#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn main() {
    let mut x: Result<u32, &str> = Ok(7);
    match x.iter_mut().next() {
        Some(v) => *v = 40,
        None => {}
    }
    assert_eq!(x, Ok(40));
    let mut x: Result<u32, &str> = Err("nothing!");
    assert_eq!(x.iter_mut().next(), None);
}