bug#22983: syntax-ppss returns wrong result.

[Dmitry Gutov <dgutov@yandex.ru>]

On 03/13/2016 08:57 PM, Alan Mackenzie wrote:

> I happen to use it frequently.  I expect other users do, to.  It's
> useful.

It might be, but it's not very well-designed. If you only want to hide 
some parts of buffer from being displayed, changing point-min and 
point-max, which affect quite a lot of Lisp functions, seems unnecessary.

Introducing a couple of global variables that would only be read by the 
display code, seems like a better approach. I don't think that 
narrow-to-region should be a user-level function. Introducing a new 
function, using a different mechanism shouldn't be too hard though, if 
we reuse the existing binding.

> What were
> strings and comments before narrowing should remain strings and comments
> after narrowing.  Otherwise, nothing would work in such a narrowed
> buffer.  font-locking, for example, behaves properly in a narrowed
> buffer.

It behaves like we tell it to behave. If I bind font-lock-dont-widen to 
t, font-lock won't look beyond the narrowing.

>> Hence, so far, I didn't make any effort to try and "do the right thing"
>> for user-activated narrowing, since these are just not well defined
>> enough to even determine what is "the right thing".
>
> Lets define them as I said in the previous paragraph.  Or can you
> conceive of a use case where one would want narrowing to invert strings
> and non-strings, leaving comments totally random?

At risk of inviting further confusion, yes, mmm-mode and polymode (new 
example!) use narrowing to persuade font-lock and indentation code that 
there's nothing beyond the narrowed region. We might declare such usages 
invalid, and that's a possible choice, but I think keeping support for 
them wouldn't be too hard, at least for a while.

Note that if your comment cache always widens the buffer before 
calculating the values to save, its result might conflict with 
syntax-ppss in mmm-mode and polymode (right?). Leading to font-lock, 
indentation and certain commands behaving in different, conflicting 
ways. That's just conjecture at this point, of course.

> Do you have any views on how the bug should be resolved?

Stefan probably has another opinion, but I'd either ignore the issue of 
narrowing, or introduce syntax-ppss-dont-widen like proposed (and thus 
make syntax-ppss widen by default). Together with adding a command that 
would replace interactive use of narrow-to-region.