Fix lifetime rules for 'if' conditions

[KiChjang]

Fixes #12033.

Changes the temporary scope rules to make the condition of an if-then-else a terminating scope. This is a [breaking-change].

[rust-highfive]

r? @arielb1

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[hanna-kruppe]

This doesn't seem to belong here.

[nagisa]

The issue title refers to fixing both if and while but there seem to be no changes or tests pertaining to the while expression. Is it intentional?

EDIT: reading the issue, its clear that only thing this PR ought to be fixing is if. Would be nice to have PR and commit titles updated correspondingly.

[KiChjang]

Whoops, I made a couple of stupids, now it should look fine. Not sure if it behaves fine though.

[arielb1]

@bors r+

[bors]

📌 Commit 4853456 has been approved by arielb1

[bors]

⌛ Testing commit 4853456 with merge 312734c...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-no-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-no-mir, auto-win-msvc-64-opt-rustbuild
Approved by: arielb1
Pushing 312734c to master...

[eddyb]

To explain how this can be a breaking-change, we can look at the RefCell-based examples: with if-else, temporaries in the conditions can live longer than variables above the if-else, whereas with just if, the "terminating" aspect means that temporaries live shorter than the variables they borrow from.

The opposite can be produced by having a local variable borrow the temporary instead:

struct Foo;

impl Foo {
    fn save<'a>(&'a self, dest: &mut &'a Foo) -> bool {
        *dest = self;
        true
    }
}

fn main() {
    let mut dest = &Foo;
    if Foo.save(&mut dest) {} else {}
}

This example compiles before this PR, but not if we remove else {}. With this PR, the if-else case behaves like the if case, so neither will compile, the temporary being to short to be borrowed by dest.

EDIT: The other breaking change here is destructor ordering, which affects all the code using if-else in the tail of a block, even if it continues to compile.
Temporaries in the condition expression of if and if-else will now always be dropped before branching based on the condition, and thus before all of the variables in the enclosing block.