#[derive(Debug)]
enum Mutant {
    Val(usize),
}

impl Mutant {
    fn mutate(&mut self, new_val: usize) {
        *self = Mutant::Val(new_val);
    }
}

fn main() {
    let mut m = Mutant::Val(1);
    eprintln!("{:?}", m);
    m.mutate(2);
    eprintln!("{:?}", m);
}