bug#36492: c-mode fails with errors during fontification

[Juri Linkov <juri@linkov.net>]

>> > I wonder if ignore-errors around font-lock-ensure in mm-display-inline-fontify
>> > is still necessary to ensure no more bugs in other modes disrupt Gnus
>> > by erroneous code in attachments?  Or maybe raising such errors will help
>> > to find fontification bugs sooner?
>>
>> My feeling is that it's better to let the errors happen, so that we can
>> debug them.  But on the other hand, it's not me that gets inconvenienced
>> by such an error (I don't use gnus).
>
> Rather than ignore-errors, perhaps using with-demoted-errors would
> still let the errors be noticed without too much inconvenience?

I tried this change:

diff --git a/lisp/gnus/mm-view.el b/lisp/gnus/mm-view.el
index 6ffa1fc168..c833f2ed01 100644
--- a/lisp/gnus/mm-view.el
+++ b/lisp/gnus/mm-view.el
@@ -500,7 +500,7 @@ mm-display-inline-fontify
 	      (setq mode major-mode)))
 	  ;; Do not fontify if the guess mode is fundamental.
 	  (unless (eq major-mode 'fundamental-mode)
-	    (font-lock-ensure))))
+	    (with-demoted-errors (font-lock-ensure)))))
       (setq text (buffer-string))
       (when (eq mode 'diff-mode)
 	(setq ovs (mapcar (lambda (ov) (list ov (overlay-start ov)

But it pops up the same error backtrace buffer.