bug#60643: 29.0.50; set-buffer-major-mode resets buffer local variables

[Eli Zaretskii]

> Date: Sat, 07 Jan 2023 20:48:54 -0800
> From:  Joseph Turner via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> It appears that set-buffer-major-mode resets buffer local variables.
> 
> To reproduce with `emacs -Q`:
> 
> (defvar-local test-var nil)
> (setq test-var t)
> (message "%s" test-var) ;; t
> ;; (set-auto-mode)
> (set-buffer-major-mode (current-buffer))
> (message "%s" test-var) ;; nil
> 
> Is this behavior expected?

Yes, I think so.  Changing the major mode kills all buffer-local
variables.  If you don't want some variable to be killed, give it the
permanent-local property.