Allow creation of sentinel features in rule-based toolchains

BEGIN_PUBLIC
Makes the `args` attribute of `cc_feature` optional so that users may declare sentinel features that do not have any inherent arguments.
END_PUBLIC

PiperOrigin-RevId: 646545838
Change-Id: I470a496aec951608f21cc8ea693387c5b551c8a9

cc/toolchains/feature.bzl

@@ -125,7 +125,6 @@ Example:
             doc = """Whether or not this feature is enabled by default.""",
         ),
         "args": attr.label_list(
-            mandatory = True,
             doc = """Args that, when expanded, implement this feature.""",
             providers = [ArgsListInfo],
         ),

tests/rule_based_toolchain/features/BUILD

@@ -109,6 +109,13 @@ util.helper_target(
     overrides = ":builtin_feature",
 )
 
+util.helper_target(
+    cc_feature,
+    name = "sentinel_feature",
+    enabled = True,
+    feature_name = "sentinel_feature_name",
+)
+
 analysis_test_suite(
     name = "test_suite",
     targets = TARGETS,

tests/rule_based_toolchain/features/features_test.bzl

@@ -37,6 +37,12 @@ visibility("private")
 
 _C_COMPILE_FILE = "tests/rule_based_toolchain/testdata/file1"
 
+def _sentinel_feature_test(env, targets):
+    sentinel_feature = env.expect.that_target(targets.sentinel_feature).provider(FeatureInfo)
+    sentinel_feature.name().equals("sentinel_feature_name")
+    sentinel_feature.args().args().contains_exactly([])
+    sentinel_feature.enabled().equals(True)
+
 def _simple_feature_test(env, targets):
     simple = env.expect.that_target(targets.simple).provider(FeatureInfo)
     simple.name().equals("feature_name")
@@ -154,13 +160,15 @@ TARGETS = [
     ":mutual_exclusion_feature",
     ":overrides",
     ":requires",
+    ":sentinel_feature",
     ":simple",
     ":simple2",
     ":transitive_constraint",
 ]
 
 # @unsorted-dict-items
 TESTS = {
+    "sentinel_feature_test": _sentinel_feature_test,
     "simple_feature_test": _simple_feature_test,
     "feature_collects_requirements_test": _feature_collects_requirements_test,
     "feature_collects_implies_test": _feature_collects_implies_test,