Use [[nodiscard]] instead of PROTOBUF_NODISCARD

With C++17 being baseline, PROTOBUF_NODISCARD is no longer necessary. Directly
using [[nodiscard]] makes port_def.inc smaller.

PiperOrigin-RevId: 693808783

src/google/protobuf/arenastring.h

@@ -344,7 +344,7 @@ struct PROTOBUF_EXPORT ArenaStringPtr {
   // Own()'d by any arena. If the field is not set, this returns nullptr. The
   // caller retains ownership. Clears this field back to the default state.
   // Used to implement release_<field>() methods on generated classes.
-  PROTOBUF_NODISCARD std::string* Release();
+  [[nodiscard]] std::string* Release();
 
   // Takes a std::string that is heap-allocated, and takes ownership. The
   // std::string's destructor is registered with the arena. Used to implement

src/google/protobuf/compiler/cpp/field_generators/message_field.cc

@@ -164,7 +164,7 @@ void SingularMessage::GenerateAccessorDeclarations(io::Printer* p) const {
 
   p->Emit(R"cc(
     $DEPRECATED$ const $Submsg$& $name$() const;
-    $DEPRECATED$ PROTOBUF_NODISCARD $Submsg$* $release_name$();
+    $DEPRECATED$ [[nodiscard]] $Submsg$* $release_name$();
     $DEPRECATED$ $Submsg$* $mutable_name$();
     $DEPRECATED$ void $set_allocated_name$($Submsg$* value);
     $DEPRECATED$ void $unsafe_arena_set_allocated_name$($Submsg$* value);

src/google/protobuf/compiler/cpp/field_generators/string_field.cc

@@ -270,7 +270,7 @@ void SingularString::GenerateAccessorDeclarations(io::Printer* p) const {
             template <typename Arg_ = const std::string&, typename... Args_>
             $DEPRECATED$ void $set_name$(Arg_&& arg, Args_... args);
             $DEPRECATED$ std::string* $mutable_name$();
-            $DEPRECATED$ PROTOBUF_NODISCARD std::string* $release_name$();
+            $DEPRECATED$ [[nodiscard]] std::string* $release_name$();
             $DEPRECATED$ void $set_allocated_name$(std::string* value);
 
             private:

src/google/protobuf/compiler/cpp/helpers.h

@@ -939,12 +939,10 @@ class PROTOC_EXPORT Formatter {
     Formatter* format_;
   };
 
-  PROTOBUF_NODISCARD ScopedIndenter ScopedIndent() {
-    return ScopedIndenter(this);
-  }
+  [[nodiscard]] ScopedIndenter ScopedIndent() { return ScopedIndenter(this); }
   template <typename... Args>
-  PROTOBUF_NODISCARD ScopedIndenter ScopedIndent(const char* format,
-                                                 const Args&&... args) {
+  [[nodiscard]] ScopedIndenter ScopedIndent(const char* format,
+                                            const Args&&... args) {
     (*this)(format, static_cast<Args&&>(args)...);
     return ScopedIndenter(this);
   }

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

@@ -960,11 +960,9 @@ void MessageGenerator::GenerateFieldAccessorDeclarations(io::Printer* p) {
       }
       template <typename _proto_TypeTraits, $pbi$::FieldType _field_type,
                 bool _is_packed>
-      PROTOBUF_NODISCARD inline
-          typename _proto_TypeTraits::Singular::MutableType
-          ReleaseExtension(
-              const $pbi$::ExtensionIdentifier<$Msg$, _proto_TypeTraits,
-                                               _field_type, _is_packed>& id) {
+      [[nodiscard]] inline typename _proto_TypeTraits::Singular::MutableType
+      ReleaseExtension(const $pbi$::ExtensionIdentifier<
+                       $Msg$, _proto_TypeTraits, _field_type, _is_packed>& id) {
         $WeakDescriptorSelfPin$;
         $annotate_extension_release$;
         return _proto_TypeTraits::Release(id.number(), _field_type, &$extensions$);