Run "TZ=UTC0 admin/update-copyright".

* lisp/ibuf-ext.el (ibuffer-old-saved-filters-warning)
(ibuffer-maybe-save-stuff): Assume customize-save-variable is
bound; it is autoloaded.
* lisp/cedet/semantic/symref/grep.el (semantic-symref-perform-search):
* lisp/password-cache.el (password-cache-remove):
* lisp/cedet/semantic/bovine/el.el (semantic-dependency-tag-file):
Remove Emacs 21 compat code.
* lisp/cedet/semantic/sort.el (semantic-string-lessp-ci):
Remove Emacs 20 compat code.
* test/lisp/cedet/semantic-utest.el (semantic-utest-temp-directory):
* lisp/mail/supercite.el (sc-ask): Remove XEmacs compat code.
* lisp/progmodes/idlw-shell.el (idlwave-shell-mode):
* lisp/progmodes/idlwave.el (idlwave-mode): Remove commented out
compat code.

Emacs 23.1 was five major releases and over a decade ago.
This list can be reviewed before to the next release, but for now
hopefully this motivates any needed external updates.
Ref: https://lists.gnu.org/archive/html/emacs-devel/2020-05/msg02198.html

* lisp/abbrev.el (pre-abbrev-expand-hook):
* lisp/bookmark.el (bookmark-read-annotation-text-func)
(bookmark-jump-noselect):
* lisp/buff-menu.el (buffer-menu-mode-hook):
* lisp/cus-edit.el (custom-mode-hook, custom-mode):
* lisp/dirtrack.el (dirtrack-debug-toggle, dirtrack-debug):
* lisp/emacs-lisp/crm.el (crm-minibuffer-complete)
(crm-minibuffer-completion-help)
(crm-minibuffer-complete-and-exit):
* lisp/emacs-lisp/easymenu.el
(easy-menu-precalculate-equivalent-keybindings):
* lisp/emacs-lisp/lisp-mode.el (lisp-mode-auto-fill):
* lisp/epa.el (epa-display-verify-result):
* lisp/epg.el (epg-passphrase-callback-function):
* lisp/eshell/eshell.el (eshell-report-bug):
* lisp/ffap.el (ffap-bug, ffap-submit-bug):
* lisp/files.el (locate-file-completion):
* lisp/hi-lock.el (hi-lock-face-history, hi-lock-regexp-history):
* lisp/hilit-chg.el (highlight-changes-initial-state)
(highlight-changes-active-string)
(highlight-changes-passive-string, global-highlight-changes):
* lisp/international/mule-cmds.el (nonascii-insert-offset)
(nonascii-translation-table):
* lisp/international/mule-diag.el (non-iso-charset-alist):
* lisp/international/mule-util.el (detect-coding-with-priority):
* lisp/international/mule.el (charset-id, charset-bytes)
(charset-list, char-valid-p, generic-char-p)
(char-coding-system-table, make-coding-system)
(set-coding-priority)
* lisp/mail/rmail.el (rmail-message-filter):
* lisp/minibuffer.el (complete-in-turn, dynamic-completion-table)
(completion-common-substring)
(minibuffer-local-must-match-filename-map):
* lisp/mouse.el (mouse-major-mode-menu, mouse-popup-menubar)
(mouse-popup-menubar-stuff):
* lisp/net/newst-treeview.el (newsticker-groups-filename):
* lisp/obsolete/tpu-edt.el (tpu-have-ispell, GOLD-map):
* lisp/password-cache.el (password-read-and-add):
* lisp/shell.el (shell-dirtrack-toggle):
* lisp/subr.el (forward-point, redisplay-end-trigger-functions)
(process-filter-multibyte-p, set-process-filter-multibyte):
* lisp/t-mouse.el (t-mouse-mode):
* lisp/term/w32-win.el (w32-focus-frame, w32-select-font):
* lisp/textmodes/ispell.el (ispell-aspell-supports-utf8):
* lisp/textmodes/remember.el (remember-buffer):
* lisp/tooltip.el (tooltip-hook):
* lisp/url/url-util.el (url-generate-unique-filename):
* lisp/url/url-vars.el (url-temporary-directory):
* lisp/vc/vc-hooks.el (vc-workfile-version)
(vc-default-working-revision):
* lisp/vc/vc-mtn.el (vc-mtn-command):
* lisp/vc/vc.el (vc-revert-buffer):
* lisp/vcursor.el (vcursor-toggle-vcursor-map):
Remove items, obsolete since Emacs 23.1.
* lisp/abbrev.el (expand-abbrev):
* lisp/epg.el (epg-context): Change
'epg-passphrase-callback-function' call to 'epa-' alternative.
* lisp/eshell/em-rebind.el (eshell-cannot-leave-input-list): Don't
refer to removed function 'forward-point'.
* test/manual/etags/c-src/abbrev.c (Fexpand_abbrev):
(syms_of_abbrev): Don't run removed hook 'pre-abbrev-expand-hook'.
* lisp/international/mule.el (transform-make-coding-system-args):
Declare obsolete.
* lisp/progmodes/idlwave.el:
Update reference to removed function 'char-valid-p'.
* lisp/gnus/mml2015.el (epg-encrypt-string):
* lisp/gnus/mml1991.el (epg-make-context):
* lisp/gnus/mml-smime.el (autoload):
Remove autoload of removed 'epg-passphrase-callback-function'.
* lisp/minibuffer.el (completion-extra-properties):
Remove support for `completion-common-substring'.
* lisp/obsolete/tpu-edt.el (tpu-toggle-overwrite-mode)
Remove support for removed `spell' package.
* src/coding.c (syms_of_coding):
* doc/misc/efaq.texi:
* doc/emacs/frames.texi (Menu Mouse Clicks):
* doc/misc/url.texi (Customization): Doc fixes.
; * etc/NEWS: List removed items.

* lisp/password-cache.el: Improve comment.  See Andreas Schwab in:
https://lists.gnu.org/r/emacs-devel/2020-05/msg02422.html

* lisp/bookmark.el (bookmark-bmenu-set-header):
Use copy-sequence instead of concat, for clarity.
Also, the byte-compiler optimizes (concat "a" "b") into "ab".
* lisp/button.el (make-text-button):
* test/lisp/erc/erc-track-tests.el (erc-track--erc-faces-in):
* test/lisp/password-cache-tests.el:
(password-cache-tests-add-and-remove)
(password-cache-tests-read-from-cache)
(password-cache-tests-in-cache-p, password-cache-tests-read)
(password-cache-tests-reset)
(password-cache-tests-add/expires-key)
(password-cache-tests-no-password-cache):
Don’t attempt to modify constant strings.
* lisp/progmodes/elisp-mode.el (elisp--xref-format)
(elisp--xref-format-extra):
Don’t attempt to modify constant strings via put-text-property.
* test/lisp/emacs-lisp/cl-macs-tests.el (cl-macs-loop-across-ref):
Don’t attempt to modify constant vectors or strings.

Run "TZ=UTC0 admin/update-copyright $(git ls-files)".

* password-cache.el (password-in-cache-p, password-cache-add):
  properly detect non-existent entry.  (Bug#36834)

* lisp/password-cache.el: Use lexical-binding.
* test/lisp/password-cache-tests.el: New file.