Feedback Rule-based Toolchain: Environment variables in cc_args

[hofbi]

I started using the rule based toolchain following the examples in rules_cc. Some compilers require custom environment variables to be set. Usually this is done in some wrapper scripts, but I was thinking if this could be done with the env attribute of cc_args. I would imagine it work somehow like

cc_args(
    name = "special_env",
    actions = [
        "@rules_cc//cc/toolchains/actions:c_compile",
        "@rules_cc//cc/toolchains/actions:cpp_compile_actions",
    ],
    env = {
        "CUSTOM_ENV_VAR_TO_PATH": "@clang-linux-x86_64//:lib-clang-include",
    },
)

Is there a way to expand the directory or subdirectory path of @clang-linux-x86_64//:lib-clang-include, see

rules_cc/examples/rule_based_toolchain/toolchain/clang.BUILD

Lines 40 to 44 in 5be90a4

	subdirectory(
	name = "lib-clang-include",
	parent = ":toolchain_root",
	path = "lib/clang/19/include",
	)

---

While working on this, I filed #290

[armandomontanez]

That'd definitely be nice to have, we'd need to sort out how to determine raw strings vs labels and add some logic to properly capture that. Perhaps we can support this the same way we support label expansions in args?

[hofbi]

Could we use the same implementation used for label expansions in args?