Re: Major modes using `widen' is a good, even essential, programming practice.

[Eli Zaretskii]

> Date: Sun, 7 Aug 2022 17:01:09 +0000
> Cc: gregory@heytings.org, emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > > Consider the second jit-lock chunk
> > > at the beginning of xdisp.c.  Fontifying that chunk involves looking
> > > back 1500 characters before BEG to see that it needs
> > > font-lock-comment-face.  You might argue that that information will be
> > > in a cache anyway, but that's not dependable.
> 
> > Either in the cache or in the buffer: the previous chunk was
> > fontified, so its end has the font-lock-comment-face.  So you know.
> 
> No, you don't.  The buffer might be being opened by desktop in a large
> comment in the middle of the file.

You've changed the scenario, yes?

> What jit-lock/font-lock actually do at the moment is to widen, then use
> syntax-ppss, i.e. in effect scan from BOB.

Yes, and that's SLOOOWWWW!

> > > Also, the (BEG END) region will typically get rounded up to whole lines,
> > > again "violating" that chunk.
> 
> > That's a far cry from going to BOB.  And if you ask nicely, we could
> > arrange that jit-lock calls you only on line boundaries (unless lines
> > are longer than some reasonable value).
> 
> The search for line boundaries is done by font-lock.el.

I don't trust it to DTRT when lines are very long.

> > > In principle, font-lock needs to look outside of (BEG END).
> 
> > No, it doesn't.  A string cannot begin before a beginning of a
> > function, for example.  And if you need to go too far, just give up
> > and blame the user who writes such code.  It is much better than
> > letting every use of CC Mode wait because once in a blue moon someone
> > could have a very long string.
> 
> That "needing to go too far" is an instantaneous jump, not a scanning.

Please tell that to someone who doesn't edit C sources as frequently
as I do.

> The string start will be in a parse-partial-sexp result somewhere.
> Sometimes people write long strings.  They certainly write long comments.

Why do I have top suffer every day just because someone, somewhere,
might do that?  I'd rather we "punish" those few people who do it
(rarely).