> It isn't acceptable to just permit memory-leaks in case of cycles.

That's only in the new --gc:arc which is still somewhat experimental. The old gc, --gc:markandsweep IIRC, does detect cycles, and has a deadline schedule (i.e. you can tell it "I now have 5ms, collect as much as you can"); But it has per-thread heaps, which means passing data between threads often entails a copy. You also have --gc:boehm (no cycles, shared heap), and --gc:none (no gc, you should take care of freeing memory yourself).