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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_mir_transform/src/lib.rs
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@ -48,6 +48,7 @@ mod add_retag;
mod check_const_item_mutation;
mod check_packed_ref;
pub mod check_unsafety;
mod remove_place_mention;
// This pass is public to allow external drivers to perform MIR cleanup
pub mod cleanup_post_borrowck;
mod const_debuginfo;
@ -460,8 +461,11 @@ fn run_runtime_lowering_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
/// Returns the sequence of passes that do the initial cleanup of runtime MIR.
fn run_runtime_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let passes: &[&dyn MirPass<'tcx>] =
&[&lower_intrinsics::LowerIntrinsics, &simplify::SimplifyCfg::ElaborateDrops];
let passes: &[&dyn MirPass<'tcx>] = &[
&lower_intrinsics::LowerIntrinsics,
&remove_place_mention::RemovePlaceMention,
&simplify::SimplifyCfg::ElaborateDrops,
];
pm::run_passes(tcx, body, passes, Some(MirPhase::Runtime(RuntimePhase::PostCleanup)));