bug#52459: 28.0.90; prin1-to-string does not escape bidi control characters despite print-escape-control-characters=t

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 52459@debbugs.gnu.org
> From: Daniel Mendler <mail@daniel-mendler.de>
> Date: Mon, 13 Dec 2021 20:16:54 +0100
> 
> > That will solve only a small fraction of situations where these
> > characters are displayed, because the vast majority of them don't use
> > prin1 to produce a string to display.  Most of the stuff displayed by
> > Emacs doesn't come from strings produced by prin1, it comes from
> > displaying the text of some buffer.
> 
> Yes, it solves a small fraction of situations. This issue does not
> address a generic setting, it only addresses the behavior of `prin1`. As
> I described at length there are packages which are affected by this and
> which could improve from an improvement of `prin1`.
> 
> To break it down once more:
> 
> 1. We have the function `prin1-to-string` which can be used to produce a
> string representation for an Emacs lisp value.
> 
> 2. The behavior of the function can be adjusted via configuration
> variables, in particular `print-escape-multibyte` and
> `print-escape-control-characters`. `print-escape-multibyte` is very
> aggressive, it escapes every multibyte character.
> `print-escape-control-characters` only escapes ASCII control characters.
> 
> 3. I am asking for a way to configure `prin1-to-string` such that it
> escapes control and other glyphless characters but not all multibyte
> characters, such that text still stays somewhat readable. I want less
> aggressive escaping than `print-escape-multibyte`. If I set only
> `print-escape-control-characters=t` is not sufficient since it escapes
> only ASCII control characters.

And, to reiterate once more, I'm against partial solutions that affect
only some functions that produce strings, and don't affect at all any
text displayed from a buffer.  It would be a broken solution, because
we will never be able to explain why 'prin1' produces escapes whereas
'format' and 'message' don't.  It is unreasonable to require Lisp
programs which will want to use something like that to use only
'prin1' and not the other functions routinely used for producing text
for display.