You're correct, that crate doesn't solve the cycle problem. There is a different way of doing arenas in Rust that does though. What you do is you put all of yo

You're correct, that crate doesn't solve the cycle problem. There is a different way of doing arenas in Rust that does though. What you do is you put all of your tree nodes in a big Vec<Node> and instead of referring to children and parents via pointers, you do so via indices. It's less convenient because you have to pass around a reference to your "arena" everywhere (the Vec or a slice of it), and it incurs bounds checks (pretty cheap though). But, it solves the problem in a way that is guaranteed safe.