This question is a bit subtle, it depends on exactly what you mean. You could make a tree using only borrow checked references and the compiler would make sure that parent nodes go out of scope at the same time or before the child nodes they point to, but I don't think that's what you're talking about.

In general, if it's a datastructure where you have to use pointers, you'll have them Box'ed, but you would try to avoid that if you can. In your example of a heap, you'd want to use an array-based implementation, probably backed by a growable Vec, and use indexes internally. A peek function would still return a normal Rust reference to the data, and the borrow checker would make sure that you don't mutate the heap's backing array while that reference was still in use, etc.