Rust Basics

Rust Variables, mut, const and Shadowing

Variables in Rust cannot change unless you ask. Learn let, let mut, const and shadowing.

Beginner
2 min read
1 runnable example

What you'll learn

let and let mut
Constants with const
Shadowing a variable

Run code →

Declaration → what it allows

let language = "Rust"cannot change

let mut score = 10can change

const MAX_SCORE: u32 = 100fixed for the whole program

let score = score * 2 → a new variable, 30

Immutable by default

Variables in Rust cannot change unless you say so. let creates a variable, let mut creates one you can change, and const creates a value that is fixed for the whole program.

score is declared with mut, so score += 5 is allowed. The second let score creates a new variable with the same name, which is called shadowing. The braces in println! print the variables named inside them.

Key points

let creates a variable that cannot change.
let mut allows the value to change.
A second let with the same name creates a new variable (shadowing).

Runnable Rust example

Rust

Run code →

main.rs

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

    const MAX_SCORE: u32 = 100;
    let score = score * 2; // shadowing creates a new variable

    println!("{language}: {score} of {MAX_SCORE}");
}

Get a Run button for this example Result of running the code

Rust: 30 of 100

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: Data Types →

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