Re: when options)mule)set-coding-system is unselectable

[Richard Stallman]

    Eli> Wrong move.  You should use "C-x RET c" before "C-x C-f" instead.
    Eli> That's because the file is already decoded by the time it is
    Eli> displayed, and so no tinkering with options>mule>set-coding-system
    Eli> will make any difference as to how it is displayed.

    OK, this is exactly what the user needs to know at that point.  Can we
    somehow arrange to get your above sentences told to the user when he
    goes reaching for that button, instead of just finding, of all the
    buttons, amazingly, just the one he thinks he needs has its lights
    turned off

I agree that this is something important to do.  I don't see precisely
where or how Emacs would present that information.  In response to
what action or command, when?