Merge b66dc7b into 0a04bbc

src/python/pants/backend/python/target_types.py

@@ -12,6 +12,7 @@
 
 from pants.backend.python.macros.python_artifact import PythonArtifact
 from pants.backend.python.subsystems.pytest import PyTest
+from pants.base.deprecated import warn_or_error
 from pants.core.goals.package import OutputPathField
 from pants.engine.addresses import Address, Addresses, UnparsedAddressInputs
 from pants.engine.fs import PathGlobs, Paths
@@ -189,6 +190,21 @@ class PexPlatformsField(StringOrStringSequenceField):
 
     alias = "platforms"
 
+    @classmethod
+    def compute_value(
+        cls, raw_value: Optional[Iterable[str]], *, address: Address
+    ) -> Optional[Tuple[str, ...]]:
+        if isinstance(raw_value, str) and address.is_base_target:
+            warn_or_error(
+                deprecated_entity_description=f"Using a bare string for the `{cls.alias}` field",
+                removal_version="2.2.0.dev0",
+                hint=(
+                    f"Using a bare string for the `{cls.alias}` field for {address}. Please "
+                    f"instead use a list of strings, i.e. use `[{raw_value}]`."
+                ),
+            )
+        return super().compute_value(raw_value, address=address)
+
 
 class PexInheritPathField(StringField):
     """Whether to inherit the `sys.path` of the environment that the binary runs in.