Use [[maybe_unused]] instead of PROTOBUF_UNUSED.

With C++17 being the baseline, we should use [[maybe_unused]]
to shrink port_def.inc.

PiperOrigin-RevId: 693862958

Author: protobuf-github-bot

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs

@@ -855,7 +855,7 @@ void FileGenerator::GenerateStaticInitializer(io::Printer* p) {
           }}},
         R"cc(
           PROTOBUF_ATTRIBUTE_INIT_PRIORITY$priority$ static ::std::false_type
-              _static_init$priority$_ PROTOBUF_UNUSED =
+              _static_init$priority$_ [[maybe_unused]] =
                   ($expr$, ::std::false_type{});
         )cc");
     // Reset the vector because we might be generating many files.

src/google/protobuf/compiler/cpp/file.cc

@@ -4807,7 +4807,7 @@ DescriptorBuilder::DescriptorBuilder(
   // have to avoid registering these pre-main, because we need to ensure that
   // the linker --gc-sections step can strip out the full runtime if it is
   // unused.
-  PROTOBUF_UNUSED static std::true_type lazy_register =
+  [[maybe_unused]] static std::true_type lazy_register =
       (internal::ExtensionSet::RegisterMessageExtension(
            &FeatureSet::default_instance(), pb::cpp.number(),
            FieldDescriptor::TYPE_MESSAGE, false, false,

src/google/protobuf/compiler/java/java_features.pb.cc

@@ -1566,8 +1566,8 @@ void EnsureArenaStringIsNotDefault(const MessageLite* msg,
   }
 }
 // The rest do nothing.
-PROTOBUF_UNUSED void EnsureArenaStringIsNotDefault(const MessageLite* msg,
-                                                   void*) {}
+[[maybe_unused]] void EnsureArenaStringIsNotDefault(const MessageLite* msg,
+                                                    void*) {}
 
 }  // namespace
 

src/google/protobuf/compiler/plugin.pb.cc

@@ -1572,7 +1572,7 @@ TEST(MESSAGE_TEST_NAME, TestEnumParsers) {
 
   constexpr int kInvalidValue = 0x900913;
   auto* ref = obj.GetReflection();
-  PROTOBUF_UNUSED auto* descriptor = obj.descriptor();
+  [[maybe_unused]] auto* descriptor = obj.descriptor();
   for (bool use_packed : {false, true}) {
     SCOPED_TRACE(use_packed);
     for (bool use_tail_field : {false, true}) {
@@ -1669,7 +1669,7 @@ TEST(MESSAGE_TEST_NAME, TestEnumParserForUnknownEnumValue) {
   // For unknown enum values, for consistency we must include the
   // int32_t enum value in the unknown field set, which might not be exactly the
   // same as the input.
-  PROTOBUF_UNUSED auto* descriptor = non_dynamic.descriptor();
+  [[maybe_unused]] auto* descriptor = non_dynamic.descriptor();
 
   const std::vector<const FieldDescriptor*> fields =
       GetFields<UNITTEST::EnumParseTester>();
@@ -1722,7 +1722,7 @@ TEST(MESSAGE_TEST_NAME, TestBoolParsers) {
       GetFields<UNITTEST::BoolParseTester>();
 
   auto* ref = obj.GetReflection();
-  PROTOBUF_UNUSED auto* descriptor = obj.descriptor();
+  [[maybe_unused]] auto* descriptor = obj.descriptor();
   for (bool use_tail_field : {false, true}) {
     SCOPED_TRACE(use_tail_field);
     for (int non_canonical_bytes = 0; non_canonical_bytes < 10;
@@ -1786,7 +1786,7 @@ TEST(MESSAGE_TEST_NAME, TestInt32Parsers) {
       GetFields<UNITTEST::Int32ParseTester>();
 
   auto* ref = obj.GetReflection();
-  PROTOBUF_UNUSED auto* descriptor = obj.descriptor();
+  [[maybe_unused]] auto* descriptor = obj.descriptor();
   for (bool use_tail_field : {false, true}) {
     SCOPED_TRACE(use_tail_field);
     for (int non_canonical_bytes = 0; non_canonical_bytes < 10;
@@ -1851,7 +1851,7 @@ TEST(MESSAGE_TEST_NAME, TestInt64Parsers) {
       GetFields<UNITTEST::Int64ParseTester>();
 
   auto* ref = obj.GetReflection();
-  PROTOBUF_UNUSED auto* descriptor = obj.descriptor();
+  [[maybe_unused]] auto* descriptor = obj.descriptor();
   for (bool use_tail_field : {false, true}) {
     SCOPED_TRACE(use_tail_field);
     for (int non_canonical_bytes = 0; non_canonical_bytes < 10;
@@ -1955,7 +1955,7 @@ TEST(MESSAGE_TEST_NAME, TestRepeatedStringParsers) {
       "abcdefghijklmnopqrstuvwxyz"
       "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 
-  PROTOBUF_UNUSED const auto* const descriptor =
+  [[maybe_unused]] const auto* const descriptor =
       UNITTEST::StringParseTester::descriptor();
 
   const std::vector<const FieldDescriptor*> fields =

src/google/protobuf/descriptor.cc

@@ -598,17 +598,6 @@ static_assert(PROTOBUF_ABSL_MIN(20230125, 3),
 #define PROTOBUF_TC_PARAM_NO_DATA_PASS \
   msg, ptr, ctx, ::google::protobuf::internal::TcFieldData::DefaultInit(), table, hasbits
 
-#ifdef PROTOBUF_UNUSED
-#error PROTOBUF_UNUSED was previously defined
-#endif
-#if ABSL_HAVE_CPP_ATTRIBUTE(maybe_unused) || (PROTOBUF_MSC_VER_MIN(1911))
-#define PROTOBUF_UNUSED [[maybe_unused]]
-#elif ABSL_HAVE_ATTRIBUTE(unused) || defined(__GNUC__)
-#define PROTOBUF_UNUSED __attribute__((__unused__))
-#else
-#define PROTOBUF_UNUSED
-#endif
-
 // For the toolchains that support it, evaluates to `true` if the input is
 // determined to be a compile time constant after optimizations have happened
 // (like inlining and constant propagation). A `false` result means that the