Issue #1674958 by das-peter | webchick: Fixed In Drupal 8, drush cc menu gives 'function 'menu_rebuild()' not found or invalid function name' error.
Since Drupal 8 is still a moving target, don't support it in Drush 6. Drupal 8 users should use Drush's development branch. That branch will soon be master, hosted at Github. Stay tuned.