As for having tight coupling between moving GC and hashmap implementation there is another reason why you want to do that: weak pointers and hashmaps that are either weak keyed or weak valued which both are things that are useful for the VM implementation itself (symbol tables, method dispatch caches, identity maps for FFI...).
I believe the GP was referring to PyPy.
From https://morepypy.blogspot.co.uk/2009/10/gc-improvements.html
> The hash field is not necessarily there; it is only present in classes whose hash is ever taken in the RPython program (which includes being keys in a dictionary). It is an "identity hash": it works like object.__hash__() in Python, but it cannot just be the address of the object in case of a GC that moves objects around.
.NET at one point stored an extra per-object word, which was either (via its LSB) a random hash code or a pointer to a metadata object that held the lock, etc.
Python did this cute thing where moved objects would get an extra word allocated to store the moved-from address, which was where the hash was stored.
Apple's plan for its (not-released) ObjC moving GC was to directly teach the GC about hash tables, so it could rehash on collection (!).
Do you know how other implementations handle this?
BEAM's GC is indeed a marvel both because of the immutability and also the enforced process isolation, so that each process can do its own GC independently. As you say, constrain the mutator...