These collections are of somewhat limited use outside of concurrent scenarios, and I don't think you can (currently?) be generic over allocation in Rust. Since these structures need some sort of automatic memory management for structural sharing, you need to use a GC of some sort, having both atomic and non-atomic Rc would require duplicating the entire thing, once with Rc nodes and one with Arc node, for more or less no value.

> I may be misunderstanding the implications of Arc and how it is implemented, and how it behaves in the absence of multiple threads

All Arc means is that refcounting operations use atomic increment/decrement instead of regular ones.