clang-cl.exe incompatible with passing both -E and -showIncludes

[fxb]

Currently sccache is unable to cache compilations that pass -showIncludes when using clang-cl.exe.

Compilations fail with the following error:
clang-cl.exe: error: argument unused during compilation: '--show-includes' [-Werror,-Wunused-command-line-argument]

This is caused by sccache passing both -E and -showIncludes to clang-cl.exe with the expectation that -E output will be on stdout and -showIncludes output will be on stderr (See: msvc.rs#L120). However clang-cl.exe outputs both on stdout, so this combination of flags is currently not supported (See: lib/Driver/ToolChains/Clang.cpp#L5080).

  // Both /showIncludes and /E (and /EP) write to stdout. Allowing both
  // would produce interleaved output, so ignore /showIncludes in such cases.

[luser]

That's a bummer. I guess we should just bail early with a CannotCache response when using clang-cl with -showIncludes? Note that one of my colleagues determined that you can coerce clang-cl to emit dependency files in the gcc style with the right set of options. We could certainly support handling -deps to do that for clang like we do with MSVC.

[fxb]

That would be unfortunate, since CMake / GN + Ninja builds always pass -showIncludes (See #47), so any projects using Ninja would not be cacheable at all when using clang-cl.exe as the compiler.

It seems like the only solution in this case would be to raise this issue on the LLVM bug tracker and contribute a fix to clang-cl.exe

[fxb]

I submitted a patch to clang, which makes clang-cl.exe behave the same as cl.exe in this case: https://reviews.llvm.org/D46394

[fxb]

My patch has now landed in clang trunk (https://reviews.llvm.org/rC331533).