Provide `sysroot` with rules based toolchain, or deprecate it in bazel

[keith]

Currently the new rules based CC toolchain implementation never sets builtin_sysroot https://github.com/bazelbuild/rules_cc/blob/d74915024017250e46d95e91a3defc34174effe0/cc/toolchains/impl/toolchain_config.bzl#L69-L89 and instead just treats sysroot as any other flag https://github.com/bazelbuild/rules_cc/blob/d74915024017250e46d95e91a3defc34174effe0/cc/toolchains/args/sysroot.bzl

This diverges from the default CC toolchain behavior where the sysroot can be provided https://github.com/bazel-contrib/toolchains_llvm/blob/d557de66f23651046a6c870d44df1cb06f80910e/toolchain/cc_toolchain_config.bzl#L394 and thus other rules potentially fetch the sysroot through the public API https://github.com/bazel-contrib/rules_cuda/blob/000d119fda8ebbe02b3a94ec2ef6543ef10b8a6d/cuda/private/cuda_helper.bzl#L299

This has lead to confusion over what the "right" way to fetch the sysroot is https://github.com/bazel-contrib/rules_cuda/pull/313

I think we should either set the sysroot in the rules based toolchain, if it's provided, or deprecate the sysroot attributes in bazel, so that it's clear that they should no longer be fetched. If we deprecated the attributes in bazel I think it would be useful to provide a recommendation for how users should fetch them. One option would be requiring the user to duplicate the sysroot location knowledge and pass it to each ruleset that needs that value, for example for rules_cuda that could be --@rules_cuda//:sysroot="the same thing you pass to the cc toolchain config"

[cerisier]

Anyone has any thoughts on this ?

In rules_zig, we need to align the sysroot of the zig compiler (libc installation) with that of a CC toolchain if it exists. Currently this is not possible with the rule based toolchain.

[cloudhan]

@cerisier This is what we do to query the sysroot in rules_cuda https://github.com/bazel-contrib/rules_cuda/commit/a862f9b1cb7b6cf15c775fd31e4a1cf56f148a82

[cerisier]

Thanks @cloudhan

I'm actually doing the same thing for now haha:

• https://github.com/zml/rules_zig/blob/zml/zig/private/common/zig_translate_c.bzl

but I'd like to avoid parsing the command line.