Yeah, but if you actually need to retain a live subgraph of the allocated heap, the arena can't help you. So you make an arena allocator that only frees its sla

Yeah, but if you actually need to retain a live subgraph of the allocated heap, the arena can't help you. So you make an arena allocator that only frees its slab after moving out the reachable set to a new compacted arena. Congratulations, you've implemented a Cheney-style compacting GC!