Hah, all sane modern languages needs to patch LLVM for GC reasons :) The guys writing it did not really know a lot about that area and how to get those intrinsic features into the LLVM proper.

Are "the guys" the LLVM people or the Rust implementers?

I imagine the LLVM people.

If you look at the LLVM documentation there is still a lot to do to be able to support most performant GC algorithms.