Now that we support rust-bindgen with clang 4.0 (bug 1363666), Xidorn recommends that mach bootstrap and tooltool install clang 4.0.x instead of 3.9 because 4.0 is the current stable release. There is some workaround code for clang 3.9 we can remove once we require 4.0.

Why not 5.0.x?

Because Clang 5.0 hasn't been released yet. Are you getting confused by the versioning scheme on MacOS?

I saw clang.manifest: https://dxr.mozilla.org/mozilla-central/source/browser/config/tooltool-manifests/win64/clang.manifest#35 Why are we using a pre-release version?

Because release versions of clang don't work for compiling Firefox on Windows.

I should say that they didn't, at least; I don't think all the necessary fixes for compiling Firefox on Windows made it into the 4.0 series, but I could be wrong about that.

I've been using libclang 4.0 for bindgen to build Stylo on Windows for quite a while.

Sure, parsing headers for a limited set of Firefox code is very different than compiling all of Firefox. :) We're downloading the version of clang that we use in automation for compiling Firefox because if it can compile Firefox, clearly it can parse all the headers that bindgen needs to deal with, and because it was close at hand and downloadable over HTTPS. If someone would like to show that LLVM 4.0 works to compile Firefox in automation, we can see about downloading that instead.

It seems to me that, mach bootstrap now installs clang 5.0.0, and it seems to be self-built version rather than a release version.

WONTFIX because this would be a lot of work and we don't have a pressing need to use clang 4.0 for rust-bindgen. I didn't realize that the clang 3.9 packages were already being used for Mac and Linux builds and not just for rust-bindgen. We use custom clang 3.9 packages to build Mac and Linux and custom clang 5.0 packages to build Win32 and Win64 (for the Windows clang test builds).