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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.

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.)

Cargo commands

cargo new

cargo new <project name>

cargo new hello_cargo

The first command creates a new directory and project called hello_cargo.

Inside hello_cargo folder we will have two files

  • Cargo.toml
  • src/main.rs

It has also initialized a new Git repository along with a .gitignore file.

Contents of Cargo.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]

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.

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.

cargo build

$ 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.

Command to run the executable

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

cargo run

cargo run // inside the project directory

// Outputs:-
//   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!

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.

cargo check

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

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

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.