#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn main() {
    let mut count = 0;
    let counter = std::iter::from_fn(move || {
        count += 1;
        if count < 6 { Some(count) } else { None }
    });
    assert_eq!(counter.collect::< Vec < _ >> (), & [1, 2, 3, 4, 5]);
}