(defadvice debug-on-entry (around advise-primitive activate)
  "If an error is signalled because FUNCTION is a primitive, query the user
whether to debug Lisp calls via `defadvice'."
  (condition-case condition-data
      ad-do-it
    (error (let ((error-message (format "Function %s is a primitive"
					(ad-get-arg 0))))
	     (if (and (equal (car (cdr condition-data)) error-message)
		      (interactive-p)
		      (yes-or-no-p
		       (concat error-message
			       "; debug Lisp calls via defadvice? ")))
		 (eval `(defadvice ,(ad-get-arg 0) (before debug activate)
			  "Debug (when called from an Emacs Lisp function)."
			  (debug)))
	       (signal 'error (cdr condition-data)))))))

;; (defadvice cancel-debug-on-entry (around advise-primitive activate)
;;   ...)