In-place destructive AVL tree iteration in Rust

avltree-iterator.rs

pub struct IntoIter<T> {
    current: AVLTree<T>,
}

impl<T> IntoIter<T> {
    pub fn new(tree: AVLTree<T>) -> Self {
        let mut into_iter = IntoIter { current: Empty };
        into_iter.traverse_left(tree);
        into_iter
    }

    fn traverse_left(&mut self, mut tree: AVLTree<T>) {
        while let NonEmpty(ref mut node) = tree {
            mem::swap(&mut node.left, &mut self.current);
            mem::swap(&mut self.current, &mut tree);
        }
    }
}

impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        let mut node = match mem::replace(&mut self.current, Empty) {
            Empty => return None,
            NonEmpty(node) => node,
        };
        self.current = mem::replace(&mut node.left, Empty);
        self.traverse_left(mem::replace(&mut node.right, Empty));
        Some(node.value)
    }
}