Hello, Cargo!

Cargo is a tool that Rustaceans use to help manage their Rust projects. Cargo is currently in a pre-1.0 state, and so it is still a work in progress. However, it is already good enough to use for many Rust projects, and so it is assumed that Rust projects will use Cargo from the beginning.

Cargo manages three things: building our code, downloading the dependencies our code needs, and building those dependencies. At first, our program doesn’t have any dependencies, so we’ll only be using the first part of its functionality. Eventually, we’ll add more. Since we started off by using Cargo, it'll be easy to add later.

If you installed Rust via the official installers you will also have Cargo. If you installed Rust some other way, you may want to check the Cargo README for specific instructions about installing it.

Converting to Cargo

Let’s convert Hello World to Cargo.

To Cargo-ify our project, we need to do three things: Make a Cargo.toml configuration file, put our source file in the right place, and get rid of the old executable (main.exe on Windows, main everywhere else). Let's do that part first:

$ mkdir src
$ mv main.rs src/main.rs
$ rm main  # or 'rm main.exe' on Windows

Note: since we're creating an executable, we retain main.rs as the source filename. If we want to make a library instead, we should use lib.rs. This convention is used by Cargo to successfully compile our projects, but it can be overridden if we wish. Custom file locations for the entry point can be specified with a [lib] or [[bin]] key in the TOML file.

Cargo expects our source files to live inside a src directory. That leaves the top level for other things, like READMEs, license information, and anything not related to our code. Cargo helps us keep our projects nice and tidy. A place for everything, and everything in its place.

Next, our configuration file:

$ editor Cargo.toml # or 'notepad Cargo.toml' on Windows

Make sure to get this name right: we need the capital C!

Put this inside:

[package]

name = "hello_world"
version = "0.0.1"
authors = [ "Your name <you@example.com>" ]

This file is in the TOML format. TOML is similar to INI, but has some extra goodies. According to the TOML docs,

TOML aims to be a minimal configuration file format that's easy to read due to obvious semantics. TOML is designed to map unambiguously to a hash table. TOML should be easy to parse into data structures in a wide variety of languages.

Once we have this file in place in our project's root directory, we should be ready to build! To do so, run:

$ cargo build
   Compiling hello_world v0.0.1 (file:///home/yourname/projects/hello_world)
$ ./target/debug/hello_world
Hello, world!

Bam! We built our project with cargo build, and ran it with ./target/debug/hello_world. We can do both in one step with cargo run:

$ cargo run
     Running `target/debug/hello_world`
Hello, world!

Notice that we didn’t re-build the project this time. Cargo figured out that we hadn’t changed the source file, and so it just ran the binary. If we had made a modification, we would have seen it do both:

$ cargo run
   Compiling hello_world v0.0.1 (file:///home/yourname/projects/hello_world)
     Running `target/debug/hello_world`
Hello, world!

This hasn’t bought us a whole lot over our simple use of rustc, but think about the future: when our project gets more complex, we need to do more things to get all of the parts to properly compile. With Cargo, as our project grows, we can just run cargo build, and it’ll work the right way.

When our project is finally ready for release, we can use cargo build --release to compile our project with optimizations.

You'll also notice that Cargo has created a new file: Cargo.lock.

[root]
name = "hello_world"
version = "0.0.1"

The Cargo.lock file is used by Cargo to keep track of dependencies in our application. Right now, we don’t have any, so it’s a bit sparse. We won't ever need to touch this file ourselves, just let Cargo handle it.

That’s it! We’ve successfully built hello_world with Cargo. Even though our program is simple, it’s using much of the real tooling that we’ll use for the rest of our Rust career. We can expect to do this to get started with virtually all Rust projects:

$ git clone someurl.com/foo
$ cd foo
$ cargo build

A New Project

We don’t have to go through this whole process every time we want to start a new project! Cargo has the ability to make a bare-bones project directory in which we can start developing right away.

To start a new project with Cargo, we use cargo new:

$ cargo new hello_world --bin

We’re passing --bin because our goal is to get straight to making an executable application, as opposed to a library. Executables are often called ‘binaries.’ (as in /usr/bin, if we’re on a Unix system)

Let's check out what Cargo has generated for us:

$ cd hello_world
$ tree .
.
├── Cargo.toml
└── src
    └── main.rs

1 directory, 2 files

If we don't have the tree command, we can probably get it from our distribution’s package manager. It’s not necessary, but it’s certainly useful.

This is all we need to get started. First, let’s check out Cargo.toml:

[package]

name = "hello_world"
version = "0.1.0"
authors = ["Your Name <you@example.com>"]

Cargo has populated this file with reasonable defaults based off the arguments we gave it and our git global configuration. You may notice that Cargo has also initialized the hello_world directory as a git repository.

Here’s what’s in src/main.rs:

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

Cargo has generated a "Hello World!" for us, and we’re ready to start coding! Cargo has its own guide which covers Cargo’s features in much more depth.

Now that we’ve got the tools down, let’s actually learn more about the Rust language itself. These are the basics that will serve us well through the rest of our time with Rust.

You have two options: Dive into a project with ‘Learn Rust’, or start from the bottom and work your way up with ‘Syntax and Semantics’. More experienced systems programmers will probably prefer ‘Learn Rust’, while those from dynamic backgrounds may enjoy either. Different people learn differently! Choose whatever’s right for you.