Your First Rust Program

Rust projects are usually created and run with Cargo, Rust's build tool.

Syntax

cargo new hello
cd hello
cargo run

cargo new creates a project folder with this layout:

hello/
├── Cargo.toml
└── src/
    └── main.rs

Cargo.toml describes the project. The edition line picks the version of the language's rules to use — new projects get the latest, 2024:

[package]
name = "hello"
version = "0.1.0"
edition = "2024"

[dependencies]

Example

Cargo already wrote a program into src/main.rs:

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

Run it from inside the hello folder with cargo run. Cargo first prints a few status lines about compiling, then runs the program:

   Compiling hello v0.1.0 (/home/ada/hello)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.52s
     Running `target/debug/hello`

Expected output

Hello, world!

Line by line:

• fn main() — declares a function named main. Every Rust program starts running here. It takes no arguments and returns nothing.
• { ... } — the function body is wrapped in curly braces.
• println!(...) — prints text followed by a newline. The ! means println! is a macro, not a regular function. Macros generate code at compile time; println! uses that to check your format string while compiling. For now, just remember the !.
• ; — ends the statement. Most lines of Rust end with a semicolon.

Common mistakes

• Forgetting the !. println("Hello"); fails with:

error[E0423]: expected function, found macro `println`

• Using single quotes for text. In Rust, 'a' is a single character; text needs double quotes. println!('Hello'); fails with:

error: character literal may only contain one codepoint

• Running cargo run outside the project folder — Cargo reports "could not find Cargo.toml". cd into the folder first.

Practice

Change the program to print your own name on one line and your favorite programming language on the next.

fn main() {
    println!("Ada");
    println!("Rust");
}