I entirely forgot to add Drop items to the used_mentioned_items fast path...

Author: RalfJung

compiler/rustc_monomorphize/src/collector.rs

@@ -721,6 +721,7 @@ struct MirUsedCollector<'a, 'tcx> {
     body: &'a mir::Body<'tcx>,
     used_items: &'a mut MonoItems<'tcx>,
     /// See the comment in `collect_items_of_instance` for the purpose of this set.
+    /// Note that this contains *not-monomorphized* items!
     used_mentioned_items: &'a mut FxHashSet<MentionedItem<'tcx>>,
     instance: Instance<'tcx>,
     /// Spans for move size lints already emitted. Helps avoid duplicate lints.
@@ -1002,6 +1003,8 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
             }
             mir::TerminatorKind::Drop { ref place, .. } => {
                 let ty = place.ty(self.body, self.tcx).ty;
+                // *Before* monomorphizing, record that we already handled this mention.
+                self.used_mentioned_items.insert(MentionedItem::Drop(ty));
                 let ty = self.monomorphize(ty);
                 visit_drop_use(self.tcx, ty, true, source, self.used_items);
             }