bug#61502: 29.0.60; c-ts-mode auto-indent not working

[Dmitry Gutov]

On 14/02/2023 23:00, Theodor Thornhill via Bug reports for GNU Emacs, 
the Swiss army knife of text editors wrote:
> On 14 February 2023 21:59:03 CET, Dmitry Gutov<dgutov@yandex.ru>  wrote:
> > On 14/02/2023 21:41, Theodor Thornhill via Bug reports for GNU Emacs, the Swiss 
> > army knife of text editors wrote:
> > > diff --git a/lisp/treesit.el b/lisp/treesit.el
> > > index 749781894b..300a703515 100644
> > > --- a/lisp/treesit.el
> > > +++ b/lisp/treesit.el
> > > @@ -1418,6 +1418,8 @@ treesit--indent-1
> > >             ;; encompass the whitespace.
> > >             (parent (cond ((and node parser)
> > >                            (treesit-node-parent node))
> > > +                       ((equal (treesit-node-type smallest-node) "}")
> > > +                        (treesit-node-parent smallest-node))
> > >                           (t (treesit-node-on bol bol)))))
> > >          (funcall treesit-indent-function node parent bol))))
> > Is it a good idea to add C-specific constants to generic code?
> >
> > Other modes might not have a node called "}" at all.
> Yeah this was merely an example. There may be some "block-ender" concept one 
> could envision. I need to experiment with it, and it may not be feasible at all

I was thinking we could use a more/grammar-specific override to choose 
which node to use to base the indentation on.

I.e. being able to override this piece of logic:

               (treesit-parent-while
                smallest-node
                (lambda (node)
                  (and (eq bol (treesit-node-start node))
                       (not (treesit-node-eq node root)))))

It could also help with issues like 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=60602#8, to avoid 
duplication in indentation rules.

AFAICT this is largely possible to do already by changing 
treesit-indent-function to some mode-specific wrapper, but an override 
like the above could be written more succinctly.