Mutability

When you need a variable whose value can change, declare it with let mut. The mut keyword makes the variable mutable.

Mutable variables

fn main() {
    let mut counter = 0;
    counter = counter + 1;
    counter += 1;
    println!("Counter is {}", counter); // 2
}

Without mut, reassigning counter would be a compile error.

Why immutable by default

Rust makes variables immutable by default for good reasons:

• Fewer bugs: values do not change unexpectedly behind your back.
• Easier reasoning: you know a value stays constant once set.
• Safer concurrency: immutable data can be shared across threads without races.

By forcing you to write mut, Rust makes mutation visible and intentional.

Example

fn main() {
    let mut temperature = 20.0;
    println!("Start: {}", temperature);
    temperature = 25.5;
    println!("Updated: {}", temperature);
}

Use mut only when change is truly needed; prefer immutability otherwise.