Rule classes cannot request duplicate toolchains.
PiperOrigin-RevId: 167861778
diff --git a/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java b/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java
index 7fcd60f..5570c3f 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java
@@ -60,7 +60,7 @@
   private final RequiredProviders requiredProviders;
   private final RequiredProviders requiredProvidersForAspects;
   private final ImmutableMap<String, Attribute> attributes;
-  private final ImmutableList<Label> requiredToolchains;
+  private final ImmutableSet<Label> requiredToolchains;
 
   /**
    * Which attributes aspect should propagate along:
@@ -83,7 +83,7 @@
       RequiredProviders requiredProviders,
       RequiredProviders requiredAspectProviders,
       ImmutableMap<String, Attribute> attributes,
-      ImmutableList<Label> requiredToolchains,
+      ImmutableSet<Label> requiredToolchains,
       @Nullable ImmutableSet<String> restrictToAttributes,
       @Nullable ConfigurationFragmentPolicy configurationFragmentPolicy,
       boolean applyToFiles) {
@@ -117,7 +117,7 @@
   }
 
   /** Returns the required toolchains declared by this aspect. */
-  public ImmutableList<Label> getRequiredToolchains() {
+  public ImmutableSet<Label> getRequiredToolchains() {
     return requiredToolchains;
   }
 
@@ -485,7 +485,7 @@
           requiredProviders.build(),
           requiredAspectProviders.build(),
           ImmutableMap.copyOf(attributes),
-          ImmutableList.copyOf(requiredToolchains),
+          ImmutableSet.copyOf(requiredToolchains),
           propagateAlongAttributes == null ? null : ImmutableSet.copyOf(propagateAlongAttributes),
           configurationFragmentPolicy.build(),
           applyToFiles);
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
index 3a222db..dcf6092 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
@@ -59,6 +59,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -483,7 +484,7 @@
     private boolean supportsConstraintChecking = true;
 
     private final Map<String, Attribute> attributes = new LinkedHashMap<>();
-    private final List<Label> requiredToolchains = new ArrayList<>();
+    private final Set<Label> requiredToolchains = new HashSet<>();
 
     /**
      * Constructs a new {@code RuleClassBuilder} using all attributes from all
@@ -1115,7 +1116,7 @@
    */
   private final boolean supportsConstraintChecking;
 
-  private final ImmutableList<Label> requiredToolchains;
+  private final ImmutableSet<Label> requiredToolchains;
 
   /**
    * Constructs an instance of RuleClass whose name is 'name', attributes are 'attributes'. The
@@ -1163,7 +1164,7 @@
       String ruleDefinitionEnvironmentHashCode,
       ConfigurationFragmentPolicy configurationFragmentPolicy,
       boolean supportsConstraintChecking,
-      List<Label> requiredToolchains,
+      Set<Label> requiredToolchains,
       Attribute... attributes) {
     this.name = name;
     this.isSkylark = isSkylark;
@@ -1192,7 +1193,7 @@
     this.outputsDefaultExecutable = outputsDefaultExecutable;
     this.configurationFragmentPolicy = configurationFragmentPolicy;
     this.supportsConstraintChecking = supportsConstraintChecking;
-    this.requiredToolchains = ImmutableList.copyOf(requiredToolchains);
+    this.requiredToolchains = ImmutableSet.copyOf(requiredToolchains);
 
     // Create the index and collect non-configurable attributes.
     int index = 0;
@@ -2002,7 +2003,7 @@
     return outputsDefaultExecutable;
   }
 
-  public ImmutableList<Label> getRequiredToolchains() {
+  public ImmutableSet<Label> getRequiredToolchains() {
     return requiredToolchains;
   }