bazelbuild
Implement rule extensions
Checklist:
- [x] extending from parent
- [x] calling parent's implementation function (in review)
- [x] merging public attributes
- [x] intializers in extended rules
- [x] limiting rule's extendability
- [x] stardoc support
- [x] cfg merging
- [x] subrules support
- [ ] handle executable
Design document: https://docs.google.com/document/d/1p6z-shWf9sdqo_ep7dcjZCGvqN5r2jsPkJCqHHgfRp4/edit
@comius where/how do you want bugs for this? We've kicked the tires on it a little and found some friction - our use case is decorating language rule foo_test to provide external dependencies - and had issues with both getting the parent executable cleanly, but also dealing with _allowlist_function_transition in the extended rule.
You can open separate bugs or explain more here. I'm aware there might be some problems with executables in executable rules. I'll need to figure this out.
About _allowlist_function_transition. The implicit attribute should be added automatically when a Starlark transition is used. Do you have more specific case where this is causing a problem?
Hello @comius : Thanks for proposing/implementing this feature. However, with current Bazel 7.0.2, the rule extension doesn't work for cc_library, cc_binary and all other CC rules. Bazel aborts with "Private API Usage" error.
Is this by design ? or is it caused because CC rules are still part of the builtins of Bazel and haven't moved to rules_cc ?
I can see that CC Rules are now in Starlark with some reliance on native rules.
No, it’s unintentional. C++ rules should be extendable in current form already. I’ll have a look.
@comius I was looking into creating a rule that extends java_library, which I thought would be a supported use case given the number of examples in the design doc that use java_library. Unfortunately, I found that this is not possible (at least on Bazel 7.0.1 with --experimental_rule_extension_api=true):
Error in rule: The rule 'java_library' is not extendable. Only Starlark rules not using deprecated features (like implicit outputs, output to genfiles) may be extended. Special rules like analysis tests or rules using build_settings cannot be extended.
It looks like java_library isn't extendable because it has two implicit outputs. Do you have any plans to support extending rules with implicit outputs, or else remove them entirely from java_library? We'd rather not resort to --experimental_builtins_bzl_path, as the ability to piggy-back off of native rules w/o maintaining our fork of @_builtins is a primary reason why we'd like to use rule extensions in the first place.
