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

[Filippo Argiolas]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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?
>

I believe we might have a bug here, as far as I can tell it does not
match

  ((n-p-gp nil "preproc" "translation_unit") column-0 0)

Because both parent and grand parent are preproc. So it matches one of
the `c-ts-mode--standalone-parent-skip-preproc' rules right after.

After skipping preproc nodes parent is translation_unit and indents an offset
from there. Guess this step could be made smarter to check for
translation_unit and the rule above could be removed?

>> 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.

This can be worked around as Yuan showed, but isn't it a grammar bug?
problem is with the #ifdef function and if statement become siblings, without
preproc they have a child-parent relation.

In my experience c-ts-mode is a bit fragile with preprocessor
statements, probably because the grammar itself is fragile (see
e.g. [1]) and the problem is an hard one.

Yuan, do you think c-ts-mode could some way benefit from LSP knowledge
about inactive preprocessor branches? Idea is that we would at least
have a good syntax tree in the active branches while allowing some
errors in the inactive ones.


Filippo


1. https://github.com/tree-sitter/tree-sitter-c/issues/108