Avoiding stackoverflow when dropping large List

[lulitao1997]

As #41 suggests, when dropping a large List, rust recursively calls Arc's drop function, which leads to stackoverflow. We can implement our custom Drop trait for List to prevent this, the following code seems to solve this problem:

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;
            }
        }
    }
}

[lulitao1997]

can i make a pull request for this? :)

[orium]

Hi @lulitao1997. A PR will be more than welcome :)

[orium]

Fixed by #47