I just want to point out that this optimization should also help Query.jl a lot, I also iterate like mad over small immutables that can hold references to heap allocated stuff. I just tried to find an issue or PR that gives the latest status/efforts/plans on that front but didn’t really find something. I’d be quite interested in learning about the current plans on this, any pointer would be most welcome.
This is definitely something I am looking forward to as well, but it requires someone with a deep understanding of how Julia GC is working and requires some non trivial work (e.g. stack scanning). AFAIK nobody is currently actively working on it.