Add Target.class_has_field()

[Eric-Arellano]

Problem

Currently, we have the instance method Target.has_field() (and Target.has_fields()), e.g. tgt.has_field(PythonSources).

In some cases, though, we need a classmethod rather than an instance method to be able to do this same type of check. For example, when generating an error message when running ./v2 binary on an invalid target type, we want to be able to calculate every single target type that does work with the goal. To do that, we need to be able to iterate over every Type[Target] registered and call PythonLibrary.has_fields([EntryPoint, PythonBinarySources]) so that we can decide if PythonLibrary is a valid target type or not.

The tricky part is that we must support plugin fields added to pre-existing target types, which are achieved by plugin authors registering UnionRule(PythonLibrary.PluginField, MyCustomField).

Solution

Add Target.class_has_field() and Target.class_has_fields().

Factor out Target.has_field() to deduplicate between the classmethod and instance method.

[Eric-Arellano]

This is prework to have this error message:

No registered binary implementations work with src/python/pants/util:strutil (target type "python_library"). All registered binary implementations:

• PythonBinaryImplementation, works with target types: [python_binary, my_custom_python_binary]
• JavaBinaryImplementation, works with target types: [java_binary]

And this for ./v2 lint:

All registered linters:

• BlackFormatter, works with target types: [python_binary, python_library, python_tests]
• BanditLinter, works with target types: [python_binary, python_library, python_tests]

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Credit goes to @blorente for the great names for these new class methods! Thank you!