bug#64835: ‘describe-variable’ unconditionally let-binds ‘print-circle’ to t

[Sebastian Miele]

That was not the case in Emacs 28.  It seems to have been introduced
in the following commit:

  2c18969c810f338d73beda592ee5af7103132e97
  Author:     Stefan Monnier <monnier@iro.umontreal.ca>
  AuthorDate: Thu Mar 2 02:08:08 2017 -0500
  Commit:     Stefan Monnier <monnier@iro.umontreal.ca>
  CommitDate: Thu Mar 2 02:08:32 2017 -0500

  * lisp/help-fns.el (describe-variable): Use cl-print for the value

  Use `pp-buffer' rather than `pp' so as to avoid calling prin1 twice.

2017 is a while ago.  The first pretest release of Emacs 28 was in
December 2021.  So the commit must have entered the current emacs-29
branch in some "non-linear" way.

The problem is that the effect of ‘print-circle’ being t pertains to
all sharing, not just sharing that leads to circles.  And that has the
effect, that the output can become arbitrarily unreadable.  Consider
for example:

  (require 'ispell)
  (ispell-set-spellchecker-params)
  (describe-variable 'ispell-dictionary-alist)

The display of the value in my machine contains, e.g., "(nil #1# #2# #3#
t #4# nil utf-8)".

I do not see the reason for hard-coding ‘print-circle’ to t in
‘describe-variable’.  The only reason I can imagine would be the
prevention of infinite loops.  But that cannot be the case, because
even with ‘print-circle’ nil, the printing routines seem to check for
circularity and handle it gracefully in the output.

Is this a bug or a necessity that I do not understand, yet?

The NEWS file does not mention this change.

In general (and somewhat unrelated), it would be nice if ‘print-circle’
would be split into two things: (1) make explicit all sharing (as is
done now), and (2) make explicit only a subset of sharing that resolves
all circles.  (The latter would in at least two aspects still be
different from the graceful handling mentioned above when ‘print-circle’
is nil.)

-----

In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.38,
 cairo version 1.17.8) of 2023-07-23 built on huette
Repository revision: 906ecf442c43da98e6b8c7488f549691901690e9
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Arch Linux

Configured using:
 'configure --with-x-toolkit=gtk --with-native-compilation=yes
 --with-tree-sitter --with-json --with-mailutils --with-imagemagick'

Configured features:
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XPM GTK3 ZLIB

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Major mode: Lisp Interaction

Minor modes in effect:
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Load-path shadows:
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Features:
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Memory information:
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