Kenichi Handa <handa@m17n.org> writes:

> detect-coding-string doesn't return all possible coding systems, but
> returns a possible coding systems Emacs may automatically detect in
> the current language environment.

Ah, I see.  

Do you know of any other way to decide if using a given coding system
for decoding a string would give a valid result?

A function similar to this would be really useful:

(defun possible-coding-system-for-string-p (str coding-system)
  "Return t if CODING-SYSTEM is a possible coding system for decoding
   STR."
  ...)

The issue comes from a discussion on the Gnus development list (I've
included one of the messages from that thread below).

Gnus does not work very well when using CVS Emacs in an UTF-8 locale,
because a lot of non-MIME capable clients don't include proper charset
information.  This causes Gnus to decode many Latin-1 strings as UTF-8.

It would help a lot if we could detect that a string cannot possibly be
encoded in UTF-8.  I know that it's not always possible to distinguish,
but just detecting strings that are invalid as UTF-8 would be very
helpful.  This doesn't just apply to UTF-8 but to any coding system, of
course.

> But the docstring of detect-coding-system is surely not
> good.  I've just changed the first paragraph as this.  How
> is it?

It's good.