer Types

recursion, Recursion in Macros

RefCell<T> struct, Interior Mutability

reference (ref) patterns, Reference Patterns

reference operators, Reference Operators

reference-counted pointer type, Rc and Arc: Shared Ownership

references (pointer type), References, References

"sea of objects" and, Taking Arms Against a Sea of Objects

and iteration, Handling Command-Line Arguments

as values, References as Values

assigning, Assigning References

borrowing, Closures That Borrow

borrowing to arbitrary expressions, Borrowing References to Arbitrary Expressions

comparing, Comparing References

constraints on borrowing local variables, Borrowing a Local Variable

constraints on passing references as arguments, Passing References as Arguments

constraints on receiving as parameters, Receiving References as Parameters

constraints on returning, Returning References

constraints on structs containing, Structs Containing References

need for distinct lifetime parameters, Distinct Lifetime Parameters

null pointers and, References Are Never Null

omitting lifetime parameters from, Omitting Lifetime Parameters

Rust vs. C++, Rust References Versus C++ References

safety, Reference Safety

shared vs. mutable, References, Sharing Versus Mutation

to references, References to References

to slices and trait objects, References to Slices and Trait Objects

refutable patterns, Where Patterns Are Allowed

RefWithFlag<'a, T>, Example: RefWithFlag

regular expressions (regex), Regular Expressions

basic use, Basic Regex Use

building regex values lazily, Building Regex Values Lazily

resource-constrained programming, Preface

Result type, Result

catching errors, Catching Errors

dealing with errors that “can’t happen”, Dealing with Errors That “Can’t Happen”

declaring a custom error type, Declaring a Custom Error Type

handling errors in main(), Handling Errors in main()

ignoring errors, Ignoring Errors

key points of design, Why Results?

printing errors, Printing Errors

propagating errors, Propagating Errors

type aliases, Result Type Aliases

with multiple error types, Working with Multiple Error Types

Result value, Handling Command-Line Arguments

return expressions, return Expressions

rev adapter, Reversible Iterators and rev

reversible iterators, Reversible Iterators and rev

Rhs type parameter, Equality Tests

Rng value, Buddy Traits (or How rand::random() Works)

routers, callbacks and, Callbacks

rposition method, position, rposition, and ExactSizeIterator

Rust (generally)

basics, A Tour of Rust

command-line arguments, Handling Command-Line Arguments

community, More Nice Things

concurrency, Concurrency

installation, Downloading and Installing Rust

reasons for using, Why Rust?

rules for well-behaved program, Undefined Behavior

simple function in, A Simple Function

simple web server, A Simple Web Server

type safety, Type Safety

unit testing in, Writing and Running Unit Tests

rustc, Downloading and Installing Rust, Built-In Macros, Debugging Macros

rustdoc, Downloading and Installing Rust

rustup, Downloading and Installing Rust

RwLock method, Read/Write Locks (RwLock<T>)

S

safe interface, A Safe Interface to libgit2

safety

closures and, Closures and Safety

invisibility of, Safety Is Invisible

type safe language, Type Safety

with references, Reference Safety

safety rules, unsafe code as escape from, Type Safety

scan adapter, scan

scoping, Scoping and Hygiene

search

conventions for searching/iterating text, Conventions for Searching and Iterating

patterns for searching text, Patterns for Searching Text

Seek trait, Seeking

Self keyword, Self in Traits

semicolons, Blocks and Semicolons

SEMVER variable, Building Regex Values Lazily

Send type, Thread Safety: Send and Sync, Unsafe Traits

serde library/crate, Traits and Other People’s Types, Buddy Traits (or How rand::random() Works), Binary Data, Compression, and Serialization

serialization, Binary Data, Compression, and Serialization

Serializer trait, Buddy Traits (or How rand::random() Works)

Servo, Why Rust?

sets, defined, HashSet<T> and BTreeSet<T>

shared access, Sharing Versus Mutation

shared mutable state, Shared Mutable State

atomics, Atomics

conditional variables (Condvar), Condition Variables (Condvar)

deadlock, Deadlock

global variables, Global Variables

multi-producer channels using mutex, Multi-producer Channels Using Mutexes

mut and mutex, mut and Mutex

mutex basics, What Is a Mutex?

mutex in Rust, Mutex<T>

mutex limitations, Why Mutexes Are Not Always a Good Idea

poisoned mutexes, Poisoned Mutexes

read/write locks (RwLock), Read/Write Locks (RwLock<T>)

shared references, References

C's pointers to const values vs., Sharing Versus Mutation

IntoIterator implementation, IntoIterator Implementations

mutable references vs., Sharing Versus Mutation

rules for, Sharing Versus Mutation, Sharing Versus Mutation

sized type, Sized

Sized type, Type Sizes and Alignments

size_hint method, by_ref, Building Collections: collect and FromIterator

skip adapter, skip and skip_while

skip_while adapter, skip and skip_while

slices, Slices

comparing, Comparing Slices

IntoIterator implementation, IntoIterator Implementations

references to, References to Slices and Trait Objects

searching, Sorting and Searching

sorting, Sorting and Searching

spawn function, spawn and join, Sharing Immutable Data Across Threads

src/bin directory, The src/bin Directory

stack unwinding, Unwinding

standard prelude, The Standard Prelude, Using Traits

statements, expressions vs., An Expression Language

static (value), Receiving References as Parameters, Items, the Building Blocks of Rust, Global Variables

static keyword, Items, the Building Blocks of Rust

static methods, Defining Methods with impl

static typing, Basic Types

string literals, String Literals

byte strings, Byte Strings

defined, Strings in Memory

string slice (&str), Strings in Memory

String types, String Types, String and str

accessing text as UTF-8, Accessing Text as UTF-8

adding text to, Appending and Inserting Text

as generic collections, Strings as Generic Collections

Ascii, Example: An Efficient ASCII String Type

borrowing slice's content, Borrowing as Other Text-Like Types

byte strings, Byte Strings

case conversion for, Case Conversion for Strings

conventions for searching/iterating text, Conventions for Searching and Iterating

converting nontextual values to, Converting Other Types to Strings

creating String values, Creating String Values

iterating over text, Iterating over Text

non-Unicode strings, Other String-Like Types

parsing values from, Parsing Other Types from Strings

patterns for searching text, Patterns for Searching Text

producing text from UTF-8 data, Producing Text from UTF-8 Data

putting off allocation, Putting Off Allocation

removing text from, Removing Text

searching and replacing patterns/text, Searching and Replacing

simple inspection, Simple Inspection

string literals, String Literals

strings in memory, Strings in Memory

trimming text, Trimming

using, Using Strings

UTF-8 and, Characters

strings and text, Strings and Text

characters (char), Characters (char)

formatting numbers, Formatting Numbers

formatting text values, Formatting Text Values

formatting values, Formatting Values

in memory, Strings in Memory

normalization, Normalization

passing between Rust and C, Finding Common Data Representations

regular expressions, Regular Expressions

String and str types, String and str

(see also String type)

Unicode, Some Unicode Background

Stroustrup, Bjarne, Why Rust?

struct expression, Named-Field Structs

struct patterns, Tuple and Struct Patterns

structs, Structs

defining methods with impl, Defining Methods with impl

deriving common traits for struct types, Deriving Common Traits for Struct Types

generic, Generic Structs

hash implementation, Hashing

implementing with enums, Rich Data Structures Using Enums

interior mutability, Interior Mutability

layout, Struct Layout

named-field, Named-Field Structs

references in, Structs Containing References

tuple-like, Tuple-Like Structs

unit-like, Unit-Like Structs

with lifetime parameters, Structs with Lifetime Parameters

submodules, Paths and Imports

subtraits, Subtraits

sum method, Simple Accumulation: count, sum, product

Sync type, Thread Safety: Send and Sync, Unsafe Traits

synchronous channel, Channel Features and Performance

syntax errors, macros and, Avoiding Syntax Errors During Matching

systems programming, Preface, Why Rust?

T

take adapter, take and take_while

take_while adapter, take and take_while

tests, Tests and Documentation

doc-tests, Doc-Tests

integration tests, Integration Tests

text

accessing as UTF-8, Accessing Text as UTF-8

adding to String, Appending and Inserting Text

(see also strings and text)

case conversion for, Case Conversion for Strings

conventions for searching/iterating, Conventions for Searching and Iterating

GapBuffer, Example: GapBuffer

iterating over, Iterating over Text

patterns for searching, Patterns for Searching Text

producing from UTF-8 data, Producing Text from UTF-8 Data

removing from String, Removing Text

searching and replacing, Searching and Replacing

trimming, Trimming

text directionality, Text Directionality

text values, Formatting Text Values

threads

and channels, Channels

deadlock, Deadlock

thread safety with Send and Sync, Thread Safety: Send and Sync

token tree, Fragment Types

tokens, Basics of Macro Expansion

ToOwned trait, ToOwned

trace_macros!(), Debugging Macros

trait objects, Trait Objects

about, Trait Objects

defined, Trait Objects

generic code vs., Which to Use

layout, Trait Object Layout

references to, References to Slices and Trait Objects

unsized types and, Sized

traits

and other people's types, Traits and Other People’s Types

buddy, Buddy Traits (or How rand::random() Works)

default methods for defining/implementing, Default Methods

defined, Handling Command-Line Arguments, Traits and Generics

defining and implementing, Defining and Implementing Traits

for defining relationships between types, Traits That Define Relationships Between Types

for operator overloading, Operator Overloading

for struct types, Deriving Common Traits for Struct Types

fully qualified method calls and, Fully Qualified Method Calls

generic, Generic Traits (or How Operator Overloading Works)

Iterator/IntoIterator, The Iterator and IntoIterator Traits

iterators and associated types, Associated Types (or How Iterators Work)

reverse-engineering bounds, Reverse-Engineering Bounds

Self as type, Self in Traits

static methods and, Static Methods

subtraits, Subtraits

unsafe, Unsafe Traits

using, Using Traits

utility (see utility traits)

with macros, Using Traits with Macros

Zeroable, Unsafe Traits

Travis CI, More Nice Things

trees, Ownership

trimming, Trimming

tuple patterns, Tuple and Struct Patterns

tuple-like structs, Tuple-Like Structs

tuples, Tuples, Fields and Elements

type aliases, Result Type Aliases, Items, the Building Blocks of Rust

type alignment, raw pointers and, Type Sizes and Alignments

type inference, Basic Types

type parameters, Parsing Pair Command-Line Arguments, Generic Structs, Generic Functions

type safety, Type Safety

type size, raw pointers and, Type Sizes and Alignments

type-safe language, Type Safety

types, Basic Types

and operator overloading, Operator Overloading

arrays, Arrays

casts and, Type Casts

closures and, Function and Closure Types

Copy types, Copy Types: The Exception to Moves

for representing sequence of values in memory, Arrays, Vectors, and Slices

goals of, Basic Types

IntoIterator implementations, IntoIterator Implementations

iterators and associated types, Associated Types (or How Iterators Work)

machine, Machine Types

pointer types, Pointer Types

Result type, Result

separating methods from definition, Defining Methods with impl

sized type, Sized

slices, Slices

traits for adding methods to, Traits and Other People’s Types

traits for defining relationships between, Traits That Define Relationships Between Types

tuples, Tuples

user-defined, Items, the Building Blocks of Rust

vectors, Vectors

U

unary operators, Unary Operators

undefined behavior, Type Safety, Unsafe from What?, Undefined Behavior

Unicode, Some Unicode Background

ASCII and, ASCII, Latin-1, and Unicode

character literals, Characters

Latin-1 and, ASCII, Latin-1, and Unicode

normalized forms, Normalization Forms

OsStr and, OsStr and Path

text directionality, Text Directionality

unicode-normalization crate, The unicode-normalization Crate

UTF-8, UTF-8

unicode-normalization crate, The unicode-normalization Crate

unit testing, Writing and Running Unit Tests

unit-like structs, Unit-Like Structs

Unix

backpressure, Channel Features and Performance

pipes, Channels

symlink function, Platform-Specific Features

unsafe blocks, Unsafe Blocks

Ascii string type conversion, Example: An Efficient ASCII String Type

dereferencing raw pointers with, Raw Pointers

unsafe functions vs., Unsafe Block or Unsafe Function?

unsafe code, Type Safety, Unsafe Code

foreign functions, Foreign Functions: Calling C and C++ from Rust (see foreign functions)

incorrect use of, Unsafe from What?

panic safety in, Panic Safety in Unsafe Code

raw pointers, Raw Pointers

(see also raw pointers)

undefined behavior, Undefined Behavior

unsafe blocks, Unsafe Blocks

unsafe blocks vs. unsafe functions, Unsafe Block or Unsafe Function?

unsafe functions, Unsafe Functions

unsafe traits, Unsafe Traits

unsafe functions, Unsafe Functions

unsafe traits, Unsafe Traits

unsized types, Sized

unwinding, Unwinding

user-defined types, Items, the Building Blocks of Rust

usize type, Integer Types

UTF-8, UTF-8

accessing text as, Accessing Text as UTF-8

char type and, Characters

OsStr and, OsStr and Path

producing text from data, Producing Text from UTF-8 Data

strings in memory, Strings in Memory

utility traits, Utility Traits

AsRef and AsMut, AsRef and AsMut

Borrow and BorrowMut, Borrow and BorrowMut

Clone, Clone

Copy, Copy

Cow, Borrow and ToOwned at Work: The Humble Cow

Default, Default

Deref and DerefMut, Deref and DerefMut

drop, Drop

From and Into, From and Into

sized, Sized

ToOwned, ToOwned

V

values

parsing from strings, Parsing Other Types from Strings

passing by value/by reference, References

receiving via channels, Receiving Values

references as, References as Values

sending via channels, Sending Values

variable capture, Capturing Variables

variables

assigning to, More Operations That Move

declaring, with foreign functions, Declaring Foreign Functions and Variables

global, Global Variables

in patterns, Literals, Variables, and Wildcards in Patterns

ownership (see ownership)

vec! macro, Repetition

Vec<T> collection type, Arrays, Vectors, and Slices, Vectors, Vec<T>

accessing elements, Accessing Elements

comparing slices, Comparing Slices

defined, Overview

growing/shrinking vectors, Growing and Shrinking Vectors

invalidation errors, Rust Rules Out Invalidation Errors

iteration, Iteration

joining, Joining

random elements, Random Elements

searching, Sorting and Searching

sorting, Sorting and Searching

splitting, Splitting

swapping, Swapping

VecDeque<T> collection type

about, VecDeque<T>

defined, Overview

LinkedList<T> vs., LinkedList<T>

vectors, Vectors

basics, Vectors

building element by element, Building Vectors Element by Element

slices and, Slices

versions, Versions

vertical bar (|), Matching Multiple Possibilities

virtual table (vtable), Trait Object Layout

W

weak pointers, Rc and Arc: Shared Ownership

web server, creating with Rust, A Simple Web Server

well-behaved program, Undefined Behavior

well-defined program, Type Safety

while let loop, Loops

while loop, Loops

wildcards, Versions, Literals, Variables, and Wildcards in Patterns

Windows

Rust package for, Downloading and Installing Rust

using functions from libraries, Using Functions from Libraries

work-stealing, Rayon

workspaces, Workspaces

Write trait, Input and Output, Writers

writers, Writers

defined, Readers and Writers

Seek trait, Seeking

various types, Other Reader and Writer Types

X

x @ pattern, @ patterns

Z

zero-overhead principle, Why Rust?

Zeroable trait, Unsafe Traits

zip adapter, zip

About the Authors

Jim Blandy has been programming since 1981, and writing free software since 1990. He has been the maintainer of GNU Emacs and GNU Guile, and a maintainer of GDB, the GNU Debugger. He is one of the original designers of the Subversion version control system. Jim now works on Firefox’s web developer tools for Mozilla.

Jason Orendorff hacks C++ for Mozilla, where he is module owner of the JavaScript engine that’s in Firefox. He is an active member of the Nashville developer community and an occasional organizer of homegrown tech events. He is interested in grammar, baking, time travel, and helping people learn about complicated topics.

Colophon

The animal on the cover of Programming Rust is a Montagu’s crab (Xantho hydrophilus).

This robust-looking crab has a muscly appearance with a broad carapace about 70 mm wide. The edge of the carapace is furrowed and the color is yellowish or reddish-brown. It has 10 legs (five pairs): the front pair of legs (the chelipeds) are equal in size with black-tipped claws or pincers; then there are three pairs of walking legs that are stout and relatively short; and the last pair of legs are for swimming. They walk and swim sideways.

Montagu’s crab has been found in the northeastern Atlantic ocean and in the Mediterranean Sea. It lives under rocks and boulders during low tide. If one is exposed when a rock has been lifted, it will aggressively hold its pincers up and spread them wide open to make itself appear bigger.

This crab eats algae, snails, and crabs of other species. It is mostly active at night. Egg-bearing females are found from March through July and the larvae are present on plankton for most of the summer.

Many of the animals on O’Reilly covers are endangered; all of them are important to the world. To learn more about how you can help, go to animals.oreilly.com.

The cover image is from Wood’s Natural History. The cover fonts are URW Typewriter and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag’s Ubuntu Mono.