Restrict usages of TrivialTypeInfo to the singleton.

eval/public/BUILD

@@ -29,6 +29,13 @@ licenses(["notice"])
 
 exports_files(["LICENSE"])
 
+# No new users should depend on MessageWrapper using legacy APIs.
+# This was added as a temporary workaround for implementing custom reflection operations
+# on messages that were compiled with the LITE_RUNTIME (MessageLite) option.
+# Internally, we now assume legacy Message types are always normal (Message) protobuf messages with
+# standard reflection operations.
+# Custom structs (including MessageLite protos) should use the modern API (see
+# `cel::CustomStructValue`).
 cc_library(
     name = "message_wrapper",
     hdrs = [

eval/public/structs/BUILD

@@ -374,6 +374,7 @@ cc_library(
         ":legacy_type_info_apis",
         "//eval/public:message_wrapper",
         "@com_google_absl//absl/base:no_destructor",
+        "@com_google_absl//absl/base:nullability",
         "@com_google_absl//absl/strings:string_view",
     ],
 )

eval/public/structs/trivial_legacy_type_info.h

@@ -18,6 +18,7 @@
 #include <string>
 
 #include "absl/base/no_destructor.h"
+#include "absl/base/nullability.h"
 #include "absl/strings/string_view.h"
 #include "eval/public/message_wrapper.h"
 #include "eval/public/structs/legacy_type_info_apis.h"
@@ -27,7 +28,12 @@ namespace google::api::expr::runtime {
 // Implementation of type info APIs suitable for testing where no message
 // operations need to be supported.
 class TrivialTypeInfo : public LegacyTypeInfoApis {
+ private:
+  struct Key {};
+
  public:
+  explicit TrivialTypeInfo(Key&) {}
+
   absl::string_view GetTypename(const MessageWrapper& wrapper) const override {
     return "opaque";
   }
@@ -43,10 +49,14 @@ class TrivialTypeInfo : public LegacyTypeInfoApis {
     return nullptr;
   }
 
-  static const TrivialTypeInfo* GetInstance() {
-    static absl::NoDestructor<TrivialTypeInfo> kInstance;
+  static const TrivialTypeInfo* absl_nonnull GetInstance() {
+    static absl::NoDestructor<Key> kKey;
+    static absl::NoDestructor<TrivialTypeInfo> kInstance(*kKey);
     return &*kInstance;
   }
+
+ private:
+  TrivialTypeInfo() = default;
 };
 
 }  // namespace google::api::expr::runtime

eval/public/structs/trivial_legacy_type_info_test.cc

@@ -21,35 +21,27 @@ namespace google::api::expr::runtime {
 namespace {
 
 TEST(TrivialTypeInfo, GetTypename) {
-  TrivialTypeInfo info;
   MessageWrapper wrapper;
 
-  EXPECT_EQ(info.GetTypename(wrapper), "opaque");
   EXPECT_EQ(TrivialTypeInfo::GetInstance()->GetTypename(wrapper), "opaque");
 }
 
 TEST(TrivialTypeInfo, DebugString) {
-  TrivialTypeInfo info;
   MessageWrapper wrapper;
 
-  EXPECT_EQ(info.DebugString(wrapper), "opaque");
   EXPECT_EQ(TrivialTypeInfo::GetInstance()->DebugString(wrapper), "opaque");
 }
 
 TEST(TrivialTypeInfo, GetAccessApis) {
-  TrivialTypeInfo info;
   MessageWrapper wrapper;
 
-  EXPECT_EQ(info.GetAccessApis(wrapper), nullptr);
   EXPECT_EQ(TrivialTypeInfo::GetInstance()->GetAccessApis(wrapper), nullptr);
 }
 
 
 TEST(TrivialTypeInfo, FindFieldByName) {
-  TrivialTypeInfo info;
   MessageWrapper wrapper;
 
-  EXPECT_EQ(info.FindFieldByName("foo"), std::nullopt);
   EXPECT_EQ(TrivialTypeInfo::GetInstance()->FindFieldByName("foo"),
             std::nullopt);
 }