| # Copyright 2024 The Bazel Authors. All rights reserved. |
| # |
| # Licensed under the Apache License, Version 2.0 (the "License"); |
| # you may not use this file except in compliance with the License. |
| # You may obtain a copy of the License at |
| # |
| # http://www.apache.org/licenses/LICENSE-2.0 |
| # |
| # Unless required by applicable law or agreed to in writing, software |
| # distributed under the License is distributed on an "AS IS" BASIS, |
| # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| # See the License for the specific language governing permissions and |
| # limitations under the License. |
| |
| """A rule to define a target to act as a singleton for label attributes. |
| |
| Label attributes with defaults cannot accept None, otherwise they fall |
| back to using the default. A sentinel allows detecting an intended None value. |
| """ |
| |
| SentinelInfo = provider( |
| doc = "Indicates this was the sentinel target.", |
| fields = [], |
| ) |
| |
| def _sentinel_impl(ctx): |
| _ = ctx # @unused |
| return [SentinelInfo()] |
| |
| sentinel = rule(implementation = _sentinel_impl) |
