#![allow(dead_code, deprecated, unused_variables, unused_mut)]
struct Alternate {
    state: i32,
}
impl Iterator for Alternate {
    type Item = i32;
    fn next(&mut self) -> Option<i32> {
        let val = self.state;
        self.state = self.state + 1;
        (val % 2 == 0).then_some(val)
    }
}
fn main() {
    let mut iter = Alternate { state: 0 };
    assert_eq!(iter.next(), Some(0));
    assert_eq!(iter.next(), None);
    assert_eq!(iter.next(), Some(2));
    assert_eq!(iter.next(), None);
    let mut iter = iter.fuse();
    assert_eq!(iter.next(), Some(4));
    assert_eq!(iter.next(), None);
    assert_eq!(iter.next(), None);
    assert_eq!(iter.next(), None);
    assert_eq!(iter.next(), None);
}