Hi emacs-devel,

Do bug reports with a proposed fix go on the bug tracker, or on this list? I've seen patches discussed here, so I'm not too sure. Apologies if this is the wrong place (please do let me know!)

Changing the lists of symbols prettified by prettify-symbols-mode is more tricky than it should be; I think this is a bug in prettify-symbols-mode. Here is a test case:

(with-current-buffer (get-buffer-create "fl")
  (erase-buffer)
  (emacs-lisp-mode)
  (prettify-symbols-mode -1)
  (setq prettify-symbols-alist '(("A" . ?B)))
  (prettify-symbols-mode)
  (setq prettify-symbols-alist '(("A" . ?C)))
  (prettify-symbols-mode)
  (insert "A A A")
  (pop-to-buffer (current-buffer)))

Looking at this code, I expect “C C C” to be shown. Yet it's not the case; instead, “B B B” is shown.

On the other hand, this works:

(with-current-buffer (get-buffer-create "fl")
  (erase-buffer)
  (emacs-lisp-mode)
  (prettify-symbols-mode -1)
  (setq prettify-symbols-alist '(("A" . ?B)))
  (prettify-symbols-mode)
  (prettify-symbols-mode -1)
  (setq prettify-symbols-alist '(("A" . ?C)))
  (prettify-symbols-mode)
  (insert "A A A")
  (pop-to-buffer (current-buffer)))

I think the line indicated by !!! in the following code sample is wrong:

    (define-minor-mode prettify-symbols-mode
      (...)
      (if prettify-symbols-mode
          ;; Turn on
          (when (setq prettify-symbols--keywords (prettify-symbols--make-keywords))
            (font-lock-add-keywords nil prettify-symbols--keywords) ;; !!! HERE
            (setq-local font-lock-extra-managed-props
                        (append font-lock-extra-managed-props
                                '(composition
                                  prettify-symbols-start
                                  prettify-symbols-end)))
            (when prettify-symbols-unprettify-at-point
              (add-hook 'post-command-hook
                        #'prettify-symbols--post-command-hook nil t))
            (font-lock-flush))

Indeed, turning on prettify-symbols-mode twice in a row causes two entries to be added to font-lock-add-keywords. This is not an issue in general, except if one changes prettify-symbols-alist between two calls. In that case font-lock-keywords gets two distinct entries for prettify-symbols, and the second one is ignored.

The proposed fix is to move the following snippet up, so that it runs in all cases, instead of just running when prettify-symbols-mode is turned off.

    (when prettify-symbols--keywords
      (font-lock-remove-keywords nil prettify-symbols--keywords)
      (setq prettify-symbols--keywords nil))

Cheers,
Clément.