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

[martin rudalics]

> 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