Remove unused NullTransition.

Work towards composable starlark transitions: #22248.

PiperOrigin-RevId: 636209749
Change-Id: I9ecdec8b376a9c95b06dea36bacbf9268d0c5195

src/main/java/com/google/devtools/build/lib/analysis/BUILD

@@ -77,7 +77,6 @@ java_library(
         ":config/transitions/composing_transition_factory",
         ":config/transitions/configuration_transition",
         ":config/transitions/no_transition",
-        ":config/transitions/null_transition",
         ":config/transitions/patch_transition",
         ":config/transitions/split_transition",
         ":config/transitions/transition_factory",
@@ -2050,7 +2049,6 @@ java_library(
     deps = [
         ":config/transitions/configuration_transition",
         ":config/transitions/no_transition",
-        ":config/transitions/null_transition",
         ":config/transitions/split_transition",
         ":config/transitions/transition_factory",
         "//third_party:auto_value",
@@ -2119,7 +2117,6 @@ java_library(
         ":config/build_options",
         ":config/transitions/configuration_transition",
         ":config/transitions/no_transition",
-        ":config/transitions/null_transition",
         ":required_config_fragments_provider",
         "//src/main/java/com/google/devtools/build/lib/events",
         "//third_party:guava",
@@ -2133,7 +2130,6 @@ java_library(
         ":config/transitions/composing_transition",
         ":config/transitions/configuration_transition",
         ":config/transitions/no_transition",
-        ":config/transitions/null_transition",
         ":config/transitions/transition_factory",
         "//third_party:auto_value",
         "//third_party:guava",
@@ -2172,21 +2168,6 @@ java_library(
     ],
 )
 
-java_library(
-    name = "config/transitions/null_transition",
-    srcs = ["config/transitions/NullTransition.java"],
-    deps = [
-        ":config/build_options",
-        ":config/transitions/configuration_transition",
-        ":config/transitions/patch_transition",
-        ":config/transitions/transition_factory",
-        "//src/main/java/com/google/devtools/build/lib/events",
-        "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec",
-        "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec:serialization-constant",
-        "//third_party:auto_value",
-    ],
-)
-
 java_library(
     name = "config/transitions/no_config_transition",
     srcs = ["config/transitions/NoConfigTransition.java"],

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

@@ -16,7 +16,6 @@
 import com.google.auto.value.AutoValue;
 import com.google.devtools.build.lib.analysis.config.transitions.ConfigurationTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
-import com.google.devtools.build.lib.analysis.config.transitions.NullTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.TransitionFactory;
 
@@ -33,8 +32,6 @@ public static <T extends TransitionFactory.Data> TransitionFactory<T> of(
       ConfigurationTransition transition) {
     if (transition instanceof NoTransition) {
       return NoTransition.createFactory();
-    } else if (transition instanceof NullTransition) {
-      return NullTransition.createFactory();
     } else if (transition instanceof SplitTransition splitTransition) {
       return split(splitTransition);
     }

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

@@ -119,29 +119,17 @@ public static ConfigurationTransition of(
     Preconditions.checkNotNull(transition1);
     Preconditions.checkNotNull(transition2);
 
-    if (isFinal(transition1)) {
-      // Since no other transition can be composed with transition1, use it directly.
-      return transition1;
-    } else if (transition1 == NoTransition.INSTANCE) {
+    if (transition1 == NoTransition.INSTANCE) {
       // Since transition1 causes no changes, use transition2 directly.
       return transition2;
-    }
-
-    if (transition2 == NoTransition.INSTANCE) {
+    } else if (transition2 == NoTransition.INSTANCE) {
       // Since transition2 causes no changes, use transition 1 directly.
       return transition1;
-    } else if (isFinal(transition2)) {
-      // When the second transition is null, there's no need to compose.
-      return transition2;
     }
 
     return new ComposingTransition(transition1, transition2);
   }
 
-  private static boolean isFinal(ConfigurationTransition transition) {
-    return transition == NullTransition.INSTANCE;
-  }
-
   /**
    * Composes a new key out of two given keys. Composing two split transitions is not allowed at the
    * moment, so what this essentially does are (1) make sure not both transitions are split and (2)

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

@@ -51,30 +51,17 @@ public static <T extends TransitionFactory.Data> TransitionFactory<T> of(
         !transitionFactory1.isSplit() || !transitionFactory2.isSplit(),
         "can't compose two split transition factories");
 
-    if (isFinal(transitionFactory1)) {
-      // Since no other transition can be composed with transitionFactory1, use it directly.
-      return transitionFactory1;
-    } else if (NoTransition.isInstance(transitionFactory1)) {
+    if (NoTransition.isInstance(transitionFactory1)) {
       // Since transitionFactory1 causes no changes, use transitionFactory2 directly.
       return transitionFactory2;
-    }
-
-    if (NoTransition.isInstance(transitionFactory2)) {
+    } else if (NoTransition.isInstance(transitionFactory2)) {
       // Since transitionFactory2 causes no changes, use transitionFactory1 directly.
       return transitionFactory1;
-    } else if (isFinal(transitionFactory2)) {
-      // When the second transition is null there's no need to compose.
-      return transitionFactory2;
     }
 
     return create(transitionFactory1, transitionFactory2);
   }
 
-  private static <T extends TransitionFactory.Data> boolean isFinal(
-      TransitionFactory<T> transitionFactory) {
-    return NullTransition.isInstance(transitionFactory);
-  }
-
   private static <T extends TransitionFactory.Data> TransitionFactory<T> create(
       TransitionFactory<T> transitionFactory1, TransitionFactory<T> transitionFactory2) {
     return new AutoValue_ComposingTransitionFactory<T>(transitionFactory1, transitionFactory2);

src/main/java/com/google/devtools/build/lib/query2/BUILD

@@ -48,7 +48,6 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/composing_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/configuration_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/no_transition",
-        "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/null_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/transition_factory",
         "//src/main/java/com/google/devtools/build/lib/analysis:configured_target",
         "//src/main/java/com/google/devtools/build/lib/analysis:configured_target_value",

src/main/java/com/google/devtools/build/lib/query2/cquery/CqueryTransitionResolver.java

@@ -34,7 +34,6 @@
 import com.google.devtools.build.lib.analysis.config.transitions.ComposingTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.ConfigurationTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
-import com.google.devtools.build.lib.analysis.config.transitions.NullTransition;
 import com.google.devtools.build.lib.analysis.configuredtargets.RuleConfiguredTarget;
 import com.google.devtools.build.lib.analysis.constraints.IncompatibleTargetChecker.IncompatibleTargetException;
 import com.google.devtools.build.lib.cmdline.Label;
@@ -55,9 +54,10 @@
 import javax.annotation.Nullable;
 
 /**
- * TransitionResolver resolves the dependencies of a {@link ConfiguredTarget}, reporting which
- * configurations its dependencies are actually needed in according to the transitions applied to
- * them. See {@link TransitionsOutputFormatterCallback}.
+ * TransitionResolver resolves the dependencies of a {@link
+ * com.google.devtools.build.lib.analysis.ConfiguredTarget}, reporting which configurations its
+ * dependencies are actually needed in according to the transitions applied to them. See {@link
+ * TransitionsOutputFormatterCallback}.
  */
 public class CqueryTransitionResolver {
 
@@ -187,15 +187,12 @@ public ImmutableSet<ResolvedTransition> dependencies(CqueryNode configuredTarget
         Label label = labelEntry.getKey();
         Collection<ConfiguredTargetAndData> targets = labelEntry.getValue();
 
-        ConfigurationTransition noOrNullTransition =
-            getTransitionIfNoOrNull(configuration, targets);
-        if (noOrNullTransition != null) {
+        // The most common case, so short-circuit this.
+        String transitionName = usesNoTransition(configuration, targets);
+        if (transitionName != null) {
           resolved.add(
               ResolvedTransition.create(
-                  label,
-                  /* buildOptions= */ ImmutableList.of(),
-                  dependencyName,
-                  noOrNullTransition.getName()));
+                  label, /* buildOptions= */ ImmutableList.of(), dependencyName, transitionName));
           continue;
         }
 
@@ -224,15 +221,16 @@ private EvaluateException(String message) {
   }
 
   @Nullable
-  private static ConfigurationTransition getTransitionIfNoOrNull(
+  private static String usesNoTransition(
       BuildConfigurationValue fromConfiguration, Collection<ConfiguredTargetAndData> targets) {
     ConfiguredTargetAndData first = targets.iterator().next();
+    // Check whether the configuration changed.
     if (targets.size() == 1 && Objects.equals(fromConfiguration, first.getConfiguration())) {
-      return NoTransition.INSTANCE;
+      return NoTransition.INSTANCE.getName();
     }
     // If any target has a null configuration, they all do, so it's sufficient to check the first.
     if (first.getConfiguration() == null) {
-      return NullTransition.INSTANCE;
+      return "(null transition)";
     }
     return null;
   }

src/test/java/com/google/devtools/build/lib/analysis/config/BUILD

@@ -44,7 +44,6 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/analysis:config/run_under_converter",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transition_factories",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/no_transition",
-        "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/null_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/patch_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/split_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/transition_factory",

src/test/java/com/google/devtools/build/lib/analysis/config/TransitionFactoriesTest.java

@@ -17,7 +17,6 @@
 
 import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
-import com.google.devtools.build.lib.analysis.config.transitions.NullTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.TransitionFactory;
 import org.junit.Test;
@@ -37,15 +36,6 @@ public void noTransition() {
     assertThat(factory.isSplit()).isFalse();
   }
 
-  @Test
-  public void nullTransition() {
-    TransitionFactory<TransitionFactory.Data> factory =
-        TransitionFactories.of(NullTransition.INSTANCE);
-    assertThat(factory).isNotNull();
-    assertThat(NullTransition.isInstance(factory)).isTrue();
-    assertThat(factory.isSplit()).isFalse();
-  }
-
   @Test
   public void splitTransition() {
     TransitionFactory<TransitionFactory.Data> factory =

src/test/java/com/google/devtools/build/lib/analysis/config/transitions/BUILD

@@ -29,7 +29,6 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/configuration_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/no_config_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/no_transition",
-        "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/null_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/patch_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/split_transition",
         "//src/main/java/com/google/devtools/build/lib/analysis:config/transitions/transition_factory",

src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionFactoryTest.java

@@ -157,28 +157,6 @@ public void compose_noTrans_second() {
     assertThat(composed).isEqualTo(patch);
   }
 
-  @Test
-  public void compose_nullTrans_first() {
-    StubPatch patch = new StubPatch(FLAG_1, "value");
-    TransitionFactory<StubData> composed =
-        ComposingTransitionFactory.of(
-            NullTransition.createFactory(), TransitionFactories.of(patch));
-
-    assertThat(composed).isNotNull();
-    assertThat(NullTransition.isInstance(composed)).isTrue();
-  }
-
-  @Test
-  public void compose_nullTrans_second() {
-    StubPatch patch = new StubPatch(FLAG_1, "value");
-    TransitionFactory<StubData> composed =
-        ComposingTransitionFactory.of(
-            TransitionFactories.of(patch), NullTransition.createFactory());
-
-    assertThat(composed).isNotNull();
-    assertThat(NullTransition.isInstance(composed)).isTrue();
-  }
-
   private static final class StubData implements TransitionFactory.Data {}
 
   // Helper methods and classes for the tests.