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

[Mohammed Sadiq <sadiq@sadiqpk.org>]

On 2022-12-19 18:05, Eli Zaretskii wrote:
>> 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.

Right, that's what I was suggesting.