unsafe impl<#[may_dangle] T> Drop for LinkedList<T> {
    fn drop(&mut self) {
        struct DropGuard<'a, T>(&'a mut LinkedList<T>);

        impl<'a, T> Drop for DropGuard<'a, T> {
            fn drop(&mut self) {
                // Continue the same loop we do below. This only runs when a destructor has
                // panicked. If another one panics this will abort.
                while self.0.pop_front_node().is_some() {}
            }
        }

        while let Some(node) = self.pop_front_node() {
            let guard = DropGuard(self);
            drop(node);
            mem::forget(guard);
        }
    }
}

这里为什么需要一个DropGuard的结构呢？蟹蟹