Fix buildifier warning

test/rules/provider_test.bzl

@@ -23,15 +23,19 @@ load(
     "unittest",
 )
 
-# A sentinel value returned by `_evaluate_field` when a `None` value is
-# encountered during the evaluation of a dotted path on any component other than
-# the last component. This allows the caller to distinguish between a legitimate
-# `None` value being returned by the entire path vs. an unexpected `None` in an
-# earlier component.
-#
-# A `provider` is used here because it is a simple way of getting a known unique
-# object from Bazel that cannot be equal to any other object.
-_EVALUATE_FIELD_FAILED = provider()
+_EVALUATE_FIELD_FAILED = provider(
+    doc = """
+A sentinel value returned by `_evaluate_field` when a `None` value is
+encountered during the evaluation of a dotted path on any component other than
+the last component. This allows the caller to distinguish between a legitimate
+`None` value being returned by the entire path vs. an unexpected `None` in an
+earlier component.
+
+A `provider` is used here because it is a simple way of getting a known unique
+object from Bazel that cannot be equal to any other object.
+""",
+    fields = [],
+)
 
 def _evaluate_field(env, source, field):
     """Evaluates a field or field path on an object and returns its value.