The methods Generation::performs_in_place_marking() is documented as that it should be used by biased locking. But it is currently unused and a "hg grep performs_in_place_marking" does not indicate that it was ever used.

Currently it is only used in two asserts, but these operate on a young or an old generation instance, where performs_in_place_marking() adds no value since it returns true for old generations and false for young.