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# 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 be caught
only 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
student 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.

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the big companies using Rust that I don't want to get into. I would also worry
about the list getting dated. /Carol -->

### 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. Chapters 2, 12, and 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 to 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/*.