bug#4030: forward-sexp parses character literal ?; as comment

[martin rudalics <rudalics@gmx.at>]

 >> We could try to mark any `?;' or `?"' sequences appropriately when
 >> fontifying though.
 >
 > We could/should/will improve the syntax parsing to handle those
 > things properly.  But it's a non-trivial amount of work, especially
 > since every major mode has similar issues but needs different
 > extra functionality.

But fontification is mode-specific.  So it would be sufficient to look
for ?s that are not within strings or comments and followed by a
semicolon, paren or double-quote and mark that appropriately (obviously
the parser is derailed at that time but it still might help people spot
the bug).

 >>> I'll also point out that an "Unbalanced parentheses" error from deep
 >>> inside Customize is not a very helpful error message (especially as it
 >>> does not indicate in which buffer the unbalanced parentheses were
 >>> found); but perhaps Customize should be adapted to cope if forward-sexp
 >>> cannot easily be fixed.
 >> Getting good diagnostics after a parsing error is hard.
 >
 > Agreed, but that doesn't mean we shouldn't intend to do better: the
 > current behavior (signalling an internal error to the user) is a bug
 > that needs to be fixed.

If the problem comes from the unprotected

       (save-excursion (forward-sexp (buffer-size)))) ; Test for scan errors.

call in `custom-save-delete' we could simply wrap it as in the patch
below.  But do we really have to scan the buffer in the first place?

martin

*** cus-edit.el.~1.364.~	2009-07-27 08:09:05.997162900 +0200
--- cus-edit.el	2009-08-07 14:49:22.890625000 +0200
***************
*** 4341,4347 ****
     ;; Skip all whitespace and comments.
     (while (forward-comment 1))
     (or (eobp)
!       (save-excursion (forward-sexp (buffer-size)))) ; Test for scan errors.
     (let (first)
       (catch 'found
         (while t ;; We exit this loop only via throw.
--- 4341,4349 ----
     ;; Skip all whitespace and comments.
     (while (forward-comment 1))
     (or (eobp)
!       (condition-case nil
! 	  (save-excursion (forward-sexp (buffer-size))) ; Test for scan errors.
! 	(scan-error (error "Scan error, watch out for ... "))))
     (let (first)
       (catch 'found
         (while t ;; We exit this loop only via throw.