Rust is getting support for local allocators/arenas as part of its core library. So it could become quite feasible to get support for tracing and garbage collection within a specific arena, with outside "GC roots" being managed by the borrow checker. You'd be fine as long as nothing in there tried to link back to your "GC roots". Similarly, "leaf" allocations could also live outside the GC arena provided that they did not try to link back into it. Either of these constraints seems like it could be checked during compile.