I'm statically linking our LLVM. This leads to a slightly reduced package size (see T8434). If anyone familiar with LLVM can tell me that the toolchain doesn't break its ABI often, I could even link Rust dynamically against it to save even more space.

This is fine with me. Eventually, to enable cross-lang PGO/LTO in firefox, both firefox and rust will need to link/use the same llvm/clang to maintain compatibility and reduce the risk of weird compiler mismatch errors. I think this is how you link it as a shared lib:

[llvm]
link-shared = true

@joebonrichie are you the LLVM maintainer? So that I know if your reply suffices or I have to wait for Core.

@joebonrichie please notify me when you've pushed your LLVM 9 patch.

I confirm, as @ermo anticipated, that Firefox 70 does not build using Rust 1.39.0.

Well, unless you want to patch it.