Bypass const_item_mutation if const's type has Drop impl

Author: dtolnay

compiler/rustc_mir/src/transform/check_const_item_mutation.rs

@@ -31,6 +31,15 @@ impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
             None
         }
     }
+
+    fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> {
+        let def_id = self.is_const_item(local)?;
+        match self.tcx.adt_def(def_id).destructor(self.tcx) {
+            Some(_) => None,
+            None => Some(def_id),
+        }
+    }
+
     fn lint_const_item_usage(
         &self,
         const_item: DefId,
@@ -59,7 +68,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
             // Assigning directly to a constant (e.g. `FOO = true;`) is a hard error,
             // so emitting a lint would be redundant.
             if !lhs.projection.is_empty() {
-                if let Some(def_id) = self.is_const_item(lhs.local) {
+                if let Some(def_id) = self.is_const_item_without_destructor(lhs.local) {
                     // Don't lint on writes through a pointer
                     // (e.g. `unsafe { *FOO = 0; *BAR.field = 1; }`)
                     if !matches!(lhs.projection.last(), Some(PlaceElem::Deref)) {
@@ -89,7 +98,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, loc: Location) {
         if let Rvalue::Ref(_, BorrowKind::Mut { .. }, place) = rvalue {
             let local = place.local;
-            if let Some(def_id) = self.is_const_item(local) {
+            if let Some(def_id) = self.is_const_item_without_destructor(local) {
                 // If this Rvalue is being used as the right-hand side of a
                 // `StatementKind::Assign`, see if it ends up getting used as
                 // the `self` parameter of a method call (as the terminator of our current