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## About the Authors

Carol Nichols is a member of the Rust Crates.io Team and a former member of the
Rust Core Team. She’s a co-founder of Integer 32, LLC, the world’s first
Rust-focused software consultancy. Nichols has also organized the Rust Belt
Rust Conference.

Steve Klabnik was the lead for the Rust documentation team and was one of
Rust’s core developers. A frequent speaker and a prolific open source
contributor, he previously worked on projects such as Ruby and Ruby on Rails.

## About the Technical Reviewer

JT is a Rust core team member and the co-creator of the Rust error message
format, Rust Language Server (RLS), and Nushell. They first started using Rust
in 2011, and in 2016 joined Mozilla to work on Rust full time, helping to shape
its direction for widespread use. These days, they are a freelance Rust trainer
and advocate for safe systems programming.

## Brief Contents

## Contents in Detail

## Foreword

It wasn’t always so clear, but the Rust programming language is fundamentally
about *empowerment*: no matter what kind of code you are writing now, Rust
empowers you to reach further, to program with confidence in a wider variety of
domains than you did before.

Take, for example, “systems-level” work that deals with low-level details of
memory management, data representation, and concurrency. Traditionally, this
realm of programming is seen as arcane, accessible to only a select few who
have devoted the necessary years learning it to avoid its infamous pitfalls.
And even those who practice it do so with caution, lest their code be open to
exploits, crashes, or corruption.

Rust breaks down these barriers by eliminating the old pitfalls and providing a
friendly, polished set of tools to help you along the way. Programmers who need
to “dip down” into lower-level control can do so with Rust, without taking on
the customary risk of crashes or security holes and without having to learn the
fine points of a fickle toolchain. Better yet, the language is designed to
guide you naturally toward reliable code that is efficient in terms of speed
and memory usage.

Programmers who are already working with low-level code can use Rust to raise
their ambitions. For example, introducing parallelism in Rust is a relatively
low-risk operation: the compiler will catch the classical mistakes for you. And
you can tackle more aggressive optimizations in your code with the confidence
that you won’t accidentally introduce crashes or vulnerabilities.

But Rust isn’t limited to low-level systems programming. It’s expressive and
ergonomic enough to make CLI apps, web servers, and many other kinds of code
quite pleasant to write—you’ll find simple examples later in the book. Working
with Rust allows you to build skills that transfer from one domain to another;
you can learn Rust by writing a web app, then apply those same skills to target
your Raspberry Pi.

This book fully embraces the potential of Rust to empower its users. It’s a
friendly and approachable text intended to help you level up not just your
knowledge of Rust, but also your reach and confidence as a programmer in
general. So dive in, get ready to learn—and welcome to the Rust community!

Nicholas Matsakis and Aaron Turon

## ACKNOWLEDGMENTS

We would like to thank everyone who has worked on the Rust language for
creating an amazing language worth writing a book about. We’re grateful to
everyone in the Rust community for being welcoming and creating an environment
worth welcoming more folks into.

We’re especially thankful for everyone who read early versions of this book
online and provided feedback, bug reports, and pull requests. Special thanks to
Eduard-Mihai Burtescu, Alex Crichton, and JT for providing technical review,
and to Karen Rustad Tölva for the cover art. Thank you to our team at No
Starch, including Bill Pollock, Liz Chadwick, and Janelle Ludowise, for
improving this book and bringing it to print.

Carol is grateful for the opportunity to work on this book. She thanks her
family for their constant love and support, especially her husband, Jake
Goulding, and her daughter, Vivian.

## Preface

This version of the text assumes you’re using Rust 1.62.0 (released 2022-06-30)
or later with `edition="2021"` in the *Cargo.toml* file of all projects to
configure them to use Rust 2021 edition idioms. See “Installation” on page XX
for instructions on installing or updating Rust, and see Appendix E for
information on editions.

The 2021 edition of the Rust language includes a number of improvements that
make Rust more ergonomic and that correct some inconsistencies. On top of a
general update to reflect these improvements, this rendition of the book has a
number of improvements to address specific feedback:

* Chapter 7 contains a new quick reference section on organizing your code into
multiple files with modules.
* Chapter 13 has new and improved closure examples that more clearly illustrate
captures, the `move` keyword, and the `Fn` traits.
* We fixed a number of small errors and imprecise wording throughout the book.
Thank you to the readers who reported them!

Note that any code from earlier renditions of this book that compiled will
continue to compile with the relevant edition in the project’s *Cargo.toml*,
even as you update the Rust compiler version you’re using. That’s Rust’s
backward-compatibility guarantees at work!

## Introduction

Welcome to *The Rust Programming Language*, an introductory book about Rust.
The Rust programming language helps you write faster, more reliable software.
High-level ergonomics and low-level control are often at odds in programming
language design; Rust challenges that conflict. Through balancing powerful
technical capacity and a great developer experience, Rust gives you the option
to control low-level details (such as memory usage) without all the hassle
traditionally associated with such control.

## Who Rust Is For

Rust is ideal for many people for a variety of reasons. Let’s look at a few of
the most important groups.

### Teams of Developers

Rust is proving to be a productive tool for collaborating among large teams of
developers with varying levels of systems programming knowledge. Low-level code
is prone to various subtle bugs, which in most other languages can only be
caught through extensive testing and careful code review by experienced
developers. In Rust, the compiler plays a gatekeeper role by refusing to
compile code with these elusive bugs, including concurrency bugs. By working
alongside the compiler, the team can spend their time focusing on the program’s
logic rather than chasing down bugs.

Rust also brings contemporary developer tools to the systems programming world:

* Cargo, the included dependency manager and build tool, makes adding,
compiling, and managing dependencies painless and consistent across the Rust
ecosystem.
* The `rustfmt` formatting tool ensures a consistent coding style across
developers.
* The Rust Language Server powers integrated development environment (IDE)
integration for code completion and inline error messages.

By using these and other tools in the Rust ecosystem, developers can be
productive while writing systems-level code.

### Students

Rust is for students and those who are interested in learning about systems
concepts. Using Rust, many people have learned about topics like operating
systems development. The community is very welcoming and happy to answer
students’ questions. Through efforts such as this book, the Rust teams want to
make systems concepts more accessible to more people, especially those new to
programming.

### Companies

Hundreds of companies, large and small, use Rust in production for a variety of
tasks, including command line tools, web services, DevOps tooling, embedded
devices, audio and video analysis and transcoding, cryptocurrencies,
bioinformatics, search engines, Internet of Things applications, machine
learning, and even major parts of the Firefox web browser.

### Open Source Developers

Rust is for people who want to build the Rust programming language, community,
developer tools, and libraries. We’d love to have you contribute to the Rust
language.

### People Who Value Speed and Stability

Rust is for people who crave speed and stability in a language. By speed, we
mean both how quickly Rust code can run and the speed at which Rust lets you
write programs. The Rust compiler’s checks ensure stability through feature
additions and refactoring. This is in contrast to the brittle legacy code in
languages without these checks, which developers are often afraid to modify. By
striving for zero-cost abstractions, higher-level features that compile to
lower-level code as fast as code written manually, Rust endeavors to make safe
code be fast code as well.

The Rust language hopes to support many other users as well; those mentioned
here are merely some of the biggest stakeholders. Overall, Rust’s greatest
ambition is to eliminate the trade-offs that programmers have accepted for
decades by providing safety *and* productivity, speed *and* ergonomics. Give
Rust a try and see if its choices work for you.

## Who This Book Is For

This book assumes that you’ve written code in another programming language, but
doesn’t make any assumptions about which one. We’ve tried to make the material
broadly accessible to those from a wide variety of programming backgrounds. We
don’t spend a lot of time talking about what programming *is* or how to think
about it. If you’re entirely new to programming, you would be better served by
reading a book that specifically provides an introduction to programming.

## How to Use This Book

In general, this book assumes that you’re reading it in sequence from front to
back. Later chapters build on concepts in earlier chapters, and earlier
chapters might not delve into details on a particular topic but will revisit
the topic in a later chapter.

You’ll find two kinds of chapters in this book: concept chapters and project
chapters. In concept chapters, you’ll learn about an aspect of Rust. In project
chapters, we’ll build small programs together, applying what you’ve learned so
far. Chapter 2, Chapter 12, and Chapter 20 are project chapters; the rest are
concept chapters.

**Chapter 1** explains how to install Rust, how to write a “Hello, world!”
program, and how to use Cargo, Rust’s package manager and build tool. **Chapter
2** is a hands-on introduction to writing a program in Rust, having you build
up a number-guessing game. Here, we cover concepts at a high level, and later
chapters will provide additional detail. If you want to get your hands dirty
right away, Chapter 2 is the place for that. **Chapter 3** covers Rust features
that are similar to those of other programming languages, and in **Chapter 4**
you’ll learn about Rust’s ownership system. If you’re a particularly meticulous
learner who prefers to learn every detail before moving on to the next, you
might want to skip Chapter 2 and go straight to Chapter 3, returning to Chapter
2 when you’d like to work on a project applying the details you’ve learned.

**Chapter 5** discusses structs and methods, and **Chapter 6** covers enums,
`match` expressions, and the `if let` control flow construct. You’ll use
structs and enums to make custom types in Rust.

In **Chapter 7**, you’ll learn about Rust’s module system and about privacy
rules for organizing your code and its public application programming interface
(API). **Chapter 8** discusses some common collection data structures that the
standard library provides, such as vectors, strings, and hash maps. **Chapter
9** explores Rust’s error-handling philosophy and techniques.

**Chapter 10** digs into generics, traits, and lifetimes, which give you the
power to define code that applies to multiple types. **Chapter 11** is all
about testing, which even with Rust’s safety guarantees is necessary to ensure
your program’s logic is correct. In **Chapter 12**, we’ll build our own
implementation of a subset of functionality from the `grep` command line tool
that searches for text within files. For this, we’ll use many of the concepts
we discussed in the previous chapters.

**Chapter 13** explores closures and iterators: features of Rust that come from
functional programming languages. In **Chapter 14**, we’ll examine Cargo in
more depth and talk about best practices for sharing your libraries with
others. **Chapter 15** discusses smart pointers that the standard library
provides and the traits that enable their functionality.

In **Chapter 16**, we’ll walk through different models of concurrent
programming and talk about how Rust helps you program in multiple threads
fearlessly. **Chapter 17** looks at how Rust idioms compare to object-oriented
programming principles you might be familiar with.

**Chapter 18** is a reference on patterns and pattern matching, which are
powerful ways of expressing ideas throughout Rust programs. **Chapter 19**
contains a smorgasbord of advanced topics of interest, including unsafe Rust,
macros, and more about lifetimes, traits, types, functions, and closures.

In **Chapter 20**, we’ll complete a project in which we’ll implement a
low-level multithreaded web server!

Finally, some appendices contain useful information about the language in a
more reference-like format**. Appendix A** covers Rust’s keywords, **Appendix
B** covers Rust’s operators and symbols, **Appendix C** covers derivable traits
provided by the standard library, **Appendix D** covers some useful development
tools, and **Appendix E** explains Rust editions.

There is no wrong way to read this book: if you want to skip ahead, go for it!
You might have to jump back to earlier chapters if you experience any
confusion. But do whatever works for you.

An important part of the process of learning Rust is learning how to read the
error messages the compiler displays: these will guide you toward working code.
As such, we’ll provide many examples that don’t compile along with the error
message the compiler will show you in each situation. Know that if you enter
and run a random example, it may not compile! Make sure you read the
surrounding text to see whether the example you’re trying to run is meant to
error. In most situations, we’ll lead you to the correct version of any code
that doesn’t compile.

## Resources and How to Contribute to This Book

This book is open source. If you find an error, please don’t hesitate to file
an issue or send a pull request on GitHub at
*https://github.com/rust-lang/book*. Please see *CONTRIBUTING.md* at
*https://github.com/rust-lang/book/blob/main/CONTRIBUTING.md* for more details.

The source code for the examples in this book, errata, and other information
are available at *https://www.nostarch.com/Rust2021*.