# Mutability

Variable bindings are immutable by default, but this can be overridden using the `mut` modifier.

    fn main() {
        let _immutable_binding = 1;
        let mut mutable_binding = 1;

        println!("Before mutation: {}", mutable_binding);

        // Ok
        mutable_binding += 1;

        println!("After mutation: {}", mutable_binding);

        // Error!
        _immutable_binding += 1;
        // FIXME ^ Comment out this line
    }

The compiler will throw a detailed diagnostic about mutability errors.