A proposal for the future of open-paren-in-column-0-is-defun-start.

[Alan Mackenzie]

Hello, Emacs.

The working of open-paren-in-column-0-is-defun-start is still functional
in emacs-27 and master, providing the newish variable
comment-use-syntax-ppss is set to nil.

When enabled, it still causes problems in, at the very least, CC Mode.
For example, in the following:

    /*
    (
    */
      ^
    point

, (forward-comment -1) spuriously returns nil and doesn't move point.

The root of the problem is that back_comment (in syntax.c) has no means
of distinguishing open parens in comments and strings from other open
parens.  I propose giving the function such a means.

This would be a text property `syntax-open-paren-in-literal' which would
be applied to each and every open paren at column zero inside a literal.
back_comment could easily and cheaply test this text-property.

`syntax-open-paren-in-literal' would be applied throughout the buffer
when it is first loaded into Emacs, and would be kept up to date by
"super before/after-change-functions".  (These functions would be called
at buffer changes regardless of the value of inhibit-modification-hooks.
We would want some way of inhibiting them being called on a change of
text properties, for efficiency's sake).

With this mechanism in place, o-p-i-c-0-i-d-s could safely be left
enabled, for speed's sake, without causing the all too familiar bugs in
CC Mode.

What do people think?

-- 
Alan Mackenzie (Nuremberg, Germany).