Add metadata header defining bit mask

PiperOrigin-RevId: 639836653

common/internal/BUILD

@@ -101,3 +101,9 @@ cc_test(
         "@com_google_absl//absl/strings:string_view",
     ],
 )
+
+cc_library(
+    name = "metadata",
+    hdrs = ["metadata.h"],
+    deps = ["@com_google_protobuf//:protobuf"],
+)

common/internal/metadata.h

@@ -0,0 +1,41 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef THIRD_PARTY_CEL_CPP_COMMON_INTERNAL_METADATA_H_
+#define THIRD_PARTY_CEL_CPP_COMMON_INTERNAL_METADATA_H_
+
+#include <cstdint>
+
+#include "google/protobuf/arena.h"
+
+namespace cel::common_internal {
+
+// `google::protobuf::Arena` has a minimum alignment of 8. `ReferenceCount` has a minimum
+// alignment that is guaranteed to be greater than or equal to `google::protobuf::Arena`.
+inline constexpr uintptr_t kMetadataOwnerNone = 0;
+inline constexpr uintptr_t kMetadataOwnerReferenceCountBit = uintptr_t{1} << 0;
+inline constexpr uintptr_t kMetadataOwnerArenaBit = uintptr_t{1} << 1;
+inline constexpr uintptr_t kMetadataOwnerBits = alignof(google::protobuf::Arena) - 1;
+inline constexpr uintptr_t kMetadataOwnerPointerMask = ~kMetadataOwnerBits;
+
+// Ensure kMetadataOwnerBits encompasses kMetadataOwnerReferenceCountBit and
+// kMetadataOwnerArenaBit.
+static_assert((kMetadataOwnerBits | kMetadataOwnerReferenceCountBit) ==
+              kMetadataOwnerBits);
+static_assert((kMetadataOwnerBits | kMetadataOwnerArenaBit) ==
+              kMetadataOwnerBits);
+
+}  // namespace cel::common_internal
+
+#endif  // THIRD_PARTY_CEL_CPP_COMMON_INTERNAL_METADATA_H_