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 1:56 PM, Theodor Thornhill via Bug reports for GNU Emacs, the Swiss army knife of text editors <bug-gnu-emacs@gnu.org> wrote:
> 
>>> 
>>> 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.
>> 
> 
> Makes sense!

I pushed a change that uses treesit-node-on. Now scrolling in most parts of the buffer is pretty fast. Scrolling around 194770 is still laggy, because the node we get from treesit-node-on is still too large. I tried some heuristics but they didn’t work very well, IMO because tree-sitter couldn’t parse that part of the code very well. The code should observe a structure like {{}, {}, {}, {}, {}, …} where there are tens thousands of inner brackets, so ideally we only need to grab the {}’s in the region we want to fontify. But tree-sitter seems to understand it in some weird structure and we still end up with very large nodes, which is far larger than the region we want to fontify and is slow to query.

I’ll try to improve it further in the future, but for now I think it’s good enough (because in most cases fontification is pretty fast).

Also I think we should probably disable fontifying errors in C. C’s macros just create too much errors.

Yuan