Collections: When removing keys, return which keys are actually removed

[josephjclark]

I know this is difficult to implement - but it would super useful, on delete, to know which keys were removed.

This could be controlled by a flag if there are performance concerns. Maybe sometimes we're happy to risk a big performance hit in exchange for knowing what will actually happen.

One approach would be to query all keys matching a pattern before the delete, then return that list. So it sort of predicts what it'll delete. I don't know how robust this is.