bug#60176: 29.0.60; Fallback file variable mode if treesitter is not usable

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Cc: monnier@iro.umontreal.ca,  sadiq@sadiqpk.org,  60176@debbugs.gnu.org
> Date: Mon, 16 Jan 2023 19:32:06 +0200
> 
> >> (defcustom treesit-enable-modes nil
> >>   :type '(repeat
> >>           (choice (function-item c-ts-mode)
> >>                   (function-item c++-ts-mode)
> >>                   (function-item c-or-c++-ts-mode)
> >>                   ...
> >>              ('c-ts-mode
> >>               (when (treesit-ready-p 'c t)
> >>                 (add-to-list 'major-mode-remap-alist '(c-mode . c-ts-mode))))
> >
> > And this bit is completely unacceptable, from where I stand: it
> > basically means that the user activated a certain major mode he/she
> > wanted to use, but the result could be that an entirely different mode
> > was silently activated instead.  What kind of UX is that, and for a
> > shining new feature at that??
> 
> It could update 'auto-mode-alist' instead of 'major-mode-remap-alist'.
> >From the user's point of view this would be more manageable than
> what you proposed on emacs-devel with some obscure logic of activating
> ts modes when the package is loaded or when the mode is enabled first time.

I don't understand the "obscure" part: the logic was exactly as above:
test that treesit-ready-p returns non-nil for the mode's language.

Other than that, my proposal does exactly what you say here: it
updates auto-mode-alist.  So it sounds like we are in violent agreement.