Support linking on Windows with rules-based toolchains

[calebzulawski]

Allow linking on Windows, tested with clang.exe and both link.exe and lld-link.

I didn't delve into compiler flags, so this does not add any support for MSVC/clang-cl.exe. I use -Wl,<flag> pretty heavily, but per #434 it should be possible to make e.g. an msvc_linker_args rule that reads a variable and conditionally prefixes -Wl, only when not using MSVC/clang-cl.

I also didn't account for MinGW, which I think acts more like Linux, so maybe conditioning on just the target OS won't be sufficient in the long run.

[calebzulawski]

I copied this from the legacy toolchain config, I don't know enough about how it works to be sure my implementation is correct:

rules_cc/cc/private/toolchain/unix_cc_toolchain_config.bzl

Lines 1119 to 1147 in 0692005

	build_interface_libraries_feature = feature(
	name = "build_interface_libraries",
	flag_sets = [
	flag_set(
	actions = [
	ACTION_NAMES.cpp_link_dynamic_library,
	ACTION_NAMES.cpp_link_nodeps_dynamic_library,
	ACTION_NAMES.lto_index_for_dynamic_library,
	ACTION_NAMES.lto_index_for_nodeps_dynamic_library,
	],
	flag_groups = [
	flag_group(
	flags = [
	"%{generate_interface_library}",
	"%{interface_library_builder_path}",
	"%{interface_library_input_path}",
	"%{interface_library_output_path}",
	],
	expand_if_available = "generate_interface_library",
	),
	],
	with_features = [
	with_feature_set(
	features = ["supports_interface_shared_libraries"],
	),
	],
	),
	],
	)

[calebzulawski]

I based this on the legacy toolchain config as well:

rules_cc/cc/private/toolchain/unix_cc_toolchain_config.bzl

Lines 1494 to 1517 in 0692005

	dynamic_library_linker_tool_feature = feature(
	name = "dynamic_library_linker_tool",
	flag_sets = [
	flag_set(
	actions = [
	ACTION_NAMES.cpp_link_dynamic_library,
	ACTION_NAMES.cpp_link_nodeps_dynamic_library,
	ACTION_NAMES.lto_index_for_dynamic_library,
	ACTION_NAMES.lto_index_for_nodeps_dynamic_library,
	],
	flag_groups = [
	flag_group(
	flags = [" + cppLinkDynamicLibraryToolPath + "],
	expand_if_available = "generate_interface_library",
	),
	],
	with_features = [
	with_feature_set(
	features = ["supports_interface_shared_libraries"],
	),
	],
	),
	],
	)

[calebzulawski]

I did not compare this to the legacy textproto, I'm not sure how to do that, but I'm sure it will look different if the legacy assumes only MSVC

[calebzulawski]

Sorry for the churn, getting everything right across multiple OSes is tough :) Should be good to go now.

[calebzulawski]

@armandomontanez any thoughts on this implementation? thanks!

[reutermj]

Not totally sure how this should be handled because this will not work with normal clang.exe and gcc.exe (if youre compiling with mingw).

[calebzulawski]

I hinted at this in the PR description--the idea is to use a variable. Bazel can be modified to include the compiler variable from create_cc_toolchain_config_info. After that's available, the args can be conditioned on that with something like:

cc_args(
    name = "mingw_args",
    require_equal = "//toolchain/variables:compiler",
    require_equal_value = "mingw",
    args = LINUX_ARGS,
 )
 
 cc_args(
    name = "msvc_args",
    require_equal = "//toolchain/variables:compiler",
    require_equal_value = "msvc",
    args = MSVC_ARGS,
 )
 
 cc_args_list(
     name = "windows_args",
     args = ["mingw_args", "msvc_args"],
 )

I'd plan on implementing a macro to generally support this, since there would be so many instances of it.

[calebzulawski]

Unfortunately I'm not really sure how to make Bazel support that, but if someone else is willing to do that, I'd be happy to do the rules_cc side :)