#![allow(dead_code, deprecated, unused_variables, unused_mut)]
#![feature(iter_advance_by)]
use std::num::NonZero;
fn main() {
    let a = [3, 4, 5, 6];
    let mut iter = a.iter();
    assert_eq!(iter.advance_back_by(2), Ok(()));
    assert_eq!(iter.next_back(), Some(& 4));
    assert_eq!(iter.advance_back_by(0), Ok(()));
    assert_eq!(iter.advance_back_by(100), Err(NonZero::new(99).unwrap()));
}