C++ compile flags end up being used for linking too

[attilaolah]

There seems to be an unfortunate interaction between Bazel flags and CMake when using rules_foreign_cc. Namely:

@rules_nixpkgs_cc sets CXX_FLAGS to -x ${ccLang}, which ends up being -x c++, here:

rules_nixpkgs/toolchains/cc/cc.nix

Lines 190 to 193 in 244ae50

	CXX_FLAGS=(
	-x ${ccLang}
	-std=c++0x
	)

Then rules_foreign_cc generates a CMake toolchain file, crosstool_bazel.cmake, which sets CMAKE_CXX_FLAGS_INIT, which in turn passes the flag to CMAKE_CXX_FLAGS. These flags are, however, used for both compilation and linking, as described here.

I can't seem to find a way to pass any flags to CMake that would be used for compilation only, but not for linking — having that would allow for a better toolchain file to be crafted.

This is probably a @rules_foreign_cc issue, but I'm listing it here since the way I stumbled upon this was due to the fact that the -x c++ flag is provided by @rules_nixpkgs_cc. Maybe folks here have a suggestion for how to work around the issue.

[attilaolah]

After some trial and error, I worked around this by modifying @rules_foreign_cc to include the following in the crosstool file:

add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-xc++>)

This works as long as I also remove the -x c++ from the compiler flags. That can be done either by patching @rules_nixpkgs_cc (but then any non-Cmake C++ targets would break), or by further patching @rules_foreign_cc to not set the flags that are also used for linking. That is probably the correct approach, so I'll be filing an issue with @rules_foreign_cc, just wanted to quickly note the workaround in this discussion.