Here's the relevant commit:
948e9a7360d009c2f01245238998887075f7a7d5
The only change necessary in the compiler was to use the
AUTORELEASE macro instead of directly sending the
autorelease message. We use this macro to avoid needing ARC-conditional sections everywhere; instead, just compile your j2objc_protoc-generated code with
-fobjc-arc to enable this feature.
FYI, there's no real benefit to rebuilding the j2objc protobuf runtime with ARC unless it helps you sleep at night (like the requesting engineer needed), as all reported leaks with that runtime were resolved years ago.