I think the correct long-run idiomatic solution would be exposing the building blocks of what collect does, so that users can easily write code that assumes a certain type (as @jeff.bezanson said, like a size hint, possibly deduced from the first element obtained) yet seamlessly go to a different branch if that assumption is violated. When the compiler knows a lot, if can prune these branches and emit very efficient code, but even the worst case heuristic fallbacks (widen as necessary) are not that bad.
I have used one component of this in the example code here, and I am working on a strategy that does something similar for “pre-allocated” buffers for output types, but that needs quite a bit of polish.