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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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8
compiler/rustc_borrowck/src/def_use.rs
View file

@ -52,12 +52,16 @@ pub fn categorize(context: PlaceContext) -> Option<DefUse> {
PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
// `PlaceMention` and `AscribeUserType` both evaluate the place, which must not
// contain dangling references.
PlaceContext::NonUse(NonUseContext::PlaceMention) |
PlaceContext::NonUse(NonUseContext::AscribeUserTy) |
PlaceContext::MutatingUse(MutatingUseContext::AddressOf) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
PlaceContext::NonUse(NonUseContext::AscribeUserTy) |
PlaceContext::MutatingUse(MutatingUseContext::Retag) =>
Some(DefUse::Use),
@ -72,8 +76,6 @@ pub fn categorize(context: PlaceContext) -> Option<DefUse> {
PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
Some(DefUse::Drop),
// This statement exists to help unsafeck. It does not require the place to be live.
PlaceContext::NonUse(NonUseContext::PlaceMention) => None,
// Debug info is neither def nor use.
PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,

2
compiler/rustc_borrowck/src/invalidation.rs
View file

@ -79,7 +79,7 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
}
// Only relevant for mir typeck
StatementKind::AscribeUserType(..)
// Only relevant for unsafeck
// Only relevant for liveness and unsafeck
| StatementKind::PlaceMention(..)
// Doesn't have any language semantics
| StatementKind::Coverage(..)

2
compiler/rustc_borrowck/src/lib.rs
View file

@ -665,7 +665,7 @@ impl<'cx, 'tcx> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtx
}
// Only relevant for mir typeck
StatementKind::AscribeUserType(..)
// Only relevant for unsafeck
// Only relevant for liveness and unsafeck
| StatementKind::PlaceMention(..)
// Doesn't have any language semantics
| StatementKind::Coverage(..)