Fix compile error in rustdoc.

Author: frank-king

Diff for: src/librustdoc/clean/mod.rs

@@ -1892,15 +1892,8 @@ pub(crate) fn clean_ty<'tcx>(ty: &hir::Ty<'tcx>, cx: &mut DocContext<'tcx>) -> T
         TyKind::BareFn(barefn) => BareFunction(Box::new(clean_bare_fn_ty(barefn, cx))),
         // Rustdoc handles `TyKind::Err`s by turning them into `Type::Infer`s.
         TyKind::Infer | TyKind::Err(_) | TyKind::Typeof(..) | TyKind::InferDelegation(..) => Infer,
-        TyKind::AnonAdt(item_id) => {
-            let path = external_path(
-                cx,
-                item_id.owner_id.def_id.to_def_id(),
-                false,
-                ThinVec::new(),
-                ty::Binder::dummy(ty::GenericArgs::empty()),
-            );
-            Type::Path { path }
+        TyKind::AnonAdt(..) => {
+            unimplemented!("Anonymous structs or unions are not supported yet")
         }
     }
 }