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

[Juri Linkov <juri@linkov.net>]

>> >> (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.

The difference is that an explicit option is more controllable by the user.
When the user needs to use some ts-mode then it's easier just to customize
the option instead of tweaking 'auto-mode-alist' when the user want to
start using that mode without first loading its package or calling it
the first time that modifies 'auto-mode-alist' as the side effect.
Or when the user wants to remove the mode from 'auto-mode-alist' after
accidentally loading the corresponding ts package.