1# Copyright 2024 The Bazel Authors. All rights reserved. 2# 3# Licensed under the Apache License, Version 2.0 (the "License"); 4# you may not use this file except in compliance with the License. 5# You may obtain a copy of the License at 6# 7# http://www.apache.org/licenses/LICENSE-2.0 8# 9# Unless required by applicable law or agreed to in writing, software 10# distributed under the License is distributed on an "AS IS" BASIS, 11# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12# See the License for the specific language governing permissions and 13# limitations under the License. 14 15"""A rule to define a target to act as a singleton for label attributes. 16 17Label attributes with defaults cannot accept None, otherwise they fall 18back to using the default. A sentinel allows detecting an intended None value. 19""" 20 21SentinelInfo = provider( 22 doc = "Indicates this was the sentinel target.", 23 fields = [], 24) 25 26def _sentinel_impl(ctx): 27 _ = ctx # @unused 28 return [SentinelInfo()] 29 30sentinel = rule(implementation = _sentinel_impl) 31