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

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`

compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs

@@ -24,7 +24,6 @@ impl<'tcx> MirPass<'tcx> for CleanupPostBorrowck {
             for statement in basic_block.statements.iter_mut() {
                 match statement.kind {
                     StatementKind::AscribeUserType(..)
-                    | StatementKind::PlaceMention(..)
                     | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Shallow, _)))
                     | StatementKind::FakeRead(..) => statement.make_nop(),
                     _ => (),

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -54,11 +54,10 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
                 | StatementKind::Coverage(_)
                 | StatementKind::Intrinsic(_)
                 | StatementKind::ConstEvalCounter
+                | StatementKind::PlaceMention(_)
                 | StatementKind::Nop => (),
 
-                StatementKind::FakeRead(_)
-                | StatementKind::PlaceMention(_)
-                | StatementKind::AscribeUserType(_, _) => {
+                StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
                     bug!("{:?} not found in this MIR phase!", &statement.kind)
                 }
             }

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -582,10 +582,9 @@ impl WriteInfo {
             | StatementKind::Nop
             | StatementKind::Coverage(_)
             | StatementKind::StorageLive(_)
-            | StatementKind::StorageDead(_) => (),
-            StatementKind::FakeRead(_)
-            | StatementKind::AscribeUserType(_, _)
-            | StatementKind::PlaceMention(_) => {
+            | StatementKind::StorageDead(_)
+            | StatementKind::PlaceMention(_) => (),
+            StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
                 bug!("{:?} not found in this MIR phase", statement)
             }
         }

compiler/rustc_mir_transform/src/lib.rs

@@ -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)));
 

compiler/rustc_mir_transform/src/remove_place_mention.rs

@@ -0,0 +1,23 @@
+//! This pass removes `PlaceMention` statement, which has no effect at codegen.
+
+use crate::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+pub struct RemovePlaceMention;
+
+impl<'tcx> MirPass<'tcx> for RemovePlaceMention {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        !sess.opts.unstable_opts.mir_keep_place_mention
+    }
+
+    fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        trace!("Running RemovePlaceMention on {:?}", body.source);
+        for data in body.basic_blocks.as_mut_preserves_cfg() {
+            data.statements.retain(|statement| match statement.kind {
+                StatementKind::PlaceMention(..) | StatementKind::Nop => false,
+                _ => true,
+            })
+        }
+    }
+}