Fix PluginField not working in production

src/python/pants/backend/codegen/protobuf/python/additional_fields.py

@@ -3,12 +3,11 @@
 
 from pants.backend.codegen.protobuf.target_types import ProtobufLibrary
 from pants.backend.python.target_types import PythonInterpreterCompatibility
-from pants.engine.unions import UnionRule
 
 
 class ProtobufPythonInterpreterCompatibility(PythonInterpreterCompatibility):
     alias = "python_compatibility"
 
 
 def rules():
-    return [UnionRule(ProtobufLibrary.PluginField, ProtobufPythonInterpreterCompatibility)]
+    return [ProtobufLibrary.register_plugin_field(ProtobufPythonInterpreterCompatibility)]

src/python/pants/backend/pants_info/list_target_types_test.py

@@ -8,7 +8,7 @@
 from pants.backend.pants_info.list_target_types import TargetTypesSubsystem, list_target_types
 from pants.core.util_rules.pants_bin import PantsBin
 from pants.engine.target import BoolField, IntField, RegisteredTargetTypes, StringField, Target
-from pants.engine.unions import UnionMembership, UnionRule
+from pants.engine.unions import UnionMembership
 from pants.testutil.engine.util import MockConsole, create_goal_subsystem, run_rule
 
 
@@ -121,7 +121,7 @@ class CustomField(BoolField):
 
     tests_target_stdout = run_goal(
         union_membership=UnionMembership.from_rules(
-            [UnionRule(FortranTests.PluginField, CustomField)]
+            [FortranTests.register_plugin_field(CustomField)]
         ),
         details_target=FortranTests.alias,
     )

src/python/pants/engine/fs.py

@@ -15,13 +15,10 @@
 
 @dataclass(frozen=True)
 class Digest:
-    """A Digest is a content-digest fingerprint, and a length of underlying content.
+    """A Digest is a lightweight reference to a set of files known about by the engine.
 
-    Typically, this is used as a lightweight reference to a set of files known about by the engine.
-    You can use `await Get(Snapshot, Digest)` to set the file names referred to, or use
-    `await Get(DigestContents, Digest)` to see the actual file content.
-
-    Occasionally, a Digest is a reference for other types of strings/bytes/content.
+    You can use `await Get(Snapshot, Digest)` to set the file names referred to, or use `await
+    Get(DigestContents, Digest)` to see the actual file content.
     """
 
     fingerprint: str

src/python/pants/engine/internals/graph.py

@@ -94,7 +94,9 @@
 
 @rule
 async def resolve_target(
-    address: Address, registered_target_types: RegisteredTargetTypes
+    address: Address,
+    registered_target_types: RegisteredTargetTypes,
+    union_membership: UnionMembership,
 ) -> WrappedTarget:
     if address.generated_base_target_name:
         base_target = await Get(WrappedTarget, Address, address.maybe_convert_to_base_target())
@@ -110,7 +112,7 @@ async def resolve_target(
         raise UnrecognizedTargetTypeException(
             target_adaptor.type_alias, registered_target_types, address=address
         )
-    target = target_type(target_adaptor.kwargs, address=address)
+    target = target_type(target_adaptor.kwargs, address=address, union_membership=union_membership)
     return WrappedTarget(target)
 
 

src/python/pants/engine/target.py

@@ -29,11 +29,11 @@
 from pants.engine.addresses import Address, assert_single_address
 from pants.engine.collection import Collection, DeduplicatedCollection
 from pants.engine.fs import Snapshot
-from pants.engine.unions import UnionMembership, union
+from pants.engine.unions import UnionMembership, UnionRule, union
 from pants.source.filespec import Filespec
 from pants.util.collections import ensure_list, ensure_str_list
 from pants.util.frozendict import FrozenDict
-from pants.util.memo import memoized_property
+from pants.util.memo import memoized_classproperty, memoized_property
 from pants.util.meta import frozen_after_init
 from pants.util.ordered_set import FrozenOrderedSet
 from pants.util.strutil import pluralize
@@ -294,16 +294,17 @@ def __init__(
     def field_types(self) -> Tuple[Type[Field], ...]:
         return (*self.core_fields, *self.plugin_fields)
 
-    @union
     @final
-    class PluginField:
-        """A sentinel class to allow plugin authors to add additional fields to this target type.
+    @memoized_classproperty
+    def _plugin_field_cls(cls) -> Type:
+        # NB: We ensure that each Target subtype has its own `PluginField` class so that
+        # registering a plugin field doesn't leak across target types.
 
-        Plugin authors may add additional fields by simply registering UnionRules between the
-        `Target.PluginField` and the custom field, e.g. `UnionRule(PythonLibrary.PluginField,
-        TypeChecked)`. The `Target` will then treat `TypeChecked` as a first-class citizen and
-        plugins can use that Field like any other Field.
-        """
+        @union
+        class PluginField:
+            pass
+
+        return PluginField
 
     def __repr__(self) -> str:
         fields = ", ".join(str(field) for field in self.field_values.values())
@@ -322,9 +323,7 @@ def __str__(self) -> str:
     @final
     @classmethod
     def _find_plugin_fields(cls, union_membership: UnionMembership) -> Tuple[Type[Field], ...]:
-        return cast(
-            Tuple[Type[Field], ...], tuple(union_membership.union_rules.get(cls.PluginField, ()))
-        )
+        return cast(Tuple[Type[Field], ...], tuple(union_membership.get(cls._plugin_field_cls)))
 
     @final
     @classmethod
@@ -469,6 +468,17 @@ def class_has_fields(
             fields, registered_fields=cls.class_field_types(union_membership=union_membership)
         )
 
+    @final
+    @classmethod
+    def register_plugin_field(cls, field: Type[Field]) -> UnionRule:
+        """Register a new field on the target type.
+
+        In the `rules()` register.py entry-point, include
+        `MyTarget.register_plugin_field(NewField)`. This will register `NewField` as a first-class
+        citizen. Plugins can use this new field like any other.
+        """
+        return UnionRule(cls._plugin_field_cls, field)
+
 
 @dataclass(frozen=True)
 class WrappedTarget:

src/python/pants/engine/target_test.py

@@ -39,7 +39,7 @@
     generate_subtarget,
     generate_subtarget_address,
 )
-from pants.engine.unions import UnionMembership, UnionRule
+from pants.engine.unions import UnionMembership
 from pants.testutil.engine.util import MockGet, run_rule
 from pants.util.collections import ensure_str_list
 from pants.util.frozendict import FrozenDict
@@ -263,7 +263,7 @@ class CustomField(BoolField):
         default = False
 
     union_membership = UnionMembership.from_rules(
-        [UnionRule(FortranTarget.PluginField, CustomField)]
+        [FortranTarget.register_plugin_field(CustomField)]
     )
     tgt_values = {CustomField.alias: True}
     tgt = FortranTarget(
@@ -289,6 +289,15 @@ class CustomField(BoolField):
     )
     assert default_tgt[CustomField].value is False
 
+    # Ensure that the `PluginField` is not being registered on other target types.
+    class OtherTarget(Target):
+        alias = "other_target"
+        core_fields = ()
+
+    other_tgt = OtherTarget({}, address=Address.parse(":other"))
+    assert other_tgt.plugin_fields == ()
+    assert other_tgt.has_field(CustomField) is False
+
 
 def test_override_preexisting_field_via_new_target() -> None:
     # To change the behavior of a pre-existing field, you must create a new target as it would not