bug#12867: 24.3.50; easy-to-repro crash involving mode line

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 13 Nov 2012 09:22:20 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: Eli Zaretskii <eliz@gnu.org>, 12867@debbugs.gnu.org
> 
>  > FWIW, I recently added such a cache (in Elisp) to nlinum.el, and on my
>  > test case (largish file with C-v/M-v style scrolling) the difference was
>  > very significant (but the previous code had no cache at all, not even
>  > base_line_pos).
> 
> So we now already have a syntax-ppss cache and a nlinum cache.  Couldn't
> we maintain one cache per buffer, updated by one and the same algorithm?

These two cache different information, so making such a cache
efficient is not very trivial.

At least for nlinum, if it were to use some Lisp binding of
find_next_newline_no_quit (there's no such binding at this time, but
it wouldn't be hard to provide one), they could use a newline cache
provided by region-cache.c for free.