# Introduction

Welcome to “The Rust Programming Language,” an introductory book about Rust.

Rust is a programming language that helps you write faster, more reliable software. High-level ergonomics and low-level control are often at odds with each other in programming language design; Rust stands to challenge that. 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 great for many people for a variety of reasons. Let’s discuss 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 a variety of 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 kinds of bugs--including concurrency bugs. By working alongside the compiler, the team can spend more time focusing on the logic of the program 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.
- Rustfmt ensures a consistent coding style across developers.
- The Rust Language Server powers 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 people who are interested in learning about systems concepts. Many people have learned about topics like operating systems development through Rust. The community is 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 getting started with programming.

### Companies

Rust is used in production by hundreds of companies, large and small, for a variety of tasks, such as 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 for you to contribute to the Rust language.

### People Who Value Speed and Stability

By speed, we mean both the speed of the programs that Rust lets you create and the speed at which Rust lets you write them. The Rust compiler’s checks ensure stability through feature additions and refactoring, as opposed to brittle legacy code in languages without these checks that developers are 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.

This isn’t a complete list of everyone the Rust language hopes to support, but these are some of the biggest stakeholders. Overall, Rust’s greatest ambition is to take trade-offs that have been accepted by programmers for decades and eliminate the dichotomy. 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 some other 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; someone new to programming entirely would be better served by reading a book specifically providing an introduction to programming.

## How to Use This Book

This book generally assumes that you’re reading it front-to-back, that is, later chapters build on top of concepts in earlier chapters, and earlier chapters may not dig into details on a topic, revisiting the topic in a later chapter.

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

Additionally, Chapter 2 is a hands-on introduction to Rust as a language. We’ll cover concepts at a high level, and later chapters will go into them in detail. If you’re the kind of person who likes to get their hands dirty right away, Chapter 2 is great for that. If you’re really that kind of person, you may even wish to skip over Chapter 3, which covers features that are very similar to other programming languages, and go straight to Chapter 4 to learn about Rust’s ownership system. By contrast, if you’re a particularly meticulous learner who prefers to learn every detail before moving onto the next, you may want to skip Chapter 2 and go straight to Chapter 3.

Chapter 5 discusses structs and methods, and Chapter 6 covers enums, `match` expressions, and the `if let` control flow construct. Structs and enums are the ways to make custom types in Rust.

In Chapter 7, you'll learn about Rust's module system and privacy for organizing your code and its public API. Chapter 8 discusses some common collection data structures provided by the standard library: vectors, strings, and hash maps. Chapter 9 is all about 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 is still necessary even with Rust's safety guarantees to ensure your program's logic is correct. In Chapter 12, we'll build a subset of the functionality of the `grep` command line tool that searches for text within files and 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 explore more about Cargo and talk about best practices for sharing your libraries with others. Chapter 15 discusses smart pointers provided by the standard library and the traits that enable their functionality.

In Chapter 16, we'll go through different models of concurrent programming and how Rust helps you to program using multiple threads fearlessly. Chapter 17 looks at how Rust idioms compare to Object Oriented Programming principles you may 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 is a smorgasbord of advanced topics that you might be interested in, including unsafe Rust and more about lifetimes, traits, types, functions, and closures.

In Chapter 20, we'll finish up with a project where we'll implement a low-level multithreaded web server!

Finally, there are some appendices. These contain useful information about the language in a more reference-like format.

In the end, there’s no wrong way to read a book: if you want to skip ahead, go for it! You may have to jump back if you find things confusing. Do whatever works for you.

An important part of the process of learning Rust is learning how to read the error messages that the compiler gives you. As such, we’ll be showing a lot of code that doesn’t compile, and the error message the compiler will show you in that situation. As such, if you pick a random example, it may not compile! Please read the surrounding text to make sure that you didn’t happen to pick one of the in-progress examples.

## Contributing to the 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. Please see CONTRIBUTING.md for more details.