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

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: stefankangas@gmail.com, yantar92@gmail.com, 57693@debbugs.gnu.org
> Date: Sun, 19 Feb 2023 11:31:24 +0000
> 
> 1. Allowing the char-table to be buffer-local

Trivial (should be possible right now)

> or within text-properties

Only if really needed.  Adding a text property for this will
complicate the display code which handles this, so I'd rather we
didn't unless we have a _very_ good reason.  And I don't see such a
reason, since we are talking about characters that cannot be displayed
by any available font, something that is unlikely to be limited to
just a small region of text.

> 2. Making (set-char-table-range glyphless-char-display ?A ?B) work on
>    GUI as well, not just in terminals

You mean (set-char-table-range glyphless-char-display ?A "B"), I
presume (the value cannot be a character, only a string).

> 3. Making the rules recursive.

What for?  It's a very significant complication, and I cannot see the
purpose.  The string value _must_ use only ASCII characters, precisely
_because_ we want to be sure no recursion will be needed.