Re: master 7362554: Widen around c-font-lock-fontify-region. This fixes bug #38049.

[Dmitry Gutov]

Hi Alan,

On 11.11.2019 22:34, Alan Mackenzie wrote:

> On Mon, Nov 11, 2019 at 18:52:34 +0200, Dmitry Gutov wrote:
> > On 09.11.2019 16:40, Alan Mackenzie wrote:
> > > +    (widen)
>
> > Could you try and honor font-lock-dont-widen here?
>
> Difficult.  In CC Mode it is sometimes necessary to enlarge a font lock
> region so as to get the context needed to parse some element of the
> buffer.  This was what gave rise to the bug.

Did that really happen in the reported scenario? I mean, 
font-lock-dont-widen is usually nil, so font-lock will call widen for 
you already.

> In all of Emacs, there's just one use of font-lock-dont-widen, in
> rmail.el, and that is almost certainly for a different reason than your
> use of it (which is, I believe, having several major modes in a single
> buffer).

Does RMail use it for quoted pirces of code? If so, it would be similar.

> Could you, perhaps, suggest some alternative to using widen in
> c-font-lock-fontify-region?  Some way which would still work for CC Mode
> (including the just fixed bug), but wouldn't adversely affect the
> several major mode code you're maintaining?  Would it be possible to
> widen "a bit" rather than widening to the whole buffer?  If so, how
> would I determine the bit to widen to?

Not sure what difference the supposed "bit" would make (and thus, how 
would mmm-mode determine it). But I think that would depend on your 
answer to my first question in this email. Because, I think, normally 
you don't "have to" widen at all, font-lock will do that for you.

In mmm-mode context, however, we apply definite boundaries to the region 
chunks. Here's an example of some Noweb code: 
https://en.wikipedia.org/wiki/Noweb#Example_of_a_simple_noweb_program

The inside of hello.c block would be narrowed to.

Now, I have remembered that CC Mode calls widen from many places 
already, so it already is problematic for using in a context like that. 
Thus this particular change doesn't change the picture much, and so 
please feel free to drop this conversation at any point of time.