There is no way to implement a dummy sweep that has the behavior of a real sweep if the collector is doing compaction. Maybe it isn't obvious: compaction isn't

There is no way to implement a dummy sweep that has the behavior of a real sweep if the collector is doing compaction. Maybe it isn't obvious: compaction isn't a plug-in that you put into a collector. It's a different way of writing a collector. For example, compacting collectors often avoid having any mark bits. You need mark bits to sweep (among other things).