retag return place

[RalfJung]

@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 rust-lang/rust#71100 to land.

[eddyb]

Also, do you think there is any way we can test this?

You want to a raw pointer to some place (so that borrowck doesn't catch it) then place = foo().
But it might not be possible to make the call terminator not use a temporary, without NRVO.

[RalfJung]

Yeah, this creates a temporary:

fn foo(_ptr: *mut i32) -> i32 { 4 }

fn main() {
    let mut ret = 0i32;
    let ptr = &mut ret as *mut _;
    ret = foo(ptr);
}

[eddyb]

I'm not entirely sure why we do that, don't we make temporaries for all the non-constant args?
Feels like overkill. cc @nikomatsakis

[RalfJung]

@bors r+

[bors]

📌 Commit 3548dcf has been approved by RalfJung

[bors]

⌛ Testing commit 3548dcf with merge fbea3e5...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: RalfJung
Pushing fbea3e5 to master...