bug#66825: last-coding-system-used in basic-save-buffer

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Cc: 66825@debbugs.gnu.org
> Date: Mon, 30 Oct 2023 19:20:19 +0200
> 
> >>     message_with_string ((NUMBERP (append)
> >> 			  ? "Updated %s"
> >> 			  : ! NILP (append)
> >> 			  ? "Added to %s"
> >> 			  : "Wrote %s"),
> >> 			 visit_file, 1);
> >
> > How does message_with_string update the mode line?
> 
> I'm not know, some deeper function needs to update the mode line
> when the multi-line message resizes the echo area.

I think it's because message_with_string eventually calls redisplay,
and redisplay updates the mode line as part of its job.

> > And why does last-coding-system-used get set to raw-text-unix in this
> > scenario anyway?
> 
> Because send_process needs to set it to raw-text-unix for ispell:
> 
>   send_process (Lisp_Object proc, const char *buf, ptrdiff_t len, Lisp_Object object)
>   {
>     Vlast_coding_system_used = CODING_ID_NAME (coding->id);

I think this happens because the string sent to the speller is a
plain-ASCII string, and those are almost always unibyte strings.

So I think the local binding of last-coding-system-used around the
call to project-mode-line is TRT, it just needs a better comment to
explain why it's needed.

I think this should also teach us a lesson: calling arbitrary complex
code from mode-line's :eval forms is in general risky business and
should be avoided as much as possible.