Set CMAKE_<LANG>_COMPILER_PREDEFINES_COMMAND in Windows.MSVC.toolchain.cmake

[hadogenes]

Setting "CMAKE__COMPILER_PREDEFINES_COMMAND" with the command that will produce the compiler's predefines unblocks downstream consumers, like 'automoc'.

[MarkSchofield]

I don't really follow - this doesn't really seem to be a responsibility of the toolchain..?

[hadogenes]

https://gitlab.kitware.com/cmake/cmake/-/merge_requests/8359
it is defined in ex. Modules/Compiler/GNU.cmake (as CMAKE_${lang}_COMPILER_PREDEFINES_COMMAND)

[MarkSchofield]

https://gitlab.kitware.com/cmake/cmake/-/merge_requests/8359 it is defined in ex. Modules/Compiler/GNU.cmake (as CMAKE_${lang}_COMPILER_PREDEFINES_COMMAND)

Ah, gotcha. So it's not really for generating moc_predefs.h directly - defining CMAKE__${lang}__COMPILER_PREDEFINES_COMMAND allows tooling to run that to get the predefines. And that unblocks automoc so that it can generate moc_predefs.h.

If I add, say:

execute_process(
        COMMAND ${CMAKE_CXX_COMPILER_PREDEFINES_COMMAND}
        OUTPUT_VARIABLE CXX_PREDEFINES_OUTPUT
    )
file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/cxx_predefines.txt ${CXX_PREDEFINES_OUTPUT})

Then the predefines are written, but an '.obj' file is generated into my work-tree. It looks like you can redirect to nul with /Fonul. I think that's worth adding.

Is it worth adding a C equivalent, too? i.e.

set(CMAKE_C_COMPILER_PREDEFINES_COMMAND
    ${CMAKE_C_COMPILER}
        /nologo
        /Zc:preprocessor
        /PD
        /c
        /Fonul.
        ${CMAKE_ROOT}/Modules/CMakeCCompilerABI.c
)

[MarkSchofield]

@hadogenes - I pushed a commit with my recommendations. Does that work for your use case?