Auto merge of #1330 - RalfJung:retag-return-place, r=RalfJung

Auto merge of #1330 - RalfJung:retag-return-place, r=RalfJung

retag return place

@eddyb suggested that return places should be treated like unique references for Stacked Borrows. That is implemented by this patch, but it is unfortunately quite the hack because otherwise we are retagging *references*, not places.

@eddyb does this roughly correspond to what you had in mind? (Except for whatever it is you think should happen with argument passing, which is a much bigger issue.) Also, do you think there is any way we can *test* this?

Needs https://github.com/rust-lang/rust/pull/71100 to land.