}
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.len();
        (len, Some(len))

impl ExactSizeIterator for Chain<'_> {
    fn len(&self) -> usize {
        match &self.state {
            Linked { mut next } => {
                let mut len = 0;
                while let Some(cause) = next {
                    next = cause.source();
                    len += 1;
                }
                len
            }
            Buffered { rest } => rest.len(),
        }
    }
}