Variables, Mutability, and Constants

Learn how Rust stores values and why immutability is the default.

Learning objectives

• Declare variables using let
• Use mut when a variable needs to change
• Create constants for fixed values

Immutable by Default

In Rust, variables are immutable by default. This means once a value is assigned, you cannot change it unless you explicitly mark the variable as mutable. This design helps reduce accidental changes in your program.

Using mut

When a variable must change, use the mut keyword. This tells Rust and other readers of your code that the value is intended to be updated.

Constants

Constants are declared with const. They must have a type annotation and are useful for values that should never change, such as limits, rates, and configuration values.

Example code

fn main() {
    let name = "Rust";
    let mut score = 10;
    score = score + 5;

    const MAX_SCORE: i32 = 100;

    println!("Language: {}", name);
    println!("Score: {} of {}", score, MAX_SCORE);
}