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

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, 60176@debbugs.gnu.org
> From: Juri Linkov <juri@linkov.net>
> Date: Mon, 19 Dec 2022 09:39:14 +0200
> 
> > It would be nice if the fallback mode is used when treesitter is not
> > usable.  Say for example, for the following file variable in a C file:
> >
> > /* -*- mode: c; mode: c-ts; -*- */
> >
> > If c-ts-mode is not usable (regardless of whether the mode is
> > available), activate c-mode instead.
> >
> > This bug is regarding the automatically loaded mode when a file with some
> > mode is set, and not regarding the user explicitly setting the mode.
> >
> > Supporting this can help me use multiple mode file variables regardless
> > of the GNU Emacs version used by co-developers.
> 
> 'major-mode-remap-alist' should be able to handle this requirement
> since its design is similar to MIME.TYPES and MAILCAP.  But actually
> MAILCAP supports an additional predicate TEST that is a command
> executed to determine whether or not the mailcap line applies.
> 
> So to completely cover all needs, 'major-mode-remap-alist' should
> support a predicate as well.  For example,
> 
> (setq major-mode-remap-alist '((c-mode #'treesit-available-p c-ts-mode)))
> 
> or a more complex predicate:
> 
> (setq major-mode-remap-alist '((c-mode (lambda ()
>                                          (and (treesit-available-p)
>                                               (treesit-ready-p 'c t)))
>                                 c-ts-mode)))

I think the idea is that this should happen by default, not as a
(not-so-trivial) customization by the users.