How to create a rule from within another rule in Bazel

Question

Situation

I have two Skylark extension rules: blah_library and blah_binary. All of a blah_library's transitive dependencies are propagated by returning a provider(transitive_deps=...), and are handled appropriately by any ultimate dependent blah_binary target.

What I want to do

I want each blah_library to also create a filegroup with all the transitive dependencies mentioned above, so that I can access them separately. E.g., I'd like to be able to pass them in as data dependencies to a cc_binary. In other words:

# Somehow have this automatically create a target named `foo__trans_deps`?
blah_library(
    name = "foo",
    srcs = [...],
    deps = [...],
)

cc_binary(
    ...,
    data = [":foo__trans_deps"],
)

How should I do this? Any help would be appreciated!

What I've tried

Make a macro

I tried making a macro like so:

_real_blah_library = rule(...)

def blah_library(name, *args, **kwargs):
    native.filegroup(
        name = name + "__trans_deps",
        srcs = ???,
    )
    _real_blah_library(name=name, *args, **kwargs)

But I'm not sure how to access the provider provided by _real_blah_library from within the macro, so I don't know how to populate the filegroup's srcs field...

Modify the blah_library rule's implementation

Right now I have something like:

_blah_provider = provider(fields=['transitive_deps'])

def _blah_library_impl(ctx):
    ...

    trans_deps = []
    for dep in ctx.attr.deps:
        trans_deps += dep[_blah_provider].trans_deps

    return _blah_provider(trans_deps=trans_deps)

blah_library = rule(impl=_blah_library_impl, ...)

I tried adding the following to _blah_library_impl, but it didn't work because apparently native.filegroup can't be called within a rule's implementation ("filegroup() cannot be called during the analysis phase"):

def _blah_library_impl(ctx):
    ...

    trans_deps = []
    for dep in ctx.attr.deps:
        trans_deps += dep[_blah_provider].trans_deps

    native.filegroup(
        name = ctx.attr.name + "__trans_deps",
        srcs = trans_deps,
    )

    return _blah_provider(trans_deps=trans_deps)

Answer 1

You can't easily create a filegroup like that, but you can still achieve what you want.

If you want to use the rule in genrule.srcs, filegroup.srcs, cc_binary.data, etc., then return a DefaultInfo provider (along with _blah_provider) and set the files field to the transitive closure of files.

You can refine the solution if you want a different set of files when the rule is in a data attribute vs. when in any other (e.g. srcs): just also set the runfiles-related members in DefaultInfo. (Frankly I don't know the difference between them, I'd just set all runfiles-fields to the same value.)