There is. We even test that it builds as part of our CI, to make sure it works, IIRC.
links to https://rustc-dev-guide.rust-lang.org/building/suggested.htm...
... which is broken. I'll file a docs bug. Can't do it right this moment, but will in a day or two.
It is not my area of expertise, but I believe the way you do it is to set this option: https://github.com/rust-lang/rust/blob/master/config.toml.ex...
buuut the dockerfile for the llvm 9 build passes this flag as an argument: https://github.com/rust-lang/rust/blob/5a549d36ee81b226d1672...