Probably this - https://github.com/skift-org/karm/blob/main/src/libs/karm-gc...

And this is not a garbage collector in the traditional sense, more like arena with smart pointers.

I had the same reading the source code - it's an interesting mix of traditional c++, while some of the projects use the latest c++20 features with modules. The GC::Ref seems to come form the karm-gc library. (according to copilot)