Re: Last use of defadvice in Emacs

[Alan Mackenzie]

Hello, Stefan.

On Mon, Apr 04, 2022 at 15:49:16 -0400, Stefan Monnier wrote:
> Hi Alan,

> The patch below replaces the last remaining use of `defadvice` in Emacs
> (well, except for Org where this has already been fixed upstream but
> we're waiting for the change to trickle down to `master`).

Why would we want to replace defadvice with advice-add?  Don't all the
objections to advice apply equally to both forms?

> It's guaranteed 100% untested, tho.

:-)

> It's kinda weird in that this `defadvice` is not actually used in
> current Emacsen, but we still have to macro-expand it, so the new code
> macroexpands to the corresponding use of `advice-add` even tho that also
> won't be used.

Yes, Emacs 21 was a long time ago.  Even CC Mode doesn't support it any
more.  The only thing which still uses the advice is XEmacs, and I don't
thing that has advice-add.  So I'm not sure the patch in the current
form makes much sense.  It might be better just to remove the < Emacs 22
bit altogether from the Emacs version of CC Mode.

I've spent an hour trying various combinations of eval-when-compile and
(boundp 'font-lock-extend-after-change-region-function), to try and get
the stuff compiled or ignored based on the presence/absence of that
variable.  Something like C's #ifdef.  I didn't manage it, and don't
think it's possible.  That's another C facility Emacs Lisp seems to be
missing.

>         Stefan


> diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
> index 957a0b8a7c5..759a01f1dd8 100644
> --- a/lisp/progmodes/cc-mode.el
> +++ b/lisp/progmodes/cc-mode.el
> @@ -2563,14 +2563,18 @@ c-extend-after-change-region
>    (cons c-new-BEG c-new-END))
 
>  ;; Emacs < 22 and XEmacs
> +(defun c--fl-extend-region (args)
> +  ;; Make sure that any string/regexp is completely font-locked.
> +  (if (not c-buffer-is-cc-mode)
> +      args
> +    (list c-new-BEG c-new-END (cddr args))))
> +
>  (defmacro c-advise-fl-for-region (function)
>    (declare (debug t))
> -  `(defadvice ,function (before get-awk-region activate)
> -     ;; Make sure that any string/regexp is completely font-locked.
> -     (when c-buffer-is-cc-mode
> -       (save-excursion
> -      (ad-set-arg 1 c-new-END)   ; end
> -      (ad-set-arg 0 c-new-BEG)))))   ; beg
> +  (if (fboundp 'advice-add)
> +      `(advice-add ',function :filter-args #'c--fl-extend-region)
> +    `(defadvice ,function (before get-awk-region activate)
> +       (ad-set-args 0 (c--fl-extend-region (ad-get-args 0))))))
 
>  (unless (boundp 'font-lock-extend-after-change-region-function)
>    (c-advise-fl-for-region font-lock-after-change-function)

-- 
Alan Mackenzie (Nuremberg, Germany).