I would say I don't (or now, didn't) expect collect to be able to know that there is an allocation under there, since that's not part of Iterator's interface.

I would say I don't (or now, didn't) expect collect to be able to know that there is an allocation under there, since that's not part of Iterator's interface. I.e., the docs could call out that there aren't such implied behaviors, and that in some circumstances, it might reuse the allocation, but that that's not guaranteed. (And ideally, offer me info on what to do if I do want certain guarantees about the allocation of the Vec.) Warding off what I think is a reasonable, but wrong, assumption, is well within the scope of the docs.