bug#68246: 30.0.50; Add non-TS mode as extra parent of TS modes

[Dmitry Gutov]

On 08/01/2024 06:11, Stefan Monnier via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> >     (set-language-for-mode 'foo-ts-mode 'foo)
>
> Maybe we want to introduce this concept, indeed.
>
> maybe we want to that notion of "language" from elsewhere, such as
> the one used in LSP?
> Or maybe we want to take it from MIME types?
> I'm sure there are other options out there.

I think the precise source of the mapping is not that important. We 
might as well continue maintaining auto-most-alist, 
interpreter-mode-alist and magic-mode-alist by hand. Or indeed learn to 
populate them from the MIME database later.

What we have, though, it different major modes duplicating 
auto-mode-alist entries even inside the core Emacs. Such as c-ts-mode 
having modified copies of forms that originate from CC Mode.

Or ruby-mode and ruby-ts-mode using two copies of the same regexp. Etc.

Instead, we could have a mapping of files to "languages" and a separate 
one from languages to major modes. And one could fetch the "language" 
for the current buffer using 'rassoc'.

> Problem is: they come with their own complexities and corner cases.
> After all, this is inevitable when you create a taxonomy.
> IOW, while we *may* want to add support for an explicit notion of "file
> type", it's a whole problem in itself and it will not solve all
> our problems either.
>
> In the mean time, I think `derived-mode-add-parents` is worth a try.
> As mentioned in some message up-thread, I'm not 100% confident that it
> won't introduce serious breakage.  But I think we do need more
> experience and installing my patch is a good way to do that.

This would've worked better inside the Emacs 29.1 release (which 
contains a few other "expedient" solutions).

I'm guessing it won't get into 29.2 either. So the users of such 
versions would have to deal with the existing taxonomy anyway, and 
half-measures might also serve to make people more confused about what 
works in which version and why.