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Rust Basics

Hello World in Rust: Structure of a Program

Write your first Rust program and see what each part does: fn main, the braces and println!.

  • Beginner
  • 2 min read
  • 1 runnable example
What you'll learn
  • Where a Rust program starts
  • Printing with println!
  • Why println! ends with !
Run code →
From source to output
  1. 1main.rsYou write the code
  2. 2rustcThe compiler checks and builds it
  3. 3main()The program starts in main
  4. 4Hello, Rust!The text appears

How a Rust program runs

Every Rust program starts in a function called main. The println! macro prints a line of text; the exclamation mark shows that it is a macro, not an ordinary function.

fn declares a function and the braces hold its body. Each statement ends with a semicolon. The compiler turns the file into a program, then the program runs main and prints the text.

Key points

  1. Every program starts in fn main.
  2. println! is a macro: the ! tells you so.
  3. rustc compiles the file into a program before it runs.

Runnable Rust example

Rust
Run code →
main.rs
fn main() {
    println!("Hello, Rust!");
}
Get a Run button for this example Result of running the code
Hello, Rust!

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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Next: Variables and Mutability →

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