Code shared from the Rust Playground

playground.rs

// An example to illustrate that 'mut' in front of a parameter name
// is NOT part of the signature of the function.

pub struct Point {
    x: f64,
    y: f64,
}

pub trait Translate: Sized {
    fn translate(&mut self, dx: f64, dy: f64);
    fn translated(mut self, dx: f64, dy: f64) -> Self {
    //            ^^^
    // `mut` is here because it is required by the default implementation,
    // but it is not part of the signature, as illustrated by Point's impl
    // line 27 below
        self.translate(dx, dy);
        self
    }
}

impl Translate for Point {
    fn translate(&mut self, dx: f64, dy: f64) {
        self.x += dx;
        self.y += dy;
    }
    
    fn translated(self, dx: f64, dy: f64) -> Self {
    //            ^^^
    // no `mut` here; this implementation does not need it
        Point {
            x: self.x+dx,
            y: self.y+dy,
        }
    }
}