bug#16526: 24.3.50; scroll-conservatively & c-mode regression

[Stefan Monnier]

> No.  The particular rogue invocation of scan-lists which caused this
> (sub-)thread scans from BOB to near EOB 29 times.  Inside scan-lists,
> _NO_ changes to syntax-tables, s-t text properties etc. are possible.
> What I'm suggesting is that cacheing that first scan from BOB will be a
> big win for many time-consuming scan-listses.

Here's the situation: we can add some special code to handle your case.
But adding code which will use syntax-ppss would be more beneficial for
99% of the major modes.

Adding a cache that only works within a single scan-lists instance is
too much trouble for too little gain.  Adding a cache that is flushed
less often means either it will suffer from the same limitations as
syntax-ppss (so we may as well use syntax-ppss), or it needs to
additionally hook into the text-property and set-syntax-table code to be
flushed in those extra corner cases, and there again these extra checks
will be only useful for those rare modes like CC-mode and will be at
best harmless and in some cases detrimental to performance compared to
simply using syntax-ppss.

IOW, using syntax-ppss for find_defun_start is The Right Thing to do.
And CC-mode should then be fixed to take advantage of it.


        Stefan