tweak temporary lifetime rules for if and while conditions

[nikomatsakis]

I think that if and while conditions should free any temporaries that occur within them (for while conditions it's mandatory, actually, since the condition is evaluated many times).

That is, if I write:

if my_set().borrow().contains(&key) { ... }

the temporary returned by borrow() should be freed once the condition is evaluated, before entering the body of the if.

The same holds for whiles.

What makes these expressions different from other compound forms is that the conditions are evaluated to scalar booleans, and hence can never create lasting aliases that might need to be preserved.

In the case of a while, this is needed because temporaries can never outlive conditional or repeated scopes, and the condition of a while is a repeated scope.

[flaper87]

cc @flaper87

[nikomatsakis]

Marking as mentor with myself. This is a fairly small change (if you know what to change :)

[flaper87]

I'll work on it :)

[pnkfelix]

1.0 blocker, P-backcompat-lang.

[eddyb]

The fix in #12134 was partial, if with an else is still broken:

use std::cell::RefCell;

fn main() {
    let x = RefCell::new(0);
    if *x.borrow() == 0 {} else {} // error: `x` does not live long enough
}

[nikomatsakis]

Nominated. Seems clear to me that this is a bug we should fix -- the main question is whether to be concerned about backwards compatibility, and -- if so -- what to do about it.

I think this is mostly a question for @rust-lang/compiler team to decide, but I tagged this issue with T-Lang too in case @rust-lang/lang members want to weight in on whether the behavior is indeed a bug. But it seems clear that having if and if/else behave differently is bogus.

[nikomatsakis]

Consensus in the compiler team meeting was that we should just fix this and add the resulting PR to the release notes.

triage: P-medium