#![allow(dead_code, deprecated, unused_variables, unused_mut)]
struct Counter {
    count: usize,
}
impl Counter {
    fn new() -> Counter {
        Counter { count: 0 }
    }
}
impl Iterator for Counter {
    type Item = usize;
    fn next(&mut self) -> Option<Self::Item> {
        self.count += 1;
        if self.count < 6 { Some(self.count) } else { None }
    }
}
impl ExactSizeIterator for Counter {
    fn len(&self) -> usize {
        5 - self.count
    }
}
fn main() {
    let mut counter = Counter::new();
    assert_eq!(5, counter.len());
    let _ = counter.next();
    assert_eq!(4, counter.len());
}