Support per_file_copt

[mering]

Options passed as --per_file_copt are currently not honored by rules_foreign_cc.

This is blocking us from enabling -Werror for our own code but ignoring warnings on external code where we have no control over.
We want to do this with the following .bazelrc:

build --per_file_copt=external/.*@-Wno-everything --host_per_file_copt=external/.*@-Wno-everything
build --copt=-Werror --host_copt=-Werror

See bazelbuild/bazel#24528 for further details.

[jsharpe]

I'm not sure how you'd implement this as you'd need support from the configure script or the cmake script to apply flags to specific files which I don't think there is a general solution for.
What we support though is features and so you could add a feature to your toolchain that sets the desired flags and then set the features for your rules_foreign_cc targets?

[mering]

@jsharpe thanks for your quick reply.

I do not have control over all rules_foreign_cc targets as some come in via transitive dependencies. I also get some errors for internal tools like make and pkgconfig.

Could there be a way to explicitly set additional copt for all rules_foreign_cc targets?

[mering]

Another approach could be to match the regex (here external/.*) against the BUILD file or the cmake() rule itself and if matching then appending the per_file_copt to the copts for the whole the target.

[jsharpe]

Another approach could be to match the regex (here external/.*) against the BUILD file or the cmake() rule itself and if matching then appending the per_file_copt to the copts for the whole the target.

I don't think starlark has visibility of the per_file_copt except indirectly via the cc_common API?

[mering]

I don't think starlark has visibility of the per_file_copt except indirectly via the cc_common API?

@meteorcloudy @fmeum could you answer this?