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

[martin rudalics <rudalics@gmx.at>]

 > While the workaround is good (and documented in the Ubuntu bug as well),
 > the ability of Customize depends on this code working correctly, and it
 > should handle any nominally well-formed .emacs file.  Perhaps there are
 > other pieces of code which rely on forward-sexp et alii for Emacs Lisp
 > parsing as well.

Using a backslash _is_ the canonical way for handling this problem.  If
some part of Emacs puts such a semicolon into an Elisp buffer without
escaping it, then that part of Emacs is wrong and has to be fixed.  If
you manually insert such a construct, then you are on your own (just as
when within a string you put a left paren in column zero).  We could try
to mark any `?;' or `?"' sequences appropriately when fontifying though.

 > 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.

 > It appears that src/syntax.c could perhaps be adapted to take into
 > account character literals as well as quoted strings, but I am not
 > familiar enough with Emacs internals to tell whether this is really a
 > feasible approach.

Let's say we give `?' character quote syntax in Elisp.  I suppose this
could be done.  But someone would have to rewrite the corresponding
parts of the parsing code.  I'm afraid there's hardly anyone around to
volunteer.  (And think of an `?' escaping a subsequent backslash.)

martin