bug#57693: 29.0.50; Is there a more reliable version of `char-displayable-p'?

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 57693@debbugs.gnu.org
> Date: Sun, 11 Sep 2022 12:43:52 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Ihor Radchenko <yantar92@gmail.com>
> > Cc: 57693@debbugs.gnu.org
> > Date: Sun, 11 Sep 2022 17:31:06 +0800
> > 
> > >   insert the character
> > >   call the new magic
> > >   if the new magic says NO-CAN-DO
> > >      replace the character with something else
> > >
> > > If the above is acceptable, I think it can be done, although it would
> > > not be very useful in other situations.  But if you want to know the
> > > answer before you insert the character, I don't think we know how to
> > > satisfy your requirements with 100% accuracy.  At least I cannot see
> > > how it could be done; maybe someone else will.
> > 
> > The described approach should be acceptable.
> > What I have in mind is a function like
> > 
> >   (insert-displayable '("fancy version" "backup"))
> > 
> > The function will try to insert "fancy version" first; check if all the
> > characters are displayable, and replace the inserted text with "backup"
> > if not.
> 
> OK, I will see what I can do.

Here's what I suggest for GUI frames:

  (defun insert-char-safely (ch repl)
    "Insert character CH, if it can be displayed; otherwise insert REPL."
    (insert ch)
    (unless (font-at (1- (point)))
      (delete-char -1)
      (insert repl)))

Do you need this to work for TTY frames as well?  If so, it could be a
problem, since most terminal emulators don't provide a way of
inquiring whether a certain character can be displayed.
char-displayable-p currently just checks on most terminals that the
terminal-coding-system can _encode_ the character, which isn't enough
if the terminal encoding is UTF-8.