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Auto merge of #104844 - cjgillot:mention-eval-place, r=jackh726,RalfJung

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Evaluate place expression in `PlaceMention`

https://github.com/rust-lang/rust/pull/102256 introduces a `PlaceMention(place)` MIR statement which keep trace of `let _ = place` statements from surface rust, but without semantics.

This PR proposes to change the behaviour of `let _ =` patterns with respect to the borrow-checker to verify that the bound place is live.

Specifically, consider this code:
```rust
let _ = {
    let a = 5;
    &a
};
```

This passes borrowck without error on stable. Meanwhile, replacing `_` by `_: _` or `_p` errors with "error[E0597]: `a` does not live long enough", [see playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c448d25a7c205dc95a0967fe96bccce8).

This PR *does not* change how `_` patterns behave with respect to initializedness: it remains ok to bind a moved-from place to `_`.

The relevant test is `tests/ui/borrowck/let_underscore_temporary.rs`. Crater check found no regression.

For consistency, this PR changes miri to evaluate the place found in `PlaceMention`, and report eventual dangling pointers found within it.

r? `@RalfJung`
This commit is contained in:
bors 2023-04-22 09:54:21 +00:00
commit 21fab435da
22 changed files with 201 additions and 39 deletions
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9
compiler/rustc_const_eval/src/transform/validate.rs
View file

@ -802,14 +802,6 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
}
}
}
StatementKind::PlaceMention(..) => {
if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
self.fail(
location,
"`PlaceMention` should have been removed after drop lowering phase",
);
}
}
StatementKind::AscribeUserType(..) => {
if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
self.fail(
@ -919,6 +911,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
StatementKind::StorageDead(_)
| StatementKind::Coverage(_)
| StatementKind::ConstEvalCounter
| StatementKind::PlaceMention(..)
| StatementKind::Nop => {}
}