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",
	)

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While working on this, I filed #290