Auto merge of #102256 - cjgillot:let-under, r=lcnr

Introduce a no-op `PlaceMention` statement for `let _ =`.

Fixes https://github.com/rust-lang/rust/issues/54003
Fixes https://github.com/rust-lang/rust/issues/80059
Split from https://github.com/rust-lang/rust/pull/101500

This PR introduces a new `PlaceMention` statement dedicated to matches that neither introduce bindings nor ascribe types.  Without this, all traces of the match would vanish from MIR, making it impossible to diagnose unsafety or use in #101500.

This allows to mark `let _ = <unsafe union access or dereference>` as requiring an unsafe block.
Nominating for lang team, as this introduces an extra error.

9 files changed, 17 insertions, 3 deletions

diff --git a/compiler/rustc_mir_transform/src/check_unsafety.rs b/compiler/rustc_mir_transform/src/check_unsafety.rs
index c61947c566d..a8ec568eb0d 100644
--- a/compiler/rustc_mir_transform/src/check_unsafety.rs
+++ b/compiler/rustc_mir_transform/src/check_unsafety.rs
@@ -100,13 +100,16 @@ impl<'tcx> Visitor<'tcx> for UnsafetyChecker<'_, 'tcx> {
             | StatementKind::StorageLive(..)
             | StatementKind::StorageDead(..)
             | StatementKind::Retag { .. }
-            | StatementKind::AscribeUserType(..)
+            | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
             | StatementKind::Nop => {
                 // safe (at least as emitted during MIR construction)
             }
+            // `AscribeUserType` just exists to help MIR borrowck.
+            // It has no semantics, and everything is already reported by `PlaceMention`.
+            StatementKind::AscribeUserType(..) => return,
         }
         self.super_statement(statement, location);
     }
diff --git a/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs b/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
index d435d3ee69b..0923824db48 100644
--- a/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
+++ b/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs
@@ -24,6 +24,7 @@ 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(),
                     _ => (),
diff --git a/compiler/rustc_mir_transform/src/coverage/spans.rs b/compiler/rustc_mir_transform/src/coverage/spans.rs
index d9f68f5d93a..2f120258659 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans.rs
@@ -832,6 +832,7 @@ pub(super) fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span>
         | StatementKind::SetDiscriminant { .. }
         | StatementKind::Deinit(..)
         | StatementKind::Retag(_, _)
+        | StatementKind::PlaceMention(..)
         | StatementKind::AscribeUserType(_, _) => {
             Some(statement.source_info.span)
         }
diff --git a/compiler/rustc_mir_transform/src/dead_store_elimination.rs b/compiler/rustc_mir_transform/src/dead_store_elimination.rs
index 9dbfb089dc6..18c407b42d3 100644
--- a/compiler/rustc_mir_transform/src/dead_store_elimination.rs
+++ b/compiler/rustc_mir_transform/src/dead_store_elimination.rs
@@ -56,7 +56,9 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
                 | StatementKind::ConstEvalCounter
                 | StatementKind::Nop => (),
 
-                StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
+                StatementKind::FakeRead(_)
+                | StatementKind::PlaceMention(_)
+                | StatementKind::AscribeUserType(_, _) => {
                     bug!("{:?} not found in this MIR phase!", &statement.kind)
                 }
             }
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs
index 3823b6201c3..7344ec793ea 100644
--- a/compiler/rustc_mir_transform/src/dest_prop.rs
+++ b/compiler/rustc_mir_transform/src/dest_prop.rs
@@ -583,7 +583,9 @@ impl WriteInfo {
             | StatementKind::Coverage(_)
             | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_) => (),
-            StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
+            StatementKind::FakeRead(_)
+            | StatementKind::AscribeUserType(_, _)
+            | StatementKind::PlaceMention(_) => {
                 bug!("{:?} not found in this MIR phase", statement)
             }
         }
diff --git a/compiler/rustc_mir_transform/src/generator.rs b/compiler/rustc_mir_transform/src/generator.rs
index 129f366bcd0..b7f1cdfc7f2 100644
--- a/compiler/rustc_mir_transform/src/generator.rs
+++ b/compiler/rustc_mir_transform/src/generator.rs
@@ -1647,6 +1647,7 @@ impl<'tcx> Visitor<'tcx> for EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
             | StatementKind::StorageDead(_)
             | StatementKind::Retag(..)
             | StatementKind::AscribeUserType(..)
+            | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
diff --git a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
index 6c067f45299..e962819b691 100644
--- a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
+++ b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
@@ -33,6 +33,7 @@ impl RemoveNoopLandingPads {
                 StatementKind::FakeRead(..)
                 | StatementKind::StorageLive(_)
                 | StatementKind::StorageDead(_)
+                | StatementKind::PlaceMention(..)
                 | StatementKind::AscribeUserType(..)
                 | StatementKind::Coverage(..)
                 | StatementKind::ConstEvalCounter
diff --git a/compiler/rustc_mir_transform/src/separate_const_switch.rs b/compiler/rustc_mir_transform/src/separate_const_switch.rs
index c3f5b881ab8..d76ab95faba 100644
--- a/compiler/rustc_mir_transform/src/separate_const_switch.rs
+++ b/compiler/rustc_mir_transform/src/separate_const_switch.rs
@@ -245,6 +245,7 @@ fn is_likely_const<'tcx>(mut tracked_place: Place<'tcx>, block: &BasicBlockData<
             | StatementKind::StorageLive(_)
             | StatementKind::Retag(_, _)
             | StatementKind::AscribeUserType(_, _)
+            | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(_)
             | StatementKind::StorageDead(_)
             | StatementKind::Intrinsic(_)
@@ -315,6 +316,7 @@ fn find_determining_place<'tcx>(
             | StatementKind::StorageDead(_)
             | StatementKind::Retag(_, _)
             | StatementKind::AscribeUserType(_, _)
+            | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(_)
             | StatementKind::Intrinsic(_)
             | StatementKind::ConstEvalCounter
diff --git a/compiler/rustc_mir_transform/src/simplify.rs b/compiler/rustc_mir_transform/src/simplify.rs
index 9ef55c558c6..929d229dcdf 100644
--- a/compiler/rustc_mir_transform/src/simplify.rs
+++ b/compiler/rustc_mir_transform/src/simplify.rs
@@ -525,6 +525,7 @@ impl<'tcx> Visitor<'tcx> for UsedLocals {
             | StatementKind::Retag(..)
             | StatementKind::Coverage(..)
             | StatementKind::FakeRead(..)
+            | StatementKind::PlaceMention(..)
             | StatementKind::AscribeUserType(..) => {
                 self.super_statement(statement, location);
             }