guava/src/com/google/common/collect/ImmutableCollection.java
@CanIgnoreReturnValue
  int copyIntoArray(Object[] dst, int offset) {
    for (E e : this) {
      dst[offset++] = e;
    }
    return offset;
  }
@CanIgnoreReturnValue
  int copyIntoArray(Object[] dst, int offset) {
    for (E e : this) {
      dst[offset++] = e;
    }
    return offset;
  }
Copies the contents of this immutable collection into the specified array at the specified
offset. Returns {@code offset + size()}.