Note that trait aliases cannot be recursive

compiler/rustc_query_impl/src/keys.rs

@@ -178,6 +178,10 @@ impl Key for (DefId, Option<Ident>) {
     fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
         tcx.def_span(self.0)
     }
+    #[inline(always)]
+    fn key_as_def_id(&self) -> Option<DefId> {
+        Some(self.0)
+    }
 }
 
 impl Key for (DefId, LocalDefId, Ident) {

compiler/rustc_query_system/src/query/job.rs

@@ -600,16 +600,23 @@ pub(crate) fn report_cycle<'a>(
         ));
     }
 
-    if !stack.is_empty()
-        && stack.iter().all(|entry| {
-            entry.query.def_kind.map_or(false, |def_kind| {
-                matches!(def_kind, SimpleDefKind::TyAlias | SimpleDefKind::TraitAlias)
-            })
+    if stack.iter().all(|entry| {
+        entry.query.def_kind.map_or(false, |def_kind| {
+            matches!(def_kind, SimpleDefKind::TyAlias | SimpleDefKind::TraitAlias)
         })
-    {
-        err.note("type aliases cannot be recursive");
-        err.help("consider using a struct, enum, or union instead to break the cycle");
-        err.help("see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information");
+    }) {
+        if stack.iter().all(|entry| {
+            entry
+                .query
+                .def_kind
+                .map_or(false, |def_kind| matches!(def_kind, SimpleDefKind::TyAlias))
+        }) {
+            err.note("type aliases cannot be recursive");
+            err.help("consider using a struct, enum, or union instead to break the cycle");
+            err.help("see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information");
+        } else {
+            err.note("trait aliases cannot be recursive");
+        }
     }
 
     if let Some((span, query)) = usage {

src/test/ui/infinite/infinite-trait-alias-recursion.stderr

@@ -30,6 +30,7 @@ note: ...which requires computing the super traits of `T3`...
 LL | trait T3 = T1 + T3;
    |            ^^
    = note: ...which again requires computing the super predicates of `T1`, completing the cycle
+   = note: trait aliases cannot be recursive
 note: cycle used when collecting item types in top-level module
   --> $DIR/infinite-trait-alias-recursion.rs:3:1
    |