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)))