Using clang-cl to ship Windows builds

David Major dmajor at
Tue Jul 10 20:29:47 UTC 2018

Bug 1443590 is switching our official Windows builds to use clang-cl
as the compiler.

Please keep an eye out for regressions and file a blocking bug for
anything that might be fallout from this change. I'm especially
interested in hearing about the quality of the debugging experience.

It's possible that the patch may bounce and we'll go back and forth to
MSVC for a while. You can check your build's compiler at
`about:buildconfig`. Treeherder is running an additional set of MSVC
jobs on mozilla-central to make sure we can fall back to a green MSVC
if needed.

Watch for more toolchain changes to come. The next steps after this
will be to switch to lld-link and enable ThinLTO. That will open the
door to a cross-language LTO that could inline calls between Rust and
C++. In the longer term we can look into cross-compiling from Linux.

But for now, shipping our most-used platform with an open-source
compiler is a huge milestone in and of itself. Big thanks to everyone
who has contributed to this effort on the Mozilla side, and also big
thanks to the developers of LLVM and Chromium who helped make clang on
Windows a realistic possibility.

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