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

[Yuan Fu]

> On May 21, 2024, at 4:43 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> 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?

There are actually another level of customization between setting 
treesit-simple-indent-rules and setting indent-line-function, which is setting 
treesit-indent-function. IIUC Óscar is trying to do that but found out 
treesit-major-mode-setup doesn’t automatically setup indent-line-functions when 
treesit-indent-function is set. I’ve now fixed that. (Though I didn’t expect 
someone to use this variable.)

And to Óscar, I’m curious and interested to know what indentation function you 
came up with; if it’s useful maybe we can add it to Emacs?

> 
>> 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.

I also updated the manual.

Yuan