#![allow(unused_variables)]
fn main() {
    use std::collections::VecDeque;
    let mut buf = VecDeque::new();
    buf.extend(1..5);
    buf.retain_mut(|x| {
        if *x % 2 == 0 {
            *x += 1;
            true
        } else {
            false
        }
    });
    assert_eq!(buf, [3, 5]);
}