Evaluate place expression in `PlaceMention`.

5 files changed, 17 insertions, 8 deletions

diff --git a/compiler/rustc_borrowck/src/def_use.rs b/compiler/rustc_borrowck/src/def_use.rs
index 9e9f0b4b4ad..6259722b694 100644
--- a/compiler/rustc_borrowck/src/def_use.rs
+++ b/compiler/rustc_borrowck/src/def_use.rs
@@ -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,
 
diff --git a/compiler/rustc_borrowck/src/invalidation.rs b/compiler/rustc_borrowck/src/invalidation.rs
index 498d254da65..06986f848bf 100644
--- a/compiler/rustc_borrowck/src/invalidation.rs
+++ b/compiler/rustc_borrowck/src/invalidation.rs
@@ -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(..)
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 6a5a7e08d38..5bf3e7632ac 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -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(..)
diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 9a366364e76..f9c15c50513 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -113,8 +113,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Intrinsic(box intrinsic) => self.emulate_nondiverging_intrinsic(intrinsic)?,
 
-            // Statements we do not track.
-            PlaceMention(..) | AscribeUserType(..) => {}
+            // Evaluate the place expression, without reading from it.
+            PlaceMention(box place) => {
+                let _ = self.eval_place(*place)?;
+            }
+
+            // This exists purely to guide borrowck lifetime inference, and does not have
+            // an operational effect.
+            AscribeUserType(..) => {}
 
             // Currently, Miri discards Coverage statements. Coverage statements are only injected
             // via an optional compile time MIR pass and have no side effects. Since Coverage
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index c38a347809f..303c120f795 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -331,7 +331,8 @@ pub enum StatementKind<'tcx> {
     /// This is especially useful for `let _ = PLACE;` bindings that desugar to a single
     /// `PlaceMention(PLACE)`.
     ///
-    /// When executed at runtime this is a nop.
+    /// When executed at runtime, this computes the given place, but then discards
+    /// it without doing a load. It is UB if the place is not pointing to live memory.
     ///
     /// Disallowed after drop elaboration.
     PlaceMention(Box<Place<'tcx>>),