fn main() {
    let mutating_closure1 = |x: &mut i32| { *x += 1; };
    // The closure is Copy, even though it takes a &mut argument!
    let mutating_closure2 = mutating_closure1;

    let mut x = 5;
    mutating_closure1(&mut x);
    mutating_closure2(&mut x);
    dbg!(x);
}