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

[Yuan Fu]

> 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