Provide a default for RuleClass.transitionFactory.

This removes a lot of useless null checks.

Work towards composable starlark transitions: #22248.

PiperOrigin-RevId: 639805169
Change-Id: I45b4f4b7cf71e568578956d6b7723fc999f5afc8

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

@@ -235,13 +235,11 @@ private static void addRequiredFragmentsFromRuleTransitions(
       ConfiguredAttributeMapper attributeMap,
       BuildOptionDetails optionDetails,
       @Nullable StarlarkAttributeTransitionProvider starlarkExecTransition) {
-    if (target.getRuleClassObject().getTransitionFactory() != null) {
-      target
-          .getRuleClassObject()
-          .getTransitionFactory()
-          .create(RuleTransitionData.create(target, configConditions.asProviders(), configHash))
-          .addRequiredFragments(requiredFragments, optionDetails);
-    }
+    target
+        .getRuleClassObject()
+        .getTransitionFactory()
+        .create(RuleTransitionData.create(target, configConditions.asProviders(), configHash))
+        .addRequiredFragments(requiredFragments, optionDetails);
     // We don't set the execution platform in this data because a) that doesn't affect which
     // fragments are required and b) it's one less parameter we have to pass to
     // RequiredFragmenstUtil's public interface.

src/main/java/com/google/devtools/build/lib/analysis/producers/TargetAndConfigurationProducer.java

@@ -466,13 +466,9 @@ public StateMachine computeTransition(Tasks tasks) {
               configConditions.asProviders(),
               preRuleTransitionKey.getConfigurationKey().getOptionsChecksum());
 
-      ConfigurationTransition transition = null;
-
       TransitionFactory<RuleTransitionData> transitionFactory =
           target.getAssociatedRule().getRuleClassObject().getTransitionFactory();
-      if (transitionFactory != null) {
-        transition = transitionFactory.create(transitionData);
-      }
+      ConfigurationTransition transition = transitionFactory.create(transitionData);
 
       if (trimmingTransitionFactory != null) {
         var trimmingTransition = trimmingTransitionFactory.create(transitionData);
@@ -483,11 +479,6 @@ public StateMachine computeTransition(Tasks tasks) {
         }
       }
 
-      if (transition == null) {
-        lookUpConfigurationValue(tasks);
-        return DONE;
-      }
-
       this.ruleTransition = transition;
 
       return new TransitionApplier(
@@ -695,18 +686,13 @@ public static DetailedExitCode createDetailedExitCode(String message) {
 
   // Public for Cquery.
   // TODO: @aranguyen keep cquery in sync with ConfiguredTargetFunction
-  @Nullable
   public static ConfigurationTransition computeTransition(
       Rule rule, @Nullable TransitionFactory<RuleTransitionData> trimmingTransitionFactory) {
     var transitionData = RuleTransitionData.create(rule, /* configConditions= */ null, "");
 
-    ConfigurationTransition transition = null;
-
     TransitionFactory<RuleTransitionData> transitionFactory =
         rule.getRuleClassObject().getTransitionFactory();
-    if (transitionFactory != null) {
-      transition = transitionFactory.create(transitionData);
-    }
+    ConfigurationTransition transition = transitionFactory.create(transitionData);
     boolean isAlias = rule.getAssociatedRule().getName().equals("alias");
     if (trimmingTransitionFactory != null && !isAlias) {
       var trimmingTransition = trimmingTransitionFactory.create(transitionData);

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java

@@ -779,7 +779,7 @@ public static StarlarkRuleFunction createRule(
             exposed.addToRuleFromStarlark(ruleDefinitionEnvironment, builder);
           }
         });
-    if (parent != null && parent.getTransitionFactory() != null) {
+    if (parent != null) {
       transitionFactory =
           ComposingTransitionFactory.of(transitionFactory, parent.getTransitionFactory());
     }

src/main/java/com/google/devtools/build/lib/packages/RuleClass.java

@@ -35,6 +35,7 @@
 import com.google.common.collect.Maps;
 import com.google.devtools.build.lib.analysis.config.Fragment;
 import com.google.devtools.build.lib.analysis.config.ToolchainTypeRequirement;
+import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.TransitionFactory;
 import com.google.devtools.build.lib.analysis.config.transitions.TransitionFactory.TransitionType;
 import com.google.devtools.build.lib.cmdline.Label;
@@ -721,7 +722,7 @@ public String toString() {
         ImmutableList.builder();
     private boolean ignoreLicenses = false;
     private ImplicitOutputsFunction implicitOutputsFunction = SafeImplicitOutputsFunction.NONE;
-    @Nullable private TransitionFactory<RuleTransitionData> transitionFactory;
+    private TransitionFactory<RuleTransitionData> transitionFactory = NoTransition.getFactory();
     private ConfiguredTargetFactory<?, ?, ?> configuredTargetFactory = null;
     private final AdvertisedProviderSet.Builder advertisedProviders =
         AdvertisedProviderSet.builder();
@@ -1141,7 +1142,8 @@ public Builder cfg(TransitionFactory<RuleTransitionData> transitionFactory) {
           type != RuleClassType.ABSTRACT,
           "Setting not inherited property (cfg) of abstract rule class '%s'",
           name);
-      Preconditions.checkState(this.transitionFactory == null, "Property cfg has already been set");
+      Preconditions.checkState(
+          NoTransition.isInstance(this.transitionFactory), "Property cfg has already been set");
       Preconditions.checkNotNull(transitionFactory);
       Preconditions.checkArgument(
           transitionFactory.transitionType().isCompatibleWith(TransitionType.RULE));
@@ -1674,7 +1676,7 @@ public Attribute.Builder<?> copy(String name) {
    * A factory which will produce a configuration transition that should be applied on any edge of
    * the configured target graph that leads into a target of this rule class.
    */
-  @Nullable private final TransitionFactory<RuleTransitionData> transitionFactory;
+  private final TransitionFactory<RuleTransitionData> transitionFactory;
 
   /** The factory that creates configured targets from this rule. */
   private final ConfiguredTargetFactory<?, ?, ?> configuredTargetFactory;
@@ -1782,7 +1784,7 @@ public Attribute.Builder<?> copy(String name) {
       ImmutableList<AllowlistChecker> allowlistCheckers,
       boolean ignoreLicenses,
       ImplicitOutputsFunction implicitOutputsFunction,
-      @Nullable TransitionFactory<RuleTransitionData> transitionFactory,
+      TransitionFactory<RuleTransitionData> transitionFactory,
       ConfiguredTargetFactory<?, ?, ?> configuredTargetFactory,
       AdvertisedProviderSet advertisedProviders,
       @Nullable StarlarkCallable configuredTargetFunction,
@@ -1892,7 +1894,6 @@ public ImplicitOutputsFunction getDefaultImplicitOutputsFunction() {
     return implicitOutputsFunction;
   }
 
-  @Nullable
   public TransitionFactory<RuleTransitionData> getTransitionFactory() {
     return transitionFactory;
   }

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

@@ -137,9 +137,6 @@ private static String getRuleClassTransition(CqueryNode ct, Target target) {
 
     TransitionFactory<RuleTransitionData> factory =
         rule.getRuleClassObject().getTransitionFactory();
-    if (factory == null) {
-      return "";
-    }
     return factory
         .create(
             RuleTransitionData.create(