Re: labeling MVar and STM data structures

Hi Ben,

This is generally in the direction I proposed in #21877; I agree that it would be quite useful. I don't think that we necessarily want to extend all MVars with labels as this would impose a fixed memory cost on all users. However, I think the approach that I describe in the ticket, introducing an out-of-band map for labelling heap objects, would be a reasonable way forward.

I support this. Thanks. --Kazu