Re: Display of undisplayable characters: \U01F3A8 instead of diamond

[Eli Zaretskii]

> From: Richard Stallman <rms@gnu.org>
> Cc: acm@muc.de, gregory@heytings.org, emacs-devel@gnu.org
> Date: Fri, 02 Sep 2022 22:51:27 -0400
> 
>   > How can Emacs know which characters does your Linux console actually
>   > support, out of all the Unicode range?
> 
> A few days ago you said it could use char-displayable-p
> to determine this.  If that works, it solves the problem of finding out
> which characters to display as diamond.

It works, but looping over all the Unicode characters is very slow, so
doing that at each startup would be unacceptable.  If someone needs, I
can show a simple code to be run by the user, once, on the Linux
console, that would allow then to determine the codepoint ranges for
which the U+FFFD (or any other) replacements are needed, so that users
could run that code, and then put such customizations in their init
files.  We could even have a special command to do that, if people
think it's important and helpful.

But all I hear till now is flat rejection of that solution, which is a
very weak incentive (to say the least) to work on such a command.

> As for how, it would be asy to add a variable whose value should be a
> vector containing integers, or a single-byte string, saying what to
> send.  This will be adequate for a Linux console, and it doesn't need
> to handle anything else.

You are describing what display-tables provide.  That's the solution I
recommend if you really want to see those U+FFFD "diamonds" instead of
the characters that cannot be displayed.  I see no reason for yet
another solution for a problem for which we already have two
(glyphless-char-display and display-tables).