bug#60691: 29.0.60; Slow tree-sitter font-lock in ruby-ts-mode

[Yuan Fu]

>> I should mention this in the comments, but the fast mode is only for very 
>> rare cases, where the file is mechanically generated and has some 
>> peculiarities that causes tree-sitter to work poorly. If the file is 
>> hand-written and “normal”, even huge files like xdisp.c is well below the 
>> bar. Therefore I don’t think “crossing the line” will realistically happen 
>> when editing source files.
>> Here is the stats of two “problematic files”, named packet and dec_mask, 
>> comparing to xdisp.c:
>> ;;           max-depth max-width count
>> ;; cut-off   100       4000
>> ;; packet   (98159     46581 1895137)
>> ;; dec mask (3         64301 283995)
>> ;; xdisp.c  (29        985   218971)
>> I’d say that any regular source file, even mechanically generated, wouldn’t 
>> go beyond ~50 levels in depth, and hand-written files should never has a 
>> node that has 4000+ direct children in the parse tree.
> 
> Oh, thanks for the explanation. Then the current strategy makes sense.
> 
> Is xdisp.c absolutely the largest C file in your experience?
> 
> According to the above numbers, a file that's only 4x as large could hit our 
> current cutoff.

I don’t think these stats increase linearly as the file size increases. Even if 
there is a file that has a node with 3999 direct children, and the developer 
adds another one, I’d say it’s better not to turn on “fast mode” immediately.

Yuan