bug#15659: 24.1; (wishlist) Simple method for preserving minor-mode

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> A function could take a minor-mode to preserve, and then either set up
> hooks needed to re-enable it after a major-mode change, or prevent
> `kill-all-local-variables' from disabling it in the first place. Or
> perhaps marking the minor-mode's function-symbol itself as
> `permanent-local' could protect it, along with all its buffer-locals.

We could start with something like the following (guaranteed 100% untested):

(put 'after-change-major-mode-hook 'permanent-local-hook t)

(defvar permanent-local--modes nil)

(defun permanent-local--reenable ()
  (mapc #'funcall permanent-local--modes))
(put 'permanent-local--reenable 'permanent-local-hook t)

(defun permanent-local-mode (mode)
  "Enable MODE permanently in this buffer."
  (interactive
   (list
    (intern
     (completing-read "Minor mode: "
                      obarray
                      (lambda (sym)
                        (or (memq mode minor-mode-list)
                            (string-match "-mode\\'" (symbol-name sym))))
                      t))))
  (funcall mode)                        ;Enable.
  (add-hook 'after-change-major-mode-hook #'permanent-local--reenable nil t))


-- Stefan