I believe Erlang makes this guarantee and exploits it in the GC: ...the terms are immutable and thus there can be no pointers modified on the old heap to point

I believe Erlang makes this guarantee and exploits it in the GC: ...the terms are immutable and thus there can be no pointers modified on the old heap to point to the young heap so old objects cannot refer to young objects. This means it doesn't have to deal with write barriers, etc.