#![allow(unused_variables)]
#![feature(vec_peek_mut)]
fn main() {
    let mut vec = Vec::new();
    assert!(vec.peek_mut().is_none());
    vec.push(1);
    vec.push(5);
    vec.push(2);
    assert_eq!(vec.last(), Some(& 2));
    if let Some(mut val) = vec.peek_mut() {
        *val = 0;
    }
    assert_eq!(vec.last(), Some(& 0));
}