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

[Alan Mackenzie]

Hello, Eli.

On Sun, Aug 07, 2022 at 18:13:47 +0300, Eli Zaretskii wrote:
> > Date: Sun, 7 Aug 2022 14:59:06 +0000
> > Cc: gregory@heytings.org, emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > > jit-lock calls the functions with two arguments, BEG and END, and
> > > expects them to work only on that chunk of text.

> > I don't think you really mean that.

> No, I really do.

> > 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.

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

> > 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.

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

-- 
Alan Mackenzie (Nuremberg, Germany).