#![allow(dead_code, deprecated, unused_variables, unused_mut)]
use std::iter;
fn main() {
    let mut curr = 1;
    let mut pow2 = iter::repeat_with(|| {
            let tmp = curr;
            curr *= 2;
            tmp
        })
        .take(4);
    assert_eq!(Some(1), pow2.next());
    assert_eq!(Some(2), pow2.next());
    assert_eq!(Some(4), pow2.next());
    assert_eq!(Some(8), pow2.next());
    assert_eq!(None, pow2.next());
}