impl<T, P> Drop for List<T, P>
where
    P: SharedPointerKind,
{
    fn drop(&mut self) {
        let mut head = self.head.take();
        while let Some(node) = head {
            if let Ok(mut node) = SharedPointer::try_unwrap(node) {
                head = node.next.take();
            }
            else {
                break;
            }
        }
    }
}