bug#26396: 25.1; char-displayable-p on a latin1 tty

[Eli Zaretskii]

> Cc: 26396@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Wed, 12 Apr 2017 23:19:53 -0700
> 
> Eli Zaretskii wrote:
> > I think the code in
> > char-displayable-p which looks at the result of internal-char-font
> > should only accept a non-negative value if the terminal-coding-system
> > supports the character.  IOW, the Linux console should not be
> > considered as being able to display a character unless the terminal
> > encoding can safely encode it.
> 
> Wouldn't it be better if Emacs ignored terminal-coding-system when the output 
> device is a Linux console and Emacs therefore knows exactly which characters 
> the 
> console can display? Instead, Emacs could simply display those characters 
> as-is. 

Yes, that would be better.  But it's probably a non-trivial project,
since we'd need separate code to determine double-width glyphs,
padding glyphs, and perhaps also something special for composed
characters.  Does the Linux console allow us to figure out all of
that?

And what does "display as-is" means in practice?  Should we send to
the console the glyph codes corresponding to Unicode points, or should
we send UTF-8 encoded characters?  (Is there some document which
describes these features in enough detail for us to figure out their
implications on Emacs display code?)