Hello, World!

Now that you have Rust installed, let’s write your first Rust program. It’s traditional when learning a new language to write a little program to print the text “Hello, world!” to the screen, and in this section, we’ll follow that tradition.

Note: This book assumes basic familiarity with the command line. Rust itself makes no specific demands about your editing, tooling, or where your code lives, so if you prefer an IDE (Integrated Development Environment) to 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. Enabling great IDE support has been a recent focus of the Rust team, and progress has been made rapidly on that front!

Creating a Project Directory

First, make a directory to put your Rust code in. Rust doesn’t care where your code lives, but for this book, we’d 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 inside that for the “Hello, world!” project:

Linux and Mac:

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

Windows CMD:

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

Windows PowerShell:

> mkdir $env:USERPROFILE\projects
> cd $env: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, use an underscore to separate them. For example, you’d use hello_world.rs rather than helloworld.rs.

Now open the main.rs file you just created, and enter the code shown 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. On Linux or macOS, enter the following commands:

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

On Windows, use .\main.exe instead of ./main.

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

Regardless of your operating system, you should see the string Hello, world! print to the terminal. If you did, then congratulations! You’ve officially written a Rust program. That makes you a Rust programmer! Welcome!

Anatomy of a Rust Program

Now, let’s go over what just happened in your “Hello, world!” program in detail. Here’s the first piece of the puzzle:

fn main() {


These lines define a function in Rust. The main function is special: it’s the first code that is run for every executable Rust program. The first line declares a function named main that has no parameters and returns nothing. If there were parameters, their names would go inside the parentheses, ( and ).

Also note that the function body is wrapped in curly brackets, { and }. Rust requires these around all function bodies. It’s considered good style to put the opening curly bracket on the same line as the function declaration, with one space in between.

At the time of writing, an automatic formatter, rustfmt, is under development. If you’d like to stick to a standard style across Rust projects, rustfmt is a tool that will format your code in a particular style. The plan is to eventually include it with the standard Rust distribution, like rustc, so depending on when you read this book, you may have it already installed! Check the online documentation for more details.

Inside the main function, we have this code:

# #![allow(unused_variables)]
#fn main() {
    println!("Hello, world!");

This line does all of the work in this little program: it prints text to the screen. There are a number of details to notice here. The first is that Rust style is to indent with four spaces, not a tab.

The second important part is println!. This is calling a Rust macro, which is how metaprogramming is done in Rust. If it were calling a function instead, it would look like this: println (without the !). We’ll discuss Rust macros in more detail in Appendix D, but for now you just need to know that when you see a ! that means that you’re calling a macro instead of a normal function.

Why println! is a Macro

There are multiple reasons why println! is a macro rather than a function, and we haven’t really explained Rust yet, so it’s not exactly obvious. Here are the reasons:

  • The string passed to println! can have formatting specifiers in it, and those are checked at compile-time.
  • Rust functions can only have a fixed number of arguments, but println! (and macros generally) can take a variable number.
  • The formatting specifiers can have named arguments, which Rust functions cannot.
  • It implicitly takes its arguments by reference even when they’re passed by value.

If none of this makes sense, don’t worry about it. We’ll cover these concepts in more detail later.

Next is "Hello, world!" which is a string. We pass this string as an argument to println!, which prints the string to the screen. Easy enough!

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

Compiling and Running Are Separate Steps

In the “Writing and Running a Rust Program” section, we showed you how to run a newly created program. We’ll break that process down and examine each step now.

Before running a Rust program, you have to compile it. You can use the Rust compiler by entering the rustc command and passing it the name of your source file, like this:

$ rustc main.rs

If you come from 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, Mac, and PowerShell on Windows, you can see the executable by entering the ls command in your shell as follows:

$ ls
main  main.rs

With CMD on Windows, you’d enter:

> dir /B %= the /B option says to only show the file names =%

This shows we have two files: the source code, with the .rs extension, and the executable (main.exe on Windows, main everywhere else). All that’s left to do from here is run the main or main.exe file, like this:

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

If main.rs were your “Hello, world!” program, this would print Hello, world! to your terminal.

If you come from a dynamic language like Ruby, Python, or JavaScript, you may not be used to compiling and running a program being separate steps. Rust is an ahead-of-time compiled language, which means that you can compile a program, 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, on the other hand, they need to have a Ruby, Python, or JavaScript implementation installed (respectively), but you only need one command to both compile and run your program. Everything is a tradeoff in language design.

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

Hello, Cargo!

Cargo is Rust’s build system and package manager, and Rustaceans use Cargo to manage their Rust projects because it makes a lot of tasks easier. For example, Cargo takes care of building your code, downloading the libraries your code depends on, and building those libraries. We call libraries your code needs dependencies.

The simplest Rust programs, like the one we’ve written so far, don’t have any dependencies, so right now, you’d only be using the part of Cargo that can take care of building your code. As you write more complex Rust programs, you’ll want to add dependencies, and if you start off using Cargo, that will be a lot easier to do.

As the vast majority of Rust projects use Cargo, we will assume that you’re using it for the rest of the book. Cargo comes installed with Rust itself, if you used the official installers as covered in the “Installation” section. If you installed Rust through some other means, you can check if you have Cargo installed by typing the following into your terminal:

$ cargo --version

If you see a version number, great! If you see an error like command not found, then you should 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 project in hello_world. Go back to your projects directory (or wherever you decided to put your code):

Linux, Mac, and PowerShell:

$ cd ~/projects

CMD for Windows:

> cd \d "%USERPROFILE%\projects"

And then on any operating system run:

$ cargo new hello_cargo --bin
$ cd hello_cargo

We passed the --bin argument to cargo new because our goal is to make an executable application, as opposed to a library. Executables are binary executable files often called just binaries. We’ve given hello_cargo as the name for our project, and Cargo creates its files in a directory of the same name that we can then go into.

If we list the files in the hello_cargo directory, we can see that Cargo has generated two files and one directory for us: a Cargo.toml and a src directory with a main.rs file inside. It has also initialized a new git repository in the hello_cargo directory for us, along with a .gitignore file. 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 up Cargo.toml in your text editor of choice. It should look similar to the code in Listing 1-2:

Filename: Cargo.toml

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


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

This file is in the TOML (Tom’s Obvious, Minimal Language) format. TOML is used as 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 three bits of configuration that Cargo needs to see in order to know that it should compile your program: its name, what version it is, and who wrote it. Cargo gets your name and email information from your environment. If it’s not correct, go ahead and fix that and save the file.

The last line, [dependencies], is the start of a section for you to list any crates (which is what we call packages of Rust code) that your project will depend on so that Cargo knows to download and compile those too. We won’t need any other crates for this project, but we will in the guessing game tutorial in Chapter 2.

Now let’s look at src/main.rs:

Filename: src/main.rs

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

Cargo has generated a “Hello World!” for you, just like the one we wrote in Listing 1-1! So that part is the same. The differences between our previous project and the project generated by Cargo that we’ve seen so far are:

  • Our code goes in the src directory
  • The top level contains a Cargo.toml configuration file

Cargo expects your source files to live inside the src directory so that the top-level project directory is just for READMEs, license information, configuration files, and anything else not related to your code. In this way, using Cargo helps you keep your projects nice and tidy. 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 our project in the hello_world directory, you can convert it to a project that does use Cargo by moving your code into the src directory and creating an appropriate Cargo.toml.

Building and Running a Cargo Project

Now let’s look at what’s different about building and running your Hello World program through Cargo! To do so, enter the following commands:

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

This creates an executable file in target/debug/hello_cargo (or target\debug\hello_cargo.exe on Windows), which you can run with this command:

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

Bam! If all goes well, Hello, world! should print to the terminal once more.

Running cargo build for the first time also causes Cargo to create a new file at the top level called Cargo.lock. Cargo uses Cargo.lock to keep track of the exact versions of dependencies used to build your project. This project doesn’t have dependencies, so the file is a bit sparse. You won’t ever need to touch this file yourself; Cargo will manage 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 and then run:

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

Notice that this time, we didn’t see the output telling us that Cargo was compiling hello_cargo. Cargo figured out that the files haven’t changed, so it 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 output like this:

$ 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!

Finally, there’s cargo check. This will quickly check your code to make sure that it compiles, but not bother producing an executable:

$ cargo check
   Compiling 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? cargo check is often much faster than cargo build, because Cargo can skip the entire step of producing the executable. If we’re checking our work throughout the process of writing the code, this will speed things up! As such, many Rustaceans run cargo check as they write their program to make sure that it compiles, and then run cargo build once they’re ready to give it a spin themselves.

So a few more differences we’ve now seen:

  • Instead of using rustc, build a project using cargo build or cargo check (or build and run it in one step with cargo run).
  • Instead of the result of the build being put in the same directory as our code, Cargo will put it in the target/debug directory.

The other advantage of using Cargo is that the commands are the same no matter what operating system you’re on, so at this point we will no longer be providing specific instructions for Linux and Mac versus Windows.

Building for Release

When your project is finally ready for release, you can use cargo build --release to compile your project with optimizations. This will create an executable in target/release instead of target/debug. These optimizations make your Rust code run faster, but turning them on makes your program take longer to compile. This is why there are two different profiles: one for development when you want to be able to rebuild quickly and often, and one for building the final program you’ll give to a user that won’t be rebuilt and that we want to run as fast as possible. If you’re benchmarking the running time of your code, 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 whole lot of value over just using rustc, but it will prove its worth as you continue. With complex projects composed of multiple crates, it’s much easier to let Cargo coordinate the build. With Cargo, you can just run cargo build, and it should work the right way.

Even though the hello_cargo project is simple, it now uses much of the real tooling you’ll use for the rest of your Rust career. In fact, you can get started with virtually all Rust projects you want to work on with the following commands to check out the code using Git, change into the project directory, and build:

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

If you want to look at Cargo in more detail, check out its documentation, which covers all of its features.