> though it will break if you need the same object used across two disjoint subtrees of the calll tree

Yes, while I prefer to use plain values or unique_ptrs when I can, I will use C++ references (or plain pointers) in this sort of situation. Basically in C++ I think an of old-style pointers/refs as Rust-like "borrowed" references.

This is an example of how the Rust safety model is really about compiler enforcement of good practices from C and C++ (as opposed to, say, GC which is a distinct practice). I appreciate what Rust is doing, but it is also true that good coding conventions in C++ will get you 90% of the way there.