Re: Last use of defadvice in Emacs

[Stefan Monnier via CC-Mode-help <cc-mode-help@lists.sourceforge.net>]

>> 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?

Both should be avoided, indeed.  But `defadvice` is slowly being
replaced because it cannot obey `lexical-binding` (along with a bunch
of more minor annoyances) so it gets a few more objections.

> 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.

I thought the code was written specifically to perform the test at
runtime, so that the code compiled on Emacs-21 would still skip the
advice when run on Emacs≥22 (and also so code compiled on Emacs-28 would
still activate the advice when run on some hypothetical future Emacs
where `font-lock-extend-after-change-region-function` has been removed).

If you want to perform the test at compile time then I think something
like the following should work.


        Stefan


diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
index 957a0b8a7c5..05da61dbb2f 100644
--- a/lisp/progmodes/cc-mode.el
+++ b/lisp/progmodes/cc-mode.el
@@ -2565,18 +2565,18 @@
 ;; Emacs < 22 and XEmacs
 (defmacro c-advise-fl-for-region (function)
   (declare (debug t))
+  (unless (boundp 'font-lock-extend-after-change-region-function)
   `(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
+	   (ad-set-arg 0 c-new-BEG)))))) ; beg
 
-(unless (boundp 'font-lock-extend-after-change-region-function)
-  (c-advise-fl-for-region font-lock-after-change-function)
-  (c-advise-fl-for-region jit-lock-after-change)
-  (c-advise-fl-for-region lazy-lock-defer-rest-after-change)
-  (c-advise-fl-for-region lazy-lock-defer-line-after-change))
+(c-advise-fl-for-region font-lock-after-change-function)
+(c-advise-fl-for-region jit-lock-after-change)
+(c-advise-fl-for-region lazy-lock-defer-rest-after-change)
+(c-advise-fl-for-region lazy-lock-defer-line-after-change)
 
 ;; Connect up to `electric-indent-mode' (Emacs 24.4 and later).
 (defun c-electric-indent-mode-hook ()