Add preprocessor switch to disable file system include / support

[W4RH4WK]

This is a feature request for adding a preprocessor switch that allows one to disable the inclusion of the std::filesystem header in std.h.

The problem is that some video game console SDKs do not support std::filesystem, but ship the <filesystem> header. Therefore FMT_HAS_INCLUDE returns true, but the <filesystem> header provided by the SDK contains an #error directive breaking the build.

I'd appreciate the inclusion of a preprocessor switch (e.g. FMT_DISABLE_STD_FILESYSTEM) which can be set prior including std.h to work around this issue.

[vitaut]

some video game console SDKs do not support std::filesystem, but ship the <filesystem> header

Could you provide some details about which SDKs do that?

[W4RH4WK]

Could you provide some details about which SDKs do that?

The Nintendo Switch SDK.

[vitaut]

{fmt} already has FMT_CPP_LIB_FILESYSTEM that wraps the corresponding feature-test macro and is overridable:

fmt/include/fmt/std.h

Lines 65 to 71 in 47a66c5

	#ifndef FMT_CPP_LIB_FILESYSTEM
	# ifdef __cpp_lib_filesystem
	# define FMT_CPP_LIB_FILESYSTEM __cpp_lib_filesystem
	# else
	# define FMT_CPP_LIB_FILESYSTEM 0
	# endif
	#endif

Does defining it to zero address your use case or is anything else missing?

[W4RH4WK]

The problem originates from #include <filesystem> here:

fmt/include/fmt/std.h

Lines 29 to 31 in 47a66c5

	# if FMT_CPLUSPLUS >= 201703L
	# if FMT_HAS_INCLUDE(<filesystem>)
	# include <filesystem>

This block is above the FMT_CPP_LIB_FILESYSTEM, so defining FMT_CPP_LIB_FILESYSTEM=0 does not solve the problem.

[W4RH4WK]

The SDK's <filesystem> header includes these lines (as std::filesystem is not supported) which break the build. That's why I'd like to prevent fmt from even including the header.

#if defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
# error "The <filesystem> library is not supported since libc++ has been configured without support for a filesystem."
#endif

[vitaut]

I think we can move the FMT_CPP_LIB_FILESYSTEM check before the <filesystem> include (it only needs <version>) and make the include conditional on it. A PR would be welcome.

[vitaut]

Addressed by #4258.