#![allow(unused_variables)]
#![feature(push_mut)]
fn main() {
    use std::collections::VecDeque;
    let mut vec_deque = VecDeque::from([1, 2, 3]);
    let x = vec_deque.insert_mut(1, 5);
    *x += 7;
    assert_eq!(vec_deque, & [1, 12, 2, 3]);
}