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

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@gmail.com>
> Cc: 57693@debbugs.gnu.org
> Date: Sun, 11 Sep 2022 17:31:06 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Yup. And I am asking if there is such test exposed to Elisp. Display
> >> code certainly knows when some character cannot be displayed and must be
> >> replaced by its hex code.
> >
> > The display engine only knows it retroactively, when it tried and
> > failed to display a character.
> 
> The situation is similar to `string-pixel-width'.

??? You rejected such methods in an earlier message, because it
required a temporary buffer, which you said will not reproduce the
exact environment of the buffer where you want to show the character.
Or what am I missing?

> >   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.