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

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -690,6 +690,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_)
             | StatementKind::Retag { .. }
+            | StatementKind::PlaceMention(..)
             | StatementKind::AscribeUserType(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)

compiler/rustc_const_eval/src/transform/validate.rs

@@ -679,6 +679,14 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     }
                 }
             }
+            StatementKind::PlaceMention(..) => {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
+                    self.fail(
+                        location,
+                        "`PlaceMention` should have been removed after drop lowering phase",
+                    );
+                }
+            }
             StatementKind::AscribeUserType(..) => {
                 if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(