#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn mutate(r: &mut Result<i32, i32>) {
    match r.as_mut() {
        Ok(v) => *v = 42,
        Err(e) => *e = 0,
    }
}
fn main() {
    let mut x: Result<i32, i32> = Ok(2);
    mutate(&mut x);
    assert_eq!(x.unwrap(), 42);
    let mut x: Result<i32, i32> = Err(13);
    mutate(&mut x);
    assert_eq!(x.unwrap_err(), 0);
}