[rule] Allow deleting attributes from Starlark rule() function

[Yannic]

This is useful for rules like rules_license's license() rule that are typically used via `package(default_package_metadata = [":license"]).

Since there's an implicit dependency from every target in the package to what's declared in default_package_metadata, rules intended for this purpose need to be wrapped in a macro today.

_license = rule(
  # ...
)

def license(**kwargs):
  _license(package_metadata = [], **kwargs)

With this change, wrapping the rule is no longer necessary.

license = rule(
  # ...
  deleted_attrs = ["package_metadata"],
)

[Yannic]

Converting to draft for now since this is purely a POC whether something like this would work at all.

@lberki do you see any obvious issues with exposing something like this to Starlark before I send time creating a design doc?

[fmeum]

The macro approach could be "inlined" via the new initializer attribute of rule. Maybe that's sufficient and a new attribute isn't needed?

[lberki]

Not on the implementation side. In fact, RuleClass.Builder.removeAttribute() is used quite a few times in built-in rules. So as far as the implementation goes, it's sound. I think the bigger issue is how to use this: presumably, you'll want to delegate to the implementation of the rule you inherit from in some way, but that implementation will expect the attribute you delete.

I do understand what you want to use this for in license(), but it seems to be a pretty niche use case and if it really is a niche, it's probably better off implemented as a macro as you propose above.

[Yannic]

The macro approach could be "inlined" via the new initializer attribute of rule. Maybe that's sufficient and a new attribute isn't needed?

Mhm I tried that yesterday, but my naive approach of filtering out ["package_metadata", "applicable_licenses"] didn't work, so it seems like propagating attributes from package() to the target happens later than the initializer.. Let me try again with setting them to [] explicitly and report back.

Do I understand correctly that initializer is available in 8.x, but not in 7.x? If so, is this something we could potentially backport?

[Yannic]

Nope initializer doesn't work: Error in license: Initializer can only set Starlark defined attributes, not 'package_metadata'

def _set_package_metadata(**kwargs):
    args = {k: v for k, v in kwargs.items()}
    args["package_metadata"] = []

    return args

[Yannic]

I can see that this feature becomes a foot-gun when extending an existing rule via subrules. Maybe we should only allow it only when there's no parent?

I don't have a usecase outside on license() or package metadata in mind right now. We are working on making this more extendible right now to account for things other than licenses or allow organizations to override / extend what's available in the bzlmod dependency, so I do expect there to be more rules with a need to remove package_metadata. And having this would allow rules_license (or its successor we plan to create in the next week) to create an abstract "parent" rule with the attributes removed to make it easy for users (like we would if it was a native rule)