Move to using objc_externally_retained to elide unneeded calls to retain/release in j2objc protobuf code.

I believe all of the C code that is wrapped in these blocks can assume that the objects being passed in are going to be held externally and will live for the lifetime of the call.

https://clang.llvm.org/docs/AutomaticReferenceCounting.html#externally-retained-variables

PiperOrigin-RevId: 841790480

Author: Dave MacLachlan

jre_emul/Classes/J2ObjC_common.h

@@ -127,6 +127,18 @@ __attribute__((always_inline)) inline id JreRetainedAutoreleasedReturnValue(id v
 # define WEAK_ __unsafe_unretained
 #endif
 
+#ifndef J2OBJC_EXTERNALLY_RETAINED
+#if __has_feature(objc_arc) && __has_attribute(objc_externally_retained)
+#   define J2OBJC_EXTERNALLY_RETAINED              __attribute__((objc_externally_retained))
+#   define J2OBJC_ASSUME_EXTERNALLY_RETAINED_BEGIN _Pragma("clang attribute J2OBJC_ASSUME_EXTERNALLY_RETAINED.push(__attribute__((objc_externally_retained)), apply_to=any(function, block, objc_method))")
+#   define J2OBJC_ASSUME_EXTERNALLY_RETAINED_END   _Pragma("clang attribute J2OBJC_ASSUME_EXTERNALLY_RETAINED.pop")
+#else // __OBJC__ && objc_arc && objc_externally_retained
+#   define J2OBJC_EXTERNALLY_RETAINED
+#   define J2OBJC_ASSUME_EXTERNALLY_RETAINED_BEGIN
+#   define J2OBJC_ASSUME_EXTERNALLY_RETAINED_END
+#endif // __OBJC__ && objc_arc && objc_externally_retained
+#endif // !J2OBJC_EXTERNALLY_RETAINED
+
 CF_EXTERN_C_BEGIN
 
 id JreThrowNullPointerException(void) __attribute__((noreturn));

protobuf/compiler/src/google/protobuf/compiler/j2objc/j2objc_enum.cc

@@ -398,6 +398,7 @@ void EnumGenerator::GenerateSource(io::Printer* printer) {
       "\n"
       "@end\n"
       "\n"
+      "J2OBJC_ASSUME_EXTERNALLY_RETAINED_BEGIN\n"
       "J2OBJC_CLASS_TYPE_LITERAL_SOURCE($classname$)\n"
       "\n"
       "IOSObjectArray *$classname$_values(void) {\n"
@@ -471,6 +472,7 @@ void EnumGenerator::GenerateSource(io::Printer* printer) {
         "}\n",
         "classname", ClassName(descriptor_), "name", "UNRECOGNIZED");
   }
+  printer->Print("\nJ2OBJC_ASSUME_EXTERNALLY_RETAINED_END\n");
 }
 
 }  // namespace j2objc

protobuf/compiler/src/google/protobuf/compiler/j2objc/j2objc_message.cc

@@ -518,6 +518,7 @@ void MessageGenerator::GenerateSource(io::Printer* printer) {
       "\n"
       "@end\n"
       "\n"
+      "J2OBJC_ASSUME_EXTERNALLY_RETAINED_BEGIN\n"
       "J2OBJC_CLASS_TYPE_LITERAL_SOURCE($classname$)\n"
       "\n"
       "$classname$ * _Nonnull $classname$_getDefaultInstance(void) {\n"
@@ -575,6 +576,7 @@ void MessageGenerator::GenerateSource(io::Printer* printer) {
       "$classname$_descriptor_, input, registry);\n"
       "}\n",
       "classname", ClassName(descriptor_));
+    printer->Print("\nJ2OBJC_ASSUME_EXTERNALLY_RETAINED_END\n");
 
   for (int i = 0; i < descriptor_->real_oneof_decl_count(); i++) {
     OneofGenerator(descriptor_->oneof_decl(i)).GenerateSource(printer);

protobuf/compiler/src/google/protobuf/compiler/j2objc/j2objc_message_lite.cc

@@ -398,6 +398,7 @@ void MessageLiteGenerator::GenerateSource(io::Printer* printer) {
       "\n"
       "@end\n"
       "\n"
+      "J2OBJC_ASSUME_EXTERNALLY_RETAINED_BEGIN\n"
       "J2OBJC_CLASS_TYPE_LITERAL_SOURCE($classname$)\n"
       "\n"
       "$classname$ * _Nonnull $classname$_getDefaultInstance(void) {\n"
@@ -455,6 +456,7 @@ void MessageLiteGenerator::GenerateSource(io::Printer* printer) {
       "$classname$_descriptor_, input, registry);\n"
       "}\n",
       "classname", ClassName(descriptor_));
+  printer->Print("\nJ2OBJC_ASSUME_EXTERNALLY_RETAINED_END\n");
 
   for (int i = 0; i < descriptor_->oneof_decl_count(); i++) {
     OneofGenerator(descriptor_->oneof_decl(i)).GenerateSource(printer);

protobuf/compiler/src/google/protobuf/compiler/j2objc/j2objc_oneof.cc

@@ -326,6 +326,7 @@ void OneofGenerator::GenerateSource(io::Printer* printer) {
       "\n"
       "@end\n"
       "\n"
+      "J2OBJC_ASSUME_EXTERNALLY_RETAINED_BEGIN\n"
       "J2OBJC_CLASS_TYPE_LITERAL_SOURCE($classname$)\n"
       "\n"
       "IOSObjectArray *$classname$_values(void) {\n"
@@ -379,6 +380,7 @@ void OneofGenerator::GenerateSource(io::Printer* printer) {
         "classname", CaseClassName(descriptor_),
         "name", CaseValueName(descriptor_->field(i)));
   }
+  printer->Print("\nJ2OBJC_ASSUME_EXTERNALLY_RETAINED_END\n");
 }
 
 void OneofGenerator::GenerateMessageOrBuilder(io::Printer* printer) {