How to iter this tree struct

Say I have a complete binary tree, and I want to get its height or depth. Here's my code:

 #[derive(Debug, PartialEq, Eq)]
 pub struct TreeNode {
   pub val: i32,
   pub left: Option<Rc<RefCell<TreeNode>>>,
   pub right: Option<Rc<RefCell<TreeNode>>>,
 }

 impl TreeNode {
   #[inline]
   pub fn new(val: i32) -> Self {
     TreeNode {
       val,
       left: None,
       right: None
     }
   }
 }
use std::rc::Rc;
use std::cell::RefCell;


    fn get_depth(r: &Option<Rc<RefCell<TreeNode>>>) -> i32 {
        if r.is_none() {
            return 0;
        }
        1 + Self::get_depth(&r.as_ref().unwrap().borrow().left)
    }

And then I want to use non-recursive code to solve this problem, but it cannot compile. How should I fix it:

    fn get_depth(r: &Option<Rc<RefCell<TreeNode>>>) -> i32 {
        let mut cur = r;
        let mut count = 0;
        while cur.is_some() {
            count+=1;
            cur = &cur.as_ref().unwrap().borrow().left; // err: borrow creates a temporary var...
        }
        count
    }

Option<Rc<RefCell<TreeNode>>> seems like a code smell without further context. Is there some particular reason to not use Option<Box<TreeNode>>?

It's a problem from leetcode, and I have to finish the function based on the func template. The struct define is part of the template

It's not possible to do iteratively because accessing the contents of the RefCell requires a guard with drop code, so accessing something 10 nodes deep will mean having ten guards with destructors active while moving down the tree.

It may be possible with a mutable reference, because the uniqueness guarantee allows you to skip the requirement of a drop guard.