bug#32599: 25.2; Feature request: input PUA characters by name

[Eli Zaretskii]

> From: jsbien@mimuw.edu.pl (Janusz S. Bień)
> Cc: 32599@debbugs.gnu.org
> Date: Mon, 27 May 2019 19:39:50 +0200
> 
> > Asking Emacs to maintain compatibility to various ad-hoc registries
> > outside of Unicode is not really reasonable.
> 
> Yes, but it is not what I propose. I just would the user to be able to
> use his own definition of PUA provided in the form of an additional
> UnicodeData.txt (not necessarily as a part of Unicode, it can be perhaps
> a different coding system).

We don't have infrastructure for reliably doing such customizations on
user level.  Even importing a new version of the Unicode Standard
currently requires some manual adaptations, although most of the job
is done by just rebuilding Emacs after downloading a few UCD files.

Apart from making it possible to modify/update the relevant tables
programmatically, someone should figure out which properties are
relevant to PUA codepoints, which scripts they should belong, and also
how to specify fonts for them.  And if you want users to be able to do
this stuff, all those missing infrastructure features should be
exposed as user commands which don't require rebuilding Emacs and
preferably not even restarting it.  Someone who knows enough about the
issue should do this job, for the feature to be available.