<Rc<RefCell<TreeNode>>> on Leetcode

I need help understanding this error. How do I get a mutable reference to the TreeNode in root? Root is defined as:

// Definition for a binary tree node.
// #[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
//     }
//   }
// }


Line 41, Char 26: borrow of moved value: `rootTreeNode` (solution.rs)
26 |         let mut rootTreeNode = Rc::try_unwrap(root.unwrap()).unwrap().into_inner(); // I've borrowed the RefCell instance
   |             ---------------- move occurs because `rootTreeNode` has type `tree_node::TreeNode`, which does not implement the `Copy` trait

You don't want into_inner() nor try_unwrap(). As evident from their name and documentation, they consume the wrapper (the RefCell and the Rc, respectively), returning the wrapped value by-value.

If you need a mutable reference to the inner value, use RefCell::borrow_mut() instead. That's the whole point of using a RefCell.

1 Like

Recommended reading: Introduction - Learning Rust With Entirely Too Many Linked Lists


That's the fact that into is used to denote something owning and moving. E.g. into_inner and into_string

borrow or as are used to show taking a reference. E.g. borrow_mut and as_str

If something gives a mutable reference, it's going to have _mut added to the end.

Related is the trio of iterators:
iter - borrows
into_iter - moves into an iterator of owned values
iter_mut - mutable borrows

The stdlib leads the way with these conventions, you won't go far wrong doing similar in your own crates.

At least, that's my understanding

Can someone explain why Rc and RefCell are used here? I'm not entirely sure I understand the benefit.

Perhaps to avoid unsafe code? But I don't think it is sound to expose Rc as a pub member in TreeNode.

OK, so let's take a step back. What are the purpose of Rc and RefCell?

Check out this: