detect cycle in type structure

compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs

@@ -221,6 +221,7 @@ fn extract_component_raw<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     ty: Ty<'tcx>,
+    ty_seen: &mut UnordSet<Ty<'tcx>>,
 ) -> SmallVec<[Ty<'tcx>; 4]> {
     // Droppiness does not depend on regions, so let us erase them.
     let ty = tcx.try_normalize_erasing_regions(param_env, ty).unwrap_or(ty);
@@ -232,10 +233,14 @@ fn extract_component_raw<'tcx>(
         if let Some(tys) = true_significant_drop_ty(tcx, ty) {
             // Some types can be further opened up because the drop is simply delegated
             for ty in tys {
-                out_tys.extend(extract_component_raw(tcx, param_env, ty));
+                if ty_seen.insert(ty) {
+                    out_tys.extend(extract_component_raw(tcx, param_env, ty, ty_seen));
+                }
             }
         } else {
-            out_tys.push(ty);
+            if ty_seen.insert(ty) {
+                out_tys.push(ty);
+            }
         }
     }
     out_tys
@@ -247,7 +252,7 @@ fn extract_component_with_significant_dtor<'tcx>(
     param_env: ty::ParamEnv<'tcx>,
     ty: Ty<'tcx>,
 ) -> SmallVec<[Ty<'tcx>; 4]> {
-    let mut tys = extract_component_raw(tcx, param_env, ty);
+    let mut tys = extract_component_raw(tcx, param_env, ty, &mut Default::default());
     let mut deduplicate = FxHashSet::default();
     tys.retain(|oty| deduplicate.insert(*oty));
     tys.into_iter().collect()