Hmm, I couldn't find that last time I looked at building Rust -- all I saw was an option to grab precompiled builds of Rust LLVM from the CI server, but maybe I missed something.
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...