When tree will go out of scope, their nodes memory will be reclaimed automatically ("dropping" in Rust terms) by automatic garbage collector built-in into compiler: `drop(root)->drop(leaf_a),drop(leaf_b)`. However, if leaf will have reference to its parent, then an infinite loop occurs: `drop(root)->drop(leaf_a)->drop(root)->drop(leaf_a)...`

The solution is to use an alternative automatic garbage collector instead of compiler built-in: arenas, RC with weak references, or Mark&Sweep GC. Arenas have better performance, because they drop all nodes at once. The easiest way to quickly implement an arena in safe Rust is to use vector (array) of nodes and used indices instead of direct references.