Bug 1512504 has now landed on autoland, meaning that compiling Firefox
with MSVC is now not supported anymore. MSVC is however still necessary
as a build requirement for its headers and libraries (as well as its
assembler on aarch64 and the preprocessor for midl ; and maybe a few other


On Thu, Dec 06, 2018 at 03:00:12PM -0500, Ted Mielczarek wrote:
> Hello,
> In light of the fact that we've switched to clang-cl for our Windows 
> builds[1], we are planning to drop support for compiling Firefox with MSVC in 
> the near future[2]. Our estimate is that this will happen sometime in Q1. 
> Supporting more than one compiler is a maintenance burden and we've already 
> seen developers spend considerable time getting their patches that work with 
> clang-cl to build with MSVC. We are currently blocked by the fact that our 
> aarch64-windows builds are still using MSVC and we are waiting on upstream 
> clang-cl work to switch those builds to clang-cl. Once that takes place we no 
> longer have a compelling reason to continue supporting MSVC.
> To preempt the question--when this happens we intend to make MSVC error in 
> configure, and not just move MSVC to Tier 3 "patches welcome" status. Our 
> reasoning is that Tier 3 configurations still create work: developers spend 
> time building in those configurations, and lack of CI coverage means that 
> when they inevitably break they waste time trying to fix things. Bugs get 
> filed, and other developers waste time trying to help or reviewing patches to 
> fix things. Explicitly unsupporting MSVC is the best way for us to convey the 
> fact that developers should not be using it and we will not accept patches to 
> fix issues that only affect MSVC.
> If you have specific reasons for continuing to use MSVC please let us know. 
> If there are deficiencies in clang-cl compared to MSVC we should get them 
> filed and fixed.
> Thanks,
> -Ted
> 1. https://bugzilla.mozilla.org/show_bug.cgi?id=1443590
> 2. https://bugzilla.mozilla.org/show_bug.cgi?id=1512504
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