guava/src/com/google/common/collect/Sets.java
public static <E extends Enum<E>> Collector<E, ?, ImmutableSet<E>> toImmutableEnumSet() {
    return (Collector) Accumulator.TO_IMMUTABLE_ENUM_SET;
  }
public static <E extends Enum<E>> Collector<E, ?, ImmutableSet<E>> toImmutableEnumSet() {
    return (Collector) Accumulator.TO_IMMUTABLE_ENUM_SET;
  }
Returns a {@code Collector} that accumulates the input elements into a new {@code ImmutableSet}
with an implementation specialized for enums. Unlike {@link ImmutableSet#toImmutableSet}, the
resulting set will iterate over elements in their enum definition order, not encounter order.

@since 21.0