Rust Variables

In Rust, variables are immutable by default. Add mut to allow changes. This is a design choice that makes programs easier to reason about.

ExampleRust

fn main() {
    let name = "Ada";
    let mut age = 36;
    println!("{} is {}", name, age);
    age = 37;
    println!("{name} is now {age}");

    const YEAR: u32 = 2026;
    println!("{}", YEAR);

    let x = 5;
    let x = x * 2; // shadowing: a new variable with the same name
    println!("{}", x);
}

Output

Ada is 36
Ada is now 37
2026
10

If you try to change a variable without mut, the compiler stops you with an error. Shadowing (let x = x * 2) creates a new variable rather than changing the old one.

Quick check

Are Rust variables mutable by default?

YesNo, add mutOnly numbers

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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