Display unknown non-ASCII chars as `.', `X' or `?' (was: Setup additional charsets on demand)

[Reiner Steib <reinersteib+gmane@imap.cc>]

On Tue, Oct 11 2005, Reiner Steib wrote:

> in reaction to Matthias Andree's posting[1]...
>
> ,----[ http://thread.gmane.org/gmane.emacs.gnus.general/59971 ]
> | What's up here? Does Gnus lack Windows-1251? If so, why does it not
> | replace everything by dots, X or ?. If it supports Windows-1251, why
> | doesn't it see it? [...] (Emacs 21.3 with rm'd movemail)
> `----
>
> ... I added `mm-charset-eval-alist' to the trunk now.

While at it...
| If so, why does it not replace everything by dots, X or ?.

I.e. if the charset is not available: How can we configure Gnus to
display every non-ASCII chars as `.', `X' or `?'?

One solution is to define a "dummy coding system" where all non-ASCII
chars are replaced by some place holder:

(defun rs-ucs-ascii-8bit-dumb (&optional char)
  "Create dummy coding system `rs-ucs-ascii-8bit-dumb'."
  (unless char (setq char ?\?))
  ;; We need cp-make-coding-system (Emacs 22)
  (unless (fboundp 'cp-make-coding-system)
    (error "Function requires `cp-make-coding-system'"))
  (cp-make-coding-system
   rs-ucs-ascii-8bit-dumb
   (make-vector 128 char)
   (format "Dummy coding system.  Replace all non-ASCII chars as `%c'." char)
   ?\?))
(rs-ucs-ascii-8bit-dumb ?¿)
(add-to-list 'mm-charset-override-alist
	     '(undecided . rs-ucs-ascii-8bit-dumb))

With this, also non-ASCII chars in postings without proper MIME
headers[1] will be displayed as `¿'.  (IIRC, slrn and tin replace
unknown characters with `?'.  Some people like this and turn off
fallbacks like iso-8859-1.)

Can we achieve the same without using functions from Emacs 22
(`cp-make-coding-system')?

Bye, Reiner.

[1] Some examples: <news:Usenet.qaremkmn@localhost>,
    <news:Usenet.cfkoelmf@localhost>
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      (o o)
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