bug#68493: Got to be a better way to customize c-ts-mode indentation

> From: Tomas Nordin <[email protected]>
> Cc: Eli Zaretskii <[email protected]>, Daniel Colascione <[email protected]>,
>  [email protected]
> Date: Mon, 03 Aug 2026 21:19:29 +0000
> 
> Björn Lindqvist <[email protected]> writes:
> 
> > Den mån 3 aug. 2026 kl 14:46 skrev Tomas Nordin <[email protected]>:
> >> Is the following description this bug? Or should I talk more about it in
> >> some other or new report?
> >>
> >> Its about indentation of concatanated strings like this (in a c-buffer)
> >>
> >> char *lines =
> >>   "line1\n"
> >>   "line2\n"
> >>
> >> The above is what I "get" with c-mode (in emacs -Q). If I switch over to
> >> c-ts-mode and say C-x h and TAB, I get
> >>
> >> char *lines =
> >>   "line1\n"
> >>     "line2\n"
> >>
> >> Further literals will align with "line2\n". I would like it to be
> >> aligned as with c-mode. Is there a setting for this?
> >>
> >> This has been observed with
> >>
> >> GNU Emacs 31.0.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.49, cairo version 1.18.4) of 2026-07-02
> >
> > Hello Tomas,
> >
> > I can confirm the bug in 32.0.50. However, my c-ts-mode config I
> > posted on emacs-devel fixes it:
> >
> > https://gist.github.com/bjourne/31f53043c72728cb4d8b90feedd0625b
> >
> > You can take inspiration from that. Specifically, you need to prepend the
> > rule ((parent-is "concatenated_string") parent 0) to
> > treesit-simple-indent-rules.
> 
> Thanks for inspiration. I spent time trying to understand this stuff
> better. After that time I don't understand it much better, but maybe a
> little bit. With shotgun programming I arrived at the attached patch. It
> fixes the problem I reported, and here are two examples where I get what
> I want:
> 
>     helpexit (argc, argv, 0, 2,
>               "Usage: toks [--help] [<delims> [<startchar>]]\n"
>               "Read lines from stdin and print tokens.");
> 
> and
> 
>     char *lines =
>         "line1\n"
>         "line2\n";
> 
> The patch was made on top of todays emacs-31. I re-built the whole thing
> (patch applied) and will now run with it for some time and see if it
> holds.

Thanks.  Yuan and Stefan, any comments or suggestions?

> diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
> index 5f064716a89..fa09cb21b34 100644
> --- a/lisp/progmodes/c-ts-mode.el
> +++ b/lisp/progmodes/c-ts-mode.el
> @@ -503,7 +503,8 @@ c-ts-mode--simple-indent-rules
>                                  "enum_specifier"
>                                  "union_specifier"
>                                  "function_declarator"
> -                                "template_declaration")))
> +                                "template_declaration"
> +                                "concatenated_string")))
>              standalone-parent 0)
>             ;; This is for the trailing-star stype:  int *
>             ;;                                       func()