`open-paren-in-column-0-is-defun-start` (was: cc-mode fontification feels random)

[Stefan Monnier]

> It was rather the use of the idiom "We can use hacks like this one".  I
> see `open-paren-in-column-0-is-defun-start' as a way to subdivide code
> into chunks that may be edited and processed independently.  Currently,
> we use a monolithic approach (one that works on the whole buffer from
> its beginning) for fontification and a chunk-wise approach (as in the
> default `beginning-of-defun') for editing proper.

I see two problems with `open-paren-in-column-0-is-defun-start` (opic0ids):

- The implementation was a lot simpler than what's needed for your
  notion of "chunk-wise editing", thus leading to somewhat arbitrary
  behaviors because we only used the opic0ids property when it was
  convenient, rather than using it at every place where it could change
  the behavior.

- this convention is imposed on top of the definition of the language,
  so it's like editing "C with the opic0ids convention" rather than
  editing "C".  This works fine if your file is indeed written in "C
  with the opic0ids convention", but no so well otherwise.  And that
  convention is specific to Emacs (I can imagine other editors
  supporting a similar convention, but most likely it won't be exactly
  the same one since it's not a widely known convention), so unless all
  the coders agree to use Emacs you'll probably want to enforce that
  convention via some kind of "sanity check" maybe running in a CI.

- I don't think a major mode for language Foo should default to
  assuming that the buffer is written in "Foo with the opic0ids
  convention".


        Stefan