bug#59415: 29.0.50; [feature/tree-sitter] c-ts-mode fails to fontify a portion of a large C file

[Yuan Fu <casouri@gmail.com>]

> On Nov 20, 2022, at 1:09 PM, Theodor Thornhill <theo@thornhill.no> wrote:
> 
>>> This diff fixes the font-lock issues:
>>> 
>>> diff --git a/lisp/treesit.el b/lisp/treesit.el
>>> index 674c984dfe..0f84d8b83e 100644
>>> --- a/lisp/treesit.el
>>> +++ b/lisp/treesit.el
>>> @@ -774,12 +774,12 @@ treesit-font-lock-fontify-region
>>>      ;; will give you that quote node.  We want to capture the string
>>>      ;; and apply string face to it, but querying on the quote node
>>>      ;; will not give us the string node.
>>> -      (when-let ((root (treesit-buffer-root-node language))
>>> +      (when-let (
>>>                 ;; Only activate if ENABLE flag is t.
>>>                 (activate (eq t enable)))
>>>        (ignore activate)
>>>        (let ((captures (treesit-query-capture
>>> -                         root query start end))
>>> +                         (treesit-node-on start end) query start end))
>>>              (inhibit-point-motion-hooks t))
>>>          (with-silent-modifications
>>>            (dolist (capture captures)
>>> 
>>> 
>>> However, the comment right above makes a case for why we should have
>>> this.  BUT, is this still relevant, Yuan, after the changes in treesit
>>> reporting what has changed etc?  What exact case is that an issue?  And
>>> is it more severe than the behavior this bug is exhibiting?
>> 
>> The case described by the comment is still relevant. With this patch,
>> the quote described in that case still wouldn’t be fontified. We can
>> use some heuristic to get a node “large enough” and not the root
>> node. Eg, find some top-level node. That should make query-capture
>> much faster.
>> 
> 
> I appreciate the explanation.  I think getting the root is a bit
> excessive.  I got the same results as you in the capture.  Maybe reuse
> the treesit-defun-type-regexp, and default to root if none found?

I tried the "top-level node” approach, and it didn’t help in package-rrc.c: the top-level node (a function definition) is still too large (spans 7680306-9936062). Since the case I described in the comment against using treesit-node-on is the exception rather than the norm, maybe we can go the other way around: use treesit-node-on first, and if the node seems too small (by some heuristic), enlarge it to some degree.

Yuan