const_correctness.rs

struct Vector {
    x: f64,
    y: f64,
}

impl Vector {
    fn length(&self) -> f64 {
        return (self.x * self.x + self.y * self.y).sqrt();
    }

    fn normalize(&mut self) {
        let l = self.length();
        self.x /= l;
        self.y /= l;
    }
}

fn foo(v: &mut Vector) {
    v.normalize();
}

fn bar(v: &Vector) {
    v.normalize(); // -> Compiler error
    println!("{}", v.length());
}

fn main() {
    let mut my_vector = Vector { x: 3.0, y: 4.0 };
    foo(&mut my_vector);
    bar(&my_vector);
}

// error[E0596]: cannot borrow `*v` as mutable, as it is behind a `&` reference
//   --> src/main.rs:23:5
//    |
//    | fn bar(v: &Vector) {
//    |           ------- help: consider changing this to be a mutable reference: `&mut Vector`
//    |     v.normalize();
//    |     ^ `v` is a `&` reference, so the data it refers to cannot be borrowed as mutable