src/main/java/com/google/devtools/build/lib/analysis/config/transitions/NullTransition.java

// Copyright 2018 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.
package com.google.devtools.build.lib.analysis.config.transitions;

import com.google.auto.value.AutoValue;
import com.google.devtools.build.lib.analysis.config.BuildOptions;
import com.google.devtools.build.lib.events.EventHandler;
import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;

/** A {@link PatchTransition} to a null configuration. */
public class NullTransition implements PatchTransition {

  @AutoCodec public static final NullTransition INSTANCE = new NullTransition();

  private NullTransition() {
  }

  @Override
  public BuildOptions patch(BuildOptions options, EventHandler eventHandler) {
    throw new UnsupportedOperationException(
        "This is only referenced in a few places, so it's easier and more efficient to optimize "
            + "Blaze's transition logic in the presence of null transitions vs. actually call this "
            + "method to get results we know ahead of time. If there's ever a need to properly "
            + "implement this method we can always do so.");
  }

  /** Returns a {@link TransitionFactory} instance that generates the null transition. */
  public static <T> TransitionFactory<T> createFactory() {
    return new AutoValue_NullTransition_Factory<>();
  }

  /**
   * Returns {@code true} if the given {@link TransitionFactory} is an instance of the null
   * transition.
   */
  public static <T> boolean isInstance(TransitionFactory<T> instance) {
    return instance instanceof Factory;
  }

  /** A {@link TransitionFactory} implementation that generates the null transition. */
  @AutoValue
  abstract static class Factory<T> implements TransitionFactory<T> {
    @Override
    public ConfigurationTransition create(T unused) {
      return INSTANCE;
    }
  }
}