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

[Dmitry Gutov <dgutov@yandex.ru>]

On 13.11.2019 23:19, Alan Mackenzie wrote:
>      (ii) This latter function uses vertical-motion to count the lines.
>      (iii) vertical-motion triggers jit-lock fontification.
>      (iv) This calls (eventually) c-font-lock-fontify-region.

By the way, I had a thought.

Like, normally font-lock does widen the buffer before fontifying 
anything. So I was wondering, why doesn't this happen between steps iii 
and iv somewhere?

Turns out, the (unless font-lock-dont-widen (widen)) calls are performed 
inside the default values of font-lock-fontify-region-function and 
font-lock-fontify-buffer-function.

But CC Mode sets font-lock-fontify-region-function to a different value, 
and that specific function doesn't include this piece of code. Or 
rather, didn't. The new commit added an unconditional (widen) call.

I think if you simply wrap it in (unless font-lock-dont-widen ...), it 
will improve compatibility with mmm-mode already (without adverse 
effects, I imagine). After that, you could audit if the other (widen) 
calls spread across CC Mode are still necessary. Maybe they're used in 
indentation code (that's also solvable). Maybe also by some other commands.