bug#15251: 24.3.50; do-auto-fill "continues" comment from inside a string

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> We should probably rework the code to merge comment-use-syntax and
>> comment-use-global-state.

> I wonder what modes have the former variable set to t, but the latter to
> nil, and which situation this combination handles.

> Maybe enhance the check in `comment-normalize-vars' and set
> comment-use-syntax' to nil when the syntax table values are not good enough
> for `comment-use-global-state'?

> Then make `comment-use-global-state' an obsolete alias for
> comment-use-syntax'.

Maybe a better way to do it is:
- Change code that uses comment-use-global-state to use (and
  comment-use-syntax comment-use-global-state) instead.
- Set comment-use-global-state to t by default.
- Major modes where comment-use-global-state is problematic (if those
  exist) can then set comment-use-global-state to nil.
- Maybe make comment-use-global-state obsolete.

Context: the problem with syntax-ppss is not so much performance as
correctness, because syntax-ppss can get confused if you use several
syntax-tables in the same buffer (e.g. via font-lock-syntax-table), or
if you use narrowing.

As time goes on, more and more code relies on syntax-ppss so more and
more code gets rewritten to avoid font-lock-syntax-table (or at least
use it in "harmless" ways, e.g. only changing syntax from symbol to
word) and narrowing.  In turn, this makes syntax-ppss more robust and
more attractive.


        Stefan