The only real difference between a weak and strong pointer is that weak pointers are less prone to leaking memory (in exchange for the possibility of "sorry I'm

The only real difference between a weak and strong pointer is that weak pointers are less prone to leaking memory (in exchange for the possibility of "sorry I'm dead" being a response to accesses). As soon as you upgrade such a pointer you've created a cyclic graph. He only way for Rust's type system to understand these kinds of access patterns in a way that doesn't require the insertion of "shared mutability" types (e.g. Mutexes) involves mutating the tree during traversals. A simple version of such a pattern: a "doubly linked" list which is actually two singly linked lists. One "traverses" this list by popping nodes off one and pushing them onto the other. The user has easy access to the heads of both lists. You could probably do the same with trees but this is a pretty unpleasant pattern.