Comment by accelbred

Arenas aren't the issue. Its objects with mixed lifetimes and mutability. I cant easily model the lifetimes of objects when there are cases like an instance where buffer memory reachable from the object has a different lifetime than maps/lists. Also these objects could be transively shared or mutable. In order to make a Rust friendly model, I'd have the tree be all shared or all mutable, and have all reachable memory have the same lifetime. This would often mean allocating the full tree into one arena. That is where the overhead comes from. Each arena would need enough memory to store the entire object; currently they can be smaller since they often only need to hold parts of objects, not the entire thing.