## 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”][installation]<!-- ignore --> section. If you installed Rust
through some other means, check whether Cargo is installed by entering the
following in your terminal:

```console
$ 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:

```console
$ 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.

<Listing number="1-2" file-name="Cargo.toml" caption="Contents of *Cargo.toml* generated by `cargo new`">

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

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
```

</Listing>

This file is in the [*TOML*][toml]<!-- ignore --> (*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][appendix-e]<!-- ignore -->.

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:

<span class="filename">Filename: src/main.rs</span>

```rust
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:

```console
$ 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
directory. Because the default build is a debug build, Cargo puts the binary in
a directory named *debug*. You can run the executable with this command:

```console
$ ./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 resultant executable all in one command:

```console
$ 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`.

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:

```console
$ 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:

```console
$ 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.
Once programs grow to multiple files or need a dependency, 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:

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

For more information about Cargo, check out [its documentation][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.

[installation]: ch01-01-installation.html#installation
[toml]: https://toml.io
[appendix-e]: appendix-05-editions.html
[cargo]: https://doc.rust-lang.org/cargo/
