Tracking issue for VecDeque::retain

[alexcrichton]

This is a tracking issue for the unstable vec_deque_retain feature in the standard library. This is probably ready to just be stabilized as-is.

cc @gankro

[Gankra]

I still maintain this is a fake method that doesn't actually exist. But yeah, seems fine.

[alexcrichton]

This feature is now entering its final comment period for stabilization.

[frankmcsherry]

random comment: given that drop is safe and pre-pooping is an understood technology, retain implementations generally could set their lens to zero and then run without doing all the mem::swap operations they seem to like to do.

[Gankra]

That seems fine, though something we should avoid when possible if not necessary.