bug#71086: 30.0.50; Setting treesit-indent-function is not effective is treesit-simple-indent-rules is nil

[Eli Zaretskii <eliz@gnu.org>]

> From: Óscar Fuentes <ofv@wanadoo.es>
> Date: Tue, 21 May 2024 03:32:05 +0200
> 
> 
> treesit-major-mode-setup setups indentation if and only if
> treesit-simple-indent-rules is non-nil. So the major mode author is
> forced to set that variable even when he makes no use of the
> treesit-simple-indent facilities and instead sets
> treesit-indent-function to his own function.

If your mode defines a treesit-indent-function, why do you need the
help of treesit-major-mode-setup for setting up indentation?

The idea of treesit-simple-indent, AFAIU, is to automate the simple
cases, on the assumption that the mode complex cases cannot be
second-guessed up front, and the mode programmer will need to set that
up manually.  For example, you can assign your functions to
indent-line-function and indent-region-function.  Or what am I
missing?

> OTOH, the info node for "Parser-based Indentation" says:
> 
>      To enable the parser-based indentation engine, either set
>   ‘treesit-simple-indent-rules’ and call ‘treesit-major-mode-setup’, or
>   equivalently, set the value of ‘indent-line-function’ to
>   ‘treesit-indent’.
> 
> However, treesit-major-mode-setup does more than what's described after
> "equivalently":
> 
>   (when treesit-simple-indent-rules
>     (setq-local treesit-simple-indent-rules
>                 (treesit--indent-rules-optimize
>                  treesit-simple-indent-rules))
>     (setq-local indent-line-function #'treesit-indent)
>     (setq-local indent-region-function #'treesit-indent-region))

So this is just a matter of updating the documentation to mention
indent-region-function?

I've added Yuan, in case he has comments or suggestions.