Use an enum set for the FeatureSet. ImmutableSet only uses EnumSets i…

…f given an EnumSet to start.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181516248

src/com/google/javascript/jscomp/parsing/parser/FeatureSet.java

@@ -16,11 +16,11 @@
 
 package com.google.javascript.jscomp.parsing.parser;
 
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.ImmutableSet;
-import com.google.common.collect.Sets;
 import com.google.errorprone.annotations.Immutable;
 import java.io.Serializable;
-import java.util.HashSet;
+import java.util.EnumSet;
 import java.util.Set;
 
 /**
@@ -44,7 +44,7 @@ public final class FeatureSet implements Serializable {
   private final ImmutableSet<Feature> features;
 
   /** The bare minimum set of features. */
-  public static final FeatureSet BARE_MINIMUM = new FeatureSet(ImmutableSet.<Feature>of());
+  public static final FeatureSet BARE_MINIMUM = new FeatureSet(emptyEnumSet());
 
   /** Features from ES3. */
   public static final FeatureSet ES3 = BARE_MINIMUM.with(LangVersion.ES3.features());
@@ -91,8 +91,8 @@ private enum LangVersion {
     ES_NEXT,
     TYPESCRIPT;
 
-    private Set<Feature> features() {
-      Set<Feature> set = new HashSet<>();
+    private EnumSet<Feature> features() {
+      EnumSet<Feature> set = EnumSet.noneOf(Feature.class);
       for (Feature feature : Feature.values()) {
         if (feature.version == this) {
           set.add(feature);
@@ -180,7 +180,8 @@ public String toString() {
     }
   }
 
-  private FeatureSet(Set<Feature> features) {
+  private FeatureSet(EnumSet<Feature> features) {
+    // ImmutableSet will only use an EnumSet if the set starts as an EnumSet.
     this.features = ImmutableSet.copyOf(features);
   }
 
@@ -214,22 +215,22 @@ public String version() {
   }
 
   public FeatureSet without(Feature feature) {
-    return new FeatureSet(Sets.difference(features, ImmutableSet.of(feature)));
+    return new FeatureSet(difference(features, EnumSet.of(feature)));
   }
 
   public FeatureSet without(FeatureSet other) {
-    return new FeatureSet(Sets.difference(features, other.features));
+    return new FeatureSet(difference(features, other.features));
   }
 
   public FeatureSet withoutTypes() {
-    return new FeatureSet(Sets.difference(features, LangVersion.TYPESCRIPT.features()));
+    return new FeatureSet(difference(features, LangVersion.TYPESCRIPT.features()));
   }
 
   /**
    * Returns a new {@link FeatureSet} including all features of both {@code this} and {@code other}.
    */
   public FeatureSet union(FeatureSet other) {
-    return new FeatureSet(Sets.union(features, other.features));
+    return new FeatureSet(union(features, other.features));
   }
 
   /**
@@ -239,14 +240,50 @@ public boolean contains(FeatureSet other) {
     return this.features.containsAll(other.features);
   }
 
+  private static EnumSet<Feature> emptyEnumSet() {
+    return EnumSet.noneOf(Feature.class);
+  }
+
+  private static EnumSet<Feature> enumSetOf(Set<Feature> set) {
+    return set.isEmpty() ? emptyEnumSet() : EnumSet.copyOf(set);
+  }
+
+  private static EnumSet<Feature> add(Set<Feature> features, Feature feature) {
+    EnumSet<Feature> result = enumSetOf(features);
+    result.add(feature);
+    return result;
+  }
+
+  private static EnumSet<Feature> union(Set<Feature> features, Set<Feature> newFeatures) {
+    EnumSet<Feature> result = enumSetOf(features);
+    result.addAll(newFeatures);
+    return result;
+  }
+
+  private static EnumSet<Feature> difference(Set<Feature> features, Set<Feature> removedFeatures) {
+    EnumSet<Feature> result = enumSetOf(features);
+    result.removeAll(removedFeatures);
+    return result;
+  }
+
+  /** Returns a feature set combining all the features from {@code this} and {@code feature}. */
+  public FeatureSet with(Feature feature) {
+    if (features.contains(feature)) {
+      return this;
+    }
+    return new FeatureSet(add(features, feature));
+  }
+
   /** Returns a feature set combining all the features from {@code this} and {@code newFeatures}. */
+  @VisibleForTesting
   public FeatureSet with(Feature... newFeatures) {
-    return new FeatureSet(Sets.union(features, ImmutableSet.copyOf(newFeatures)));
+    return new FeatureSet(union(features, ImmutableSet.copyOf(newFeatures)));
   }
 
   /** Returns a feature set combining all the features from {@code this} and {@code newFeatures}. */
+  @VisibleForTesting
   public FeatureSet with(Set<Feature> newFeatures) {
-    return new FeatureSet(Sets.union(features, newFeatures));
+    return new FeatureSet(union(features, newFeatures));
   }
 
   /**