struct CountingIteratorWrapper<I> {
    i: I,
    n: usize,
}

impl<I> CountingIteratorWrapper<I> {
    fn new(i: I) -> Self { Self { i, n: 0 } }
    fn so_far(&self) -> usize { self.n }
}

impl<I:Iterator> Iterator for CountingIteratorWrapper<I> {
    type Item = I::Item;
    fn next(&mut self) -> Option<Self::Item> {
        let v = self.i.next()?;
        self.n += 1;
        Some(v)
    }
}

fn main() {
    let mut i = CountingIteratorWrapper::new(0..10);
    let v: Vec<_> = i.by_ref().filter(|x| x % 3 == 0).collect();
    println!("{:?} {:?}", i.so_far(), v);
}