Rust and GCC, two different ways

IIRC PGI and pre-LLVM Intel use EDG?

That being said, it seems nowadays there's not that much interest in proprietary language extensions, and thus the focus on these proprietary LLVM forks seem to be in tuning (including new optimization passes?) for the particular vendor's CPU's. And maybe some improved runtime libraries. So in that sense it makes sense for the vendors to collaborate on an open source frontend (clang(++)) , as the secret proprietary juice is elsewhere.