Re: How does c-ts-mode, tree-sitter indentation, and preprocessor directives work?

[Yuan Fu]

> On Nov 27, 2024, at 11:30 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Björn Lindqvist <bjourne@gmail.com>
>> Date: Thu, 28 Nov 2024 00:27:17 +0100
>> 
>> I've been trying to get c-ts-mode to indent like I want, but I'm
>> running into problems related to preprocessor directives.
> 
> Preprocessor directives are difficult because the tree-sitter C/C++
> grammars include only partial support for them.
> 
>> For
>> example, consider a type definition nested in two #ifdefs:
>> 
>>    #ifdef X
>>    #ifdef Y
>>    typedef int foo;
>>    #endif
>>    #endif
>> 
>> Since both the parent and grand parent of the type_definition is a
>> preproc_ifdef no rule matches.
> 
> But if you go back (up) the parent-child hierarchy, you will
> eventually find a node which is not a preproc_SOMETHING, and can go
> from there, no?
> 
>> Another issue is that I want my
>> preprocessor directives kept at column 0, which unfortunately screws
>> up all rules that refer to the parent. E.g.:
>> 
>>    ((parent-is "if_statement") standalone-parent 4)
>> 
>> Doesn't work for
>> 
>>    int main() {
>>        if (true)
>>    #ifdef A
>>            prutt();
>>    #else
>>            fis();
>>    #endif
>>    }
>> 
>> The rule I'd like to express is "take the indent of the closest
>> *indenting* parent and add one indent". That rule would match whether
>> that parent is a "while_statement", "if_statement", "for_statement",
>> etc. You can't express such rules with tree-sitter, can you?
> 
> Not sure, but Yuan will know.

Everything is possible, it’s just elisp. The only problem is how generic you 
can make the rule. Here’s a POC that only works for this example; specifically, 
it only works for if statements and #ifdef directives. It should be extendable 
to for statement, while statement, etc, and maybe other directives too.

Speaking of indent, we need to do something with c-ts-mode’s indentation rules. 
It’s getting too long and too complex. But I don’t have any great idea at this 
point. Maybe we can replace the rules with a hand-rolled function so it has 
more structure, or try nvim’s query approach.

Yuan

preproc-indent.patch
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