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: sadiq@sadiqpk.org,  monnier@iro.umontreal.ca,  60176@debbugs.gnu.org
> Date: Mon, 19 Dec 2022 19:29:13 +0200
> 
> >> 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)))
> >
> > I think the idea is that this should happen by default, not as a
> > (not-so-trivial) customization by the users.
> 
> Then 'c-ts-mode' should fall back to 'c-mode' when treesit is not available?
> Like this:
> 
> ```
> diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
> index cf941236f82..5004b9a5e1d 100644
> --- a/lisp/progmodes/c-ts-mode.el
> +++ b/lisp/progmodes/c-ts-mode.el
> @@ -616,7 +616,7 @@ c-ts-mode
>  (define-derived-mode c-ts-mode c-ts-base-mode "C"
>  
>    (unless (treesit-ready-p 'c)
> -    (error "Tree-sitter for C isn't available"))
> +    (c-mode))

Yes.  But not literally like that, because, for example, we don't want
that to happen when c-ts-mode is invoked interactively by the user --
in that case we do want to signal an error.

The automatic fallback should IMO happen only in situations like mode
specifications in 'mode' cookies or file-local variables, where it is
better to visit in a similar mode than completely fail visiting a
file.