private static Collection<Artifact> getTargetListAttribute(
     RuleContext ruleContext, String attributeName) {
   return (ruleContext.attributes().has(attributeName, BuildType.LABEL_LIST)
           && ruleContext.getAttributeMode(attributeName) == Mode.TARGET)
       ? ruleContext.getPrerequisiteArtifacts(attributeName, Mode.TARGET).list()
       : ImmutableList.<Artifact>of();
 }