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 09:56:27 +0200
> 
> I don't understand this alternative.  The mode line updating
> that uses 'project-mode-line-format' that unintentionally
> changes 'last-coding-system-used' is called from this line
> in 'basic-save-buffer-2':
> 
>   (write-region nil nil
>                 buffer-file-name nil t buffer-file-truename)
> 
> because this call in 'write_region' updates the mode line:
> 
>     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?

And why does last-coding-system-used get set to raw-text-unix in this
scenario anyway?

> +    ;; Preserve the original value of 'last-coding-system-used'
> +    ;; that can break 'basic-save-buffer' (bug#66825)
> +    (let ((last-coding-system-used nil))
> +      (concat
> +       " "
> +       (propertize
> +        (project-name project)
> +        'face project-mode-line-face
> +        'mouse-face 'mode-line-highlight
> +        'help-echo "mouse-1: Project menu"
> +        'local-map project-mode-line-map)))))

I'm confused how this avoids the problem, probably because I don't
understand the answers to the two questions above.