in chill-scala/src/main/scala/com/twitter/chill/ClosureCleaner.scala [153:162]
def accessedFieldsOf(cls: Class[_]): Set[Field] =
accessedFieldsMap.get(cls) match {
case Some(s) => s
case None =>
// Compute and store:
val af = getAccessedFields(cls)
// Add all of af:
accessedFieldsMap ++= af
af.getOrElse(cls, Set.empty)
}