How to implement an iterator of mutable references

Question

I implemented a simple Binary Search Tree in Rust (following CIS 198, it's great), and for learning I'm doing iterators that just run through the right edges.

I could not implement an iterator that gives mutable references. I tried a lot of ways, but none were accepted by Rust compiler. The code I need help is the one below (while I made a gist with the complete code here):

#[derive(Debug)]
pub struct Tree<T>(Option<Box<Node<T>>>);

#[derive(Debug)]
pub struct Node<T> {
    elem: T,
    left: Tree<T>,
    right: Tree<T>,
}

// MUTABLE BORROW STRUCT
pub struct IterMut<'a, T: 'a> {
    next: &'a mut Tree<T>,
}

// MUTABLE BORROW NEXT (I'M STUCK HERE, NOTHING WORKS)
impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;
    fn next(&mut self) -> Option<Self::Item> {
        // 1 try: cannot infer lifetime
        self.next.0.as_mut().map(|node| {
            self.next = &mut node.right;
            &mut node.elem
        })

        // 2 try: node.right, node.elem does not live long enough
        self.next.0.take().map(|node| {
            self.next = &mut node.right;
            &mut node.elem
        })
    }
}

Answer 1

I think you cannot split self into 2 mutable objects (one for the Item, one for self itself) without using some unsafe code.

Answer 2

You need to change the type of the field IterMut::next to Option<&'a mut Node<T>>:

pub struct IterMut<'a, T: 'a> {
    next: Option<&'a mut Node<T>>,
}

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;
    fn next(&mut self) -> Option<Self::Item> {
        self.next.take().map(|node| {
            self.next = node.right.0.as_mut().map(|node| &mut **node);
            &mut node.elem
        })

    }
}

You can find more useful information about the implementation of the mutable iterator for recursive data structures in the IterMut chapter of Learning Rust With Entirely Too Many Linked Lists.