Make if and while conditions temporary

[flaper87]

Closes #12033

IR Before:

normal-return:                                    ; preds = %while_body
  %113 = load i64* %i
  %114 = sub i64 %113, 1
  store i64 %114, i64* %i
  br label %while_cond

IR After:

normal-return:                                    ; preds = %while_cond
  store i8 %11, i8* %0
  %18 = load i8* %0, !range !0
  call void @_ZN9Temporary9glue_drop19he4ee51d3c03b9cf4ajE(%struct.Temporary* %10)
  %19 = bitcast %struct.Temporary* %10 to i8*
  call void @_ZN2rt11global_heap14exchange_free_19h4fabdf24a2250163aj4v0.0E(i8* %19)
  %20 = icmp ne i8 %18, 0
  br i1 %20, label %while_body, label %while_exit

[flaper87]

@nikomatsakis r?

[huonw]

Is there a particular reason that the test case doesn't look anything like #12033 at all? (And also, isn't the while part useless, since it's not actually checking that it was dropped inside the loop?)

[flaper87]

@huonw not sure what you mean with "it doesn't look like #12033 at all"

Also, it's not necessary to check at every iteration, although we could. Before the fix, just 1 Temporary was freed at the end of the loop. After the fix, all Temporarys are freed.

[huonw]

Ah, I see; I still think that ensuring that it's dropped inside the loop would be better: make test conservative as possible.

What I mean by being different is: the code in #12033 is demonstrating the borrow checker, while this doesn't test borrowck at all. (Calling a function borrow doesn't make it a borrow automatically. :) )

... But niko has r+'d so it's presumably ok.

[huonw]

Ah, wait! I think I misread the bug.