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

[Sebastian Miele <iota@whxvd.name>]

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:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ
IMAGEMAGICK JPEG JSON LCMS2 LIBOTF LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2
XPM GTK3 ZLIB

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: C.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils pp shortdoc text-property-search
mouse-copy mouse-drag thingatpt time-date cl-print byte-opt help-fns
radix-tree ispell comp comp-cstr warnings icons subr-x rx cl-seq cl-macs
gv cl-extra help-mode cl-loaddefs cl-lib bytecomp byte-compile rmc
iso-transl tooltip cconv eldoc paren electric uniquify ediff-hook
vc-hooks lisp-float-type elisp-mode mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode lisp-mode prog-mode register
page tab-bar menu-bar rfn-eshadow isearch easymenu timer select
scroll-bar mouse jit-lock font-lock syntax font-core term/tty-colors
frame minibuffer nadvice seq simple cl-generic indonesian philippine
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite emoji-zwj charscript
charprop case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads dbusbind inotify lcms2 dynamic-setting system-font-setting
font-render-setting cairo move-toolbar gtk x-toolkit xinput2 x multi-tty
make-network-process native-compile emacs)

Memory information:
((conses 16 102484 8121)
 (symbols 48 7638 0)
 (strings 32 28748 4608)
 (string-bytes 1 869676)
 (vectors 16 19807)
 (vector-slots 8 344131 11905)
 (floats 8 148 62)
 (intervals 56 1312 1)
 (buffers 984 14))