New upstream version 1.63.0+dfsg1

[rustc.git] / src / doc / book / nostarch / chapter01.md

<!-- DO NOT EDIT THIS FILE.

This file is periodically generated from the content in the `/src/`
directory, so all fixes need to be made in `/src/`.
-->

[TOC]

# Getting Started

Let’s start your Rust journey! There’s a lot to learn, but every journey starts
somewhere. In this chapter, we’ll discuss:

* Installing Rust on Linux, macOS, and Windows
* Writing a program that prints `Hello, world!`
* Using `cargo`, Rust’s package manager and build system

## Installation

The first step is to install Rust. We’ll download Rust through `rustup`, a
command line tool for managing Rust versions and associated tools. You’ll need
an internet connection for the download.

> Note: If you prefer not to use `rustup` for some reason, please see the
> Other Rust Installation Methods page at
> *https://forge.rust-lang.org/infra/other-installation-methods.html* for more
> options.

The following steps install the latest stable version of the Rust compiler.
Rust’s stability guarantees ensure that all the examples in the book that
compile will continue to compile with newer Rust versions. The output might
differ slightly between versions, because Rust often improves error messages
and warnings. In other words, any newer, stable version of Rust you install
using these steps should work as expected with the content of this book.

> ### Command Line Notation
>
> In this chapter and throughout the book, we’ll show some commands used in the
> terminal. Lines that you should enter in a terminal all start with `$`. You
> don’t need to type in the `$` character; it’s the command line prompt shown
> to indicate the start of each command. Lines that don’t start with `$`
> typically show the output of the previous command. Additionally,
> PowerShell-specific examples will use `>` rather than `$`.

### Installing `rustup` on Linux or macOS

If you’re using Linux or macOS, open a terminal and enter the following command:

```
$ curl --proto '=https' --tlsv1.3 https://sh.rustup.rs -sSf | sh
```

The command downloads a script and starts the installation of the `rustup`
tool, which installs the latest stable version of Rust. You might be prompted
for your password. If the install is successful, the following line will appear:

```
Rust is installed now. Great!
```

You will also need a *linker*, which is a program that Rust uses to join its
compiled outputs into one file. It is likely you already have one. If you get
linker errors, you should install a C compiler, which will typically include a
linker. A C compiler is also useful because some common Rust packages depend on
C code and will need a C compiler.

On macOS, you can get a C compiler by running:

```
$ xcode-select --install
```

Linux users should generally install GCC or Clang, according to their
distribution’s documentation. For example, if you use Ubuntu, you can install
the `build-essential` package.

### Installing `rustup` on Windows

On Windows, go to *https://www.rust-lang.org/tools/install* and follow the
instructions for installing Rust. At some point in the installation, you’ll
receive a message explaining that you’ll also need the MSVC build tools for
Visual Studio 2013 or later.
<!-- is this still true: you need 2013 or later? /LC -->
<!-- Yup, I have verified /Carol -->
To acquire the build tools, you’ll need to
install Visual Studio 2022
<!-- and is VS 2019 still the right/best version? /LC -->
<!-- Just checked, there's now VS 2022! /Carol -->
from *https://visualstudio.microsoft.com/downloads/*. When asked which
workloads to install, include:

- “Desktop Development with C++”
- The Windows 10 or 11 SDK
- The English language pack component, along with any other language pack of
  your choosing

<!-- Liz: We do want to recommend the English language pack along with whatever
other languages the reader speaks-- otherwise, sometimes error messages are
printed strangely
https://github.com/rust-lang/rust/issues/35785#issuecomment-735051657 /Carol -->

The rest of this book uses commands that work in both *cmd.exe* and PowerShell.
If there are specific differences, we’ll explain which to use.

### Troubleshooting

To check whether you have Rust installed correctly, open a shell and enter this
line:

```
$ rustc --version
```

You should see the version number, commit hash, and commit date for the latest
stable version that has been released in the following format:

```
rustc x.y.z (abcabcabc yyyy-mm-dd)
```

If you see this information, you have installed Rust successfully! If you don’t
see this information and you’re on Windows, check that Rust is in your `%PATH%`
system variable. If that’s all correct and Rust still isn’t working, there are
a number of places you can get help. The easiest is the #beginners channel on
the official Rust Discord at *https://discord.gg/rust-lang*. There, you can
chat with other Rustaceans (a silly nickname we call ourselves) who can help
you out. Other great resources include the Users forum at
*https://users.rust-lang.org/* and Stack Overflow at
*https://stackoverflow.com/questions/tagged/rust*.

### Updating and Uninstalling

Once Rust is installed via `rustup`, when a new version of Rust is released, updating to the latest version is
<!-- do you mean that the reader should immediately update to the latest
version, or just that when, at some point, they want to update, it's easy to
do? /LC -->
<!-- The latter; I've clarified /Carol -->
easy. From your shell, run the following update script:

```
$ rustup update
```

To uninstall Rust and `rustup`, run the following uninstall script from your
shell:

```
$ rustup self uninstall
```

### Local Documentation

The installation of Rust also includes a local copy of the documentation, so
you can read it offline. Run `rustup doc` to open the local documentation in
your browser.

Any time a type or function is provided by the standard library and you’re not
sure what it does or how to use it, use the application programming interface
(API) documentation to find out!

## Hello, World!

Now that you’ve installed Rust, let’s write your first Rust program. It’s
traditional when learning a new language to write a little program that prints
the text `Hello, world!` to the screen, so we’ll do the same here!

> Note: This book assumes basic familiarity with the command line. Rust makes
> no specific demands about your editing or tooling or where your code lives, so
> if you prefer to use an integrated development environment (IDE) instead of
> the command line, feel free to use your favorite IDE. Many IDEs now have some
> degree of Rust support; check the IDE’s documentation for details. Recently,
> the Rust team has been focusing on enabling great IDE support, and progress
> has been made rapidly on that front!

### Creating a Project Directory

You’ll start by making a directory to store your Rust code. It doesn’t matter
to Rust where your code lives, but for the exercises and projects in this book,
we suggest making a *projects* directory in your home directory and keeping all
your projects there.

Open a terminal and enter the following commands to make a *projects* directory
and a directory for the “Hello, world!” project within the *projects* directory.

For Linux, macOS, and PowerShell on Windows, enter this:

```
$ mkdir ~/projects
$ cd ~/projects
$ mkdir hello_world
$ cd hello_world
```

For Windows CMD, enter this:

```
> mkdir "%USERPROFILE%\projects"
> cd /d "%USERPROFILE%\projects"
> mkdir hello_world
> cd hello_world
```

### Writing and Running a Rust Program

Next, make a new source file and call it *main.rs*. Rust files always end with
the *.rs* extension. If you’re using more than one word in your filename, the
convention is to use an underscore to separate them. For example, use
*hello_world.rs* rather than *helloworld.rs*.

Now open the *main.rs* file you just created and enter the code in Listing 1-1.

Filename: main.rs

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

Listing 1-1: A program that prints `Hello, world!`

Save the file and go back to your terminal window in the
*~/projects/hello_world* directory. On Linux or macOS, enter the following
commands to compile and run the file:
<!-- just to be clear, are we in the projects folder at this point? or a subfolder? /LC -->
<!-- In the hello_world subfolder, as that's where we left the terminal in the
"Creating a project directory" section. I've added the path here for clarity
/Carol -->

```
$ rustc main.rs
$ ./main
Hello, world!
```

On Windows, enter the command `.\main.exe` instead of `./main`:

```
> rustc main.rs
> .\main.exe
Hello, world!
```

Regardless of your operating system, the string `Hello, world!` should print to
the terminal. If you don’t see this output, refer back to the “Troubleshooting”
part of the Installation section for ways to get help.

If `Hello, world!` did print, congratulations! You’ve officially written a Rust
program. That makes you a Rust programmer—welcome!

### Anatomy of a Rust Program

Let’s review this “Hello, world!” program in detail. Here’s the first piece of
the puzzle:

```
fn main() {

}
```

These lines define a function named `main`. The `main` function is special: it
is always the first code that runs in every executable Rust program. Here, the
first line declares a function named `main` that has no parameters and returns
nothing. If there were parameters, they would go inside the parentheses `()`.

The function body is wrapped in `{}`. Rust requires curly brackets around all
function bodies. It’s good style to place the opening curly bracket on the same
line as the function declaration, adding one space in between.

> Note: If you want to stick to a standard style across Rust projects, you can
> use an automatic formatter tool called `rustfmt` to format your code in a
> particular style (more on `rustfmt` in Appendix D). The Rust team has
> included this tool with the standard Rust distribution, like `rustc`, so it
> should already be installed on your computer!

The body of the the `main` function holds the following code:

```
    println!("Hello, world!");
```

This line does all the work in this little program: it prints text to the
screen. There are four important details to notice here.

First, Rust style is to indent with four spaces, not a tab.

Second, `println!` calls a Rust macro. If it had called a function instead, it
would be entered as `println` (without the `!`). We’ll discuss Rust macros in
more detail in Chapter 19. For now, you just need to know that using a `!`
means that you’re calling a macro instead of a normal function, and that macros
don’t always follow the same rules as functions.

Third, you see the `"Hello, world!"` string. We pass this string as an argument
to `println!`, and the string is printed to the screen.

Fourth, we end the line with a semicolon (`;`), which indicates that this
expression is over and the next one is ready to begin. Most lines of Rust code
end with a semicolon.

### Compiling and Running Are Separate Steps

You’ve just run a newly created program, so let’s examine each step in the
process.

Before running a Rust program, you must compile it using the Rust compiler by
entering the `rustc` command and passing it the name of your source file, like
this:

```
$ rustc main.rs
```

If you have a C or C++ background, you’ll notice that this is similar to `gcc`
or `clang`. After compiling successfully, Rust outputs a binary executable.

On Linux, macOS, and PowerShell on Windows, you can see the executable by
entering the `ls` command in your shell. On Linux and macOS, you’ll see two
files. With PowerShell on Windows, you’ll see the same three files that you
would see using CMD.

```
$ ls
main  main.rs
```

With CMD on Windows, you would enter the following:

```
> dir /B %= the /B option says to only show the file names =%
main.exe
main.pdb
main.rs
```

This shows the source code file with the *.rs* extension, the executable file
(*main.exe* on Windows, but *main* on all other platforms), and, when using
Windows, a file containing debugging information with the *.pdb* extension.
From here, you run the *main* or *main.exe* file, like this:

```
$ ./main # or .\main.exe on Windows
```

If your *main.rs* is your “Hello, world!” program, this line prints `Hello,
world!` to your terminal.

If you’re more familiar with a dynamic language, such as Ruby, Python, or
JavaScript, you might not be used to compiling and running a program as
separate steps. Rust is an *ahead-of-time compiled* language, meaning you can
compile a program and give the executable to someone else, and they can run it
even without having Rust installed. If you give someone a *.rb*, *.py*, or
*.js* file, they need to have a Ruby, Python, or JavaScript implementation
installed (respectively). But in those languages, you only need one command to
compile and run your program. Everything is a trade-off in language design.

Just compiling with `rustc` is fine for simple programs, but as your project
grows, you’ll want to manage all the options and make it easy to share your
code. Next, we’ll introduce you to the Cargo tool, which will help you write
real-world Rust programs.

## Hello, Cargo!

Cargo is Rust’s build system and package manager. Most Rustaceans use this tool
to manage their Rust projects because Cargo handles a lot of tasks for you,
such as building your code, downloading the libraries your code depends on, and
building those libraries. (We call the libraries that your code needs
*dependencies*.)

The simplest Rust programs, like the one we’ve written so far, don’t have any
dependencies. If we had built the “Hello, world!” project with Cargo, it would
only use the part of Cargo that handles building your code. As you write more
complex Rust programs, you’ll add dependencies, and if you start a project
using Cargo, adding dependencies will be much easier to do.

Because the vast majority of Rust projects use Cargo, the rest of this book
assumes that you’re using Cargo too. Cargo comes installed with Rust if you
used the official installers discussed in the “Installation” section. If you
installed Rust through some other means, check whether Cargo is installed by
entering the following into your terminal:

```
$ cargo --version
```

If you see a version number, you have it! If you see an error, such as `command
not found`, look at the documentation for your method of installation to
determine how to install Cargo separately.

### Creating a Project with Cargo

Let’s create a new project using Cargo and look at how it differs from our
original “Hello, world!” project. Navigate back to your *projects* directory (or
wherever you decided to store your code). Then, on any operating system, run
the following:

```
$ cargo new hello_cargo
$ cd hello_cargo
```

The first command creates a new directory and project called *hello_cargo*.
We’ve named our project *hello_cargo*, and Cargo creates its files in a
directory of the same name.

Go into the *hello_cargo* directory and list the files. You’ll see that Cargo
has generated two files and one directory for us: a *Cargo.toml* file and a
*src* directory with a *main.rs* file inside.

It has also initialized a new Git repository along with a *.gitignore* file.
Git files won’t be generated if you run `cargo new` within an existing Git
repository; you can override this behavior by using `cargo new --vcs=git`.

> Note: Git is a common version control system. You can change `cargo new` to
> use a different version control system or no version control system by using
> the `--vcs` flag. Run `cargo new --help` to see the available options.

Open *Cargo.toml* in your text editor of choice. It should look similar to the
code in Listing 1-2.

Filename: Cargo.toml

```
[package]
name = "hello_cargo"
version = "0.1.0"
edition = "2021"

[dependencies]
```

Listing 1-2: Contents of *Cargo.toml* generated by `cargo new`

This file is in the *TOML* (*Tom’s Obvious, Minimal Language*) format, which is
Cargo’s configuration format.

The first line, `[package]`, is a section heading that indicates that the
following statements are configuring a package. As we add more information to
this file, we’ll add other sections.

The next three lines set the configuration information Cargo needs to compile
your program: the name, the version, and the edition of Rust to use. We’ll talk
about the `edition` key in Appendix E.

The last line, `[dependencies]`, is the start of a section for you to list any
of your project’s dependencies. In Rust, packages of code are referred to as
*crates*. We won’t need any other crates for this project, but we will in the
first project in Chapter 2, so we’ll use this dependencies section then.

Now open *src/main.rs* and take a look:

Filename: src/main.rs

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

Cargo has generated a “Hello, world!” program for you, just like the one we
wrote in Listing 1-1! So far, the differences between our project and the
project Cargo generated are that Cargo placed the code in the *src* directory,
and we have a *Cargo.toml* configuration file in the top directory.

Cargo expects your source files to live inside the *src* directory. The
top-level project directory is just for README files, license information,
configuration files, and anything else not related to your code. Using Cargo
helps you organize your projects. There’s a place for everything, and
everything is in its place.

If you started a project that doesn’t use Cargo, as we did with the “Hello,
world!” project, you can convert it to a project that does use Cargo. Move the
project code into the *src* directory and create an appropriate *Cargo.toml*
file.

### Building and Running a Cargo Project

Now let’s look at what’s different when we build and run the “Hello, world!”
program with Cargo! From your *hello_cargo* directory, build your project by
entering the following command:

```
$ cargo build
   Compiling hello_cargo v0.1.0 (file:///projects/hello_cargo)
    Finished dev [unoptimized + debuginfo] target(s) in 2.85 secs
```

This command creates an executable file in *target/debug/hello_cargo* (or
*target\debug\hello_cargo.exe* on Windows) rather than in your current
<!-- why does it put it in a debug folder? Interesting to know, because it seems
laborious to have to enter the whole path to run the executable /LC -->
<!-- Because `build` uses debug settings by default, and the compiler tries to
make it clear that this isn't a release build by storing it in the `debug`
path. Most people use `cargo run` instead, which is what we show in a few
paragraphs. We talk about building for release mode in the next section; do you
think it needs to be mentioned here too? I think it would be somewhat
distracting and repetitive to get into that right here... /Carol -->
directory. You can run the executable with this command:

```
$ ./target/debug/hello_cargo # or .\target\debug\hello_cargo.exe on Windows
Hello, world!
```

If all goes well, `Hello, world!` should print to the terminal. Running `cargo
build` for the first time also causes Cargo to create a new file at the top
level: *Cargo.lock*. This file keeps track of the exact versions of
dependencies in your project. This project doesn’t have dependencies, so the
file is a bit sparse. You won’t ever need to change this file manually; Cargo
manages its contents for you.

We just built a project with `cargo build` and ran it with
`./target/debug/hello_cargo`, but we can also use `cargo run` to compile the
code and then run the resulting executable all in one command:

```
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/hello_cargo`
Hello, world!
```

Using `cargo run` is more convenient than having to remember to run `cargo
build` and then use the whole path to the binary, so most developers use `cargo
run`.
<!-- Liz: I added this note here to address the above comment about
laboriousness. The idea was to show `cargo build`, talk about where the result
of that ends up, then show that `cargo run` does those two steps in one command.
Do you think that works or does this section need to be reordered somehow?
/Carol -->

Notice that this time we didn’t see output indicating that Cargo was compiling
`hello_cargo`. Cargo figured out that the files hadn’t changed, so it didn’t
rebuild but just ran the binary. If you had modified your source code, Cargo
would have rebuilt the project before running it, and you would have seen this
output:

```
$ cargo run
   Compiling hello_cargo v0.1.0 (file:///projects/hello_cargo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.33 secs
     Running `target/debug/hello_cargo`
Hello, world!
```

Cargo also provides a command called `cargo check`. This command quickly checks
your code to make sure it compiles but doesn’t produce an executable:

```
$ cargo check
   Checking hello_cargo v0.1.0 (file:///projects/hello_cargo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.32 secs
```

Why would you not want an executable? Often, `cargo check` is much faster than
`cargo build`, because it skips the step of producing an executable. If you’re
continually checking your work while writing the code, using `cargo check` will
speed up the process of letting you know if your project is still compiling! As
such, many Rustaceans run `cargo check` periodically as they write their
program to make sure it compiles. Then they run `cargo build` when they’re
ready to use the executable.

Let’s recap what we’ve learned so far about Cargo:

* We can create a project using `cargo new`.
* We can build a project using `cargo build`.
* We can build and run a project in one step using `cargo run`.
* We can build a project without producing a binary to check for errors using
  `cargo check`.
* Instead of saving the result of the build in the same directory as our code,
  Cargo stores it in the *target/debug* directory.

An additional advantage of using Cargo is that the commands are the same no
matter which operating system you’re working on. So, at this point, we’ll no
longer provide specific instructions for Linux and macOS versus Windows.

### Building for Release

When your project is finally ready for release, you can use `cargo build
--release` to compile it with optimizations. This command will create an
executable in *target/release* instead of *target/debug*. The optimizations
make your Rust code run faster, but turning them on lengthens the time it takes
for your program to compile. This is why there are two different profiles: one
for development, when you want to rebuild quickly and often, and another for
building the final program you’ll give to a user that won’t be rebuilt
repeatedly and that will run as fast as possible. If you’re benchmarking your
code’s running time, be sure to run `cargo build --release` and benchmark with
the executable in *target/release*.

### Cargo as Convention

With simple projects, Cargo doesn’t provide a lot of value over just using
`rustc`, but it will prove its worth as your programs become more intricate.
With complex projects composed of multiple crates, it’s much easier to let
Cargo coordinate the build.

Even though the `hello_cargo` project is simple, it now uses much of the real
tooling you’ll use in the rest of your Rust career. In fact, to work on any
existing projects, you can use the following commands to check out the code
using Git, change to that project’s directory, and build:

```
$ git clone example.org/someproject
$ cd someproject
$ cargo build
```

For more information about Cargo, check out its documentation at *https://doc.rust-lang.org/cargo/*.

## Summary

You’re already off to a great start on your Rust journey! In this chapter,
you’ve learned how to:

* Install the latest stable version of Rust using `rustup`
* Update to a newer Rust version
* Open locally installed documentation
* Write and run a “Hello, world!” program using `rustc` directly
* Create and run a new project using the conventions of Cargo

This is a great time to build a more substantial program to get used to reading
and writing Rust code. So, in Chapter 2, we’ll build a guessing game program.
If you would rather start by learning how common programming concepts work in
Rust, see Chapter 3 and then return to Chapter 2.