bug#56682: Fix the long lines font locking related slowdowns

[Stefan Monnier]

>>>>>> There have been many more complaints about being unable to edit a file
>>>>>> with a 5000+ line comfortably, than about editing 100+ MB files.
>>>>> Also because we have things like so-long to handle the large file case.
>>>> These things are expedients for want of anything better, aren't they?
>>> Yes, just like the narrowing your use in your long lines patch.
>> That's... unfair.  These things turn off everything (in particular, the
>> major mode and font locking) and provide a bare-bones editing
>> experience. What we'd like now is to provide Emacs users a near-complete
>> editing experience in all buffers.
> And, by the way, what could be a better solution?

A solution that is humble enough to leave an open door for ad-hoc
case-specific handling of the problem.

> Without assuming that computers have infinitely fast CPUs, that is?

If the font-lock rules to use in the buffer depend on whether the first
line of the buffer starts with #! or not, then your narrowing will break
them even tho they could work perfectly fine in a 1TB buffer without
requiring an infinitely fast CPU (since they don't need to scan the
whole buffer: just the region of interest plus the first 2 chars).


        Stefan