How do I call a rule impl explicitly?

[cbarrete]

I've heard in multiple contexts that calling a rule's implementation from another rule could make sense.

I have previously wanted to do that to get the providers from cxx_library and add my own on top, and I now want to do it to create a wrapper around cxx_library that has a transition applied to it (I wish I could just use target transitions from a macro, but they don't exist yet!). In those cases, a macro just doesn't cut it.

So my question is: how? Rule implementations take a single AnalysisContext parameter, which I cannot mutate at will, nor create myself. If I need to amend the attrs passed to the underlying impl, I am stuck; but even without that, I don't think that forwarding the context as is would be correct: for example ctx.label would likely be incorrect.

Is there something I'm missing here?

[cbarrete]

For anyone also interested in this, getting the providers from another rule can be done with something like:

my_wrapper_rule = rule(
    impl = lambda ctx: ctx.attrs.dep.providers,
    attrs = {
        "dep": attrs.dep(),
    },
)

It's not quite the same as it requires 2 targets to be defined. It can be abstracted away behind a macro, which is less than ideal, but better than nothing.