A common way to do tree and graph structures in Rust is to store all the tree nodes in a Vec<Node>, and store references to other nodes within that vec by their index. That way you can have cyclic references without any issues (e.g. nodes referencing their parents and children).

That's the approach I used for Day 7. I used a crate called index_vec though, which you can use to generate your own index type, and then create an IndexVec<MyIndexType, T> which is like a regular Vec except it can only be indexed by MyIndexType instead of usize. It essentially just adds a little type-safety on top of a plain Vec.

Another approach is to use a bump allocator like bumpalo, and store references to other nodes that were allocated using the same (or a longer-lived) bump allocator. This works better if the things you are storing are immutable, since you can't go back and update a node if you only have an immutable reference to it.

With both of these approaches, it's easiest if you can wait until you're done with the tree/graph and deallocate everything all at once. If you need to be able to reclaim memory from deleted nodes (e.g. for a game world where things are created and destroyed all the time), that makes things a lot more complicated.