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

[Juri Linkov]

>> >> >> (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.
>
> With the changes I proposed, there's no need to tweak
> auto-mode-alist.  A simple load or require of the mode will install
> the mode in auto-mode-alist.  What can be easier and simpler?

This doesn't address the problems mentioned above and below.

>> Or when the user wants to remove the mode from 'auto-mode-alist' after
>> accidentally loading the corresponding ts package.