diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 340c5a769dbc4..8bea404ccf213 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -339,7 +339,7 @@ rustc_queries! {
 
     query opaque_types_defined_by(
         key: LocalDefId
-    ) -> &'tcx [LocalDefId] {
+    ) -> &'tcx ty::List<LocalDefId> {
         desc {
             |tcx| "computing the opaque types defined by `{}`",
             tcx.def_path_str(key.to_def_id())
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index f7bce93fee55d..80620efb0b310 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -157,6 +157,7 @@ pub struct CtxtInterners<'tcx> {
     external_constraints: InternedSet<'tcx, ExternalConstraintsData<'tcx>>,
     predefined_opaques_in_body: InternedSet<'tcx, PredefinedOpaquesData<'tcx>>,
     fields: InternedSet<'tcx, List<FieldIdx>>,
+    local_def_ids: InternedSet<'tcx, List<LocalDefId>>,
 }
 
 impl<'tcx> CtxtInterners<'tcx> {
@@ -182,6 +183,7 @@ impl<'tcx> CtxtInterners<'tcx> {
             external_constraints: Default::default(),
             predefined_opaques_in_body: Default::default(),
             fields: Default::default(),
+            local_def_ids: Default::default(),
         }
     }
 
@@ -1568,6 +1570,7 @@ slice_interners!(
     place_elems: pub mk_place_elems(PlaceElem<'tcx>),
     bound_variable_kinds: pub mk_bound_variable_kinds(ty::BoundVariableKind),
     fields: pub mk_fields(FieldIdx),
+    local_def_ids: intern_local_def_ids(LocalDefId),
 );
 
 impl<'tcx> TyCtxt<'tcx> {
@@ -1798,6 +1801,13 @@ impl<'tcx> TyCtxt<'tcx> {
         self.intern_clauses(clauses)
     }
 
+    pub fn mk_local_def_ids(self, clauses: &[LocalDefId]) -> &'tcx List<LocalDefId> {
+        // FIXME consider asking the input slice to be sorted to avoid
+        // re-interning permutations, in which case that would be asserted
+        // here.
+        self.intern_local_def_ids(clauses)
+    }
+
     pub fn mk_const_list_from_iter<I, T>(self, iter: I) -> T::Output
     where
         I: Iterator<Item = T>,
diff --git a/compiler/rustc_ty_utils/src/opaque_types.rs b/compiler/rustc_ty_utils/src/opaque_types.rs
index 06a30677d2005..82c4f38511efb 100644
--- a/compiler/rustc_ty_utils/src/opaque_types.rs
+++ b/compiler/rustc_ty_utils/src/opaque_types.rs
@@ -269,7 +269,10 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
     }
 }
 
-fn opaque_types_defined_by<'tcx>(tcx: TyCtxt<'tcx>, item: LocalDefId) -> &'tcx [LocalDefId] {
+fn opaque_types_defined_by<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    item: LocalDefId,
+) -> &'tcx ty::List<LocalDefId> {
     let kind = tcx.def_kind(item);
     trace!(?kind);
     let mut collector = OpaqueTypeCollector::new(tcx, item);
@@ -327,7 +330,7 @@ fn opaque_types_defined_by<'tcx>(tcx: TyCtxt<'tcx>, item: LocalDefId) -> &'tcx [
             return tcx.opaque_types_defined_by(tcx.local_parent(item));
         }
     }
-    tcx.arena.alloc_from_iter(collector.opaques)
+    tcx.mk_local_def_ids(&collector.opaques)
 }
 
 pub(super) fn provide(providers: &mut Providers) {