use crate_info.deps in establish_cc_info

[krasimirgg]

This avoids the dependence on ctx.attr, allowing custom deps registered via custom transform_deps to be propagated correctly. I believe this should be a non-functional change for rules_rust.
Internally, we have a slightly modified rust_library that has cc_deps attribute which is processed by https://github.com/google/crubit, which posts extra deps via transform_deps. Currently, for obvious reasons, doesn't get processed in establish_cc_info.

[UebelAndre]

I think this seems fine. Would you be able to link the crubit modified version of the rules so I can better understand this change?

[scentini]

We are patching the actual rust_{library, binary, test} internally, so it's not easy to export it, but I can summarize what we do:

1. We add a cc_deps attribute that has the rust_bindings_from_cc_aspect attached to it.
2. We replace the transform_deps invocations with a custom function that transforms the RustBindingsFromCcInfo provider produced by the aspect into DepVariantInfos. It looks something like this:

def collect_transformed_deps(ctx):
    """Creates DepVariantInfos from all dependencies and from the rust_bindings_from_cc_aspect.

    Args:
        ctx (ctx): The target's context object.

    Returns:
        list[DepVariantInfo]: a list of DepVariantInfos that this target depends on.
    """
    deps = [DepVariantInfo(
        crate_info = dep[CrateInfo] if CrateInfo in dep else None,
        dep_info = dep[DepInfo] if DepInfo in dep else None,
        build_info = dep[BuildInfo] if BuildInfo in dep else None,
        cc_info = dep[CcInfo] if CcInfo in dep else None,
    ) for dep in ctx.attr.deps]

    if hasattr(ctx.attr, "cc_deps"):
        for dep in ctx.attr.cc_deps:
            if RustBindingsFromCcInfo in dep:
                deps.append(dep[RustBindingsFromCcInfo].dep_variant_info)
                deps.append(
                    DepVariantInfo(
                        cc_info = dep[RustBindingsFromCcInfo].cc_info,
                        crate_info = None,
                        dep_info = None,
                        build_info = None,
                    ),
                )

    return deps

Long term plan is to make rules_rust API stable enough and extensible enough so that we don't need to patch the rules but rather extend them.