IIRC, the Rust team recently migrated to LLVM 6, which is why I opened a pull request at HaikuPorts switching to LLVM 6 release, and another one to fix the cross-compiled version in Rust’s libc, since building Rust 1.27 in Haiku still doesn’t work yet. I was able to successfully cross-compile Rust 1.27 with Haiku’s LLVM 5, but I’d rather aim for supporting LLVM 6 at this time.
But yes after bumping LLVM 6, we still need to check that Mesa builds and works with this change.