bug#66394: 29.1; Make register-read-with-preview more useful

[Thierry Volpiatto]

When using `copy-to-register`, it is hard to see which register is
already taken in the preview buffer.
This patch highlight the register entered at prompt if it is already
taken otherwise a minibuffer message is sent to notify user the register
is available.
If any interest here is the patch, feel free to modify if needed.
Thanks.

diff --git a/lisp/register.el b/lisp/register.el
index ca6de450993..4c83264d4eb 100644
--- a/lisp/register.el
+++ b/lisp/register.el
@@ -154,27 +154,37 @@ listing existing registers after `register-preview-delay' 
seconds.
 If `help-char' (or a member of `help-event-list') is pressed,
 display such a window regardless."
   (let* ((buffer "*Register Preview*")
-        (timer (when (numberp register-preview-delay)
-                 (run-with-timer register-preview-delay nil
-                                 (lambda ()
-                                   (unless (get-buffer-window buffer)
-                                     (register-preview buffer))))))
-        (help-chars (cl-loop for c in (cons help-char help-event-list)
-                             when (not (get-register c))
-                             collect c)))
+         (pat "")
+         result timer)
+    (register-preview buffer)
     (unwind-protect
-       (progn
-         (while (memq (read-key (propertize prompt 'face 'minibuffer-prompt))
-                      help-chars)
-           (unless (get-buffer-window buffer)
-             (register-preview buffer 'show-empty)))
-          (when (or (eq ?\C-g last-input-event)
-                    (eq 'escape last-input-event)
-                    (eq ?\C-\[ last-input-event))
-            (keyboard-quit))
-         (if (characterp last-input-event) last-input-event
-           (error "Non-character input-event")))
-      (and (timerp timer) (cancel-timer timer))
+         (progn
+           (minibuffer-with-setup-hook
+               (lambda ()
+                 (setq timer
+                       (run-with-idle-timer
+                        0.3 'repeat
+                        (lambda ()
+                          (with-selected-window (minibuffer-window)
+                            (let ((input (minibuffer-contents)))
+                              (when (not (string= input pat))
+                                (setq pat input))))
+                          (with-current-buffer buffer
+                            (let ((ov (make-overlay (point-min) (point-min))))
+                              (goto-char (point-min))
+                              (if (string= pat "")
+                                  (remove-overlays)
+                                (if (re-search-forward (concat "^" pat) nil t)
+                                    (progn (move-overlay
+                                            ov
+                                            (match-beginning 0) (match-end 0))
+                                           (overlay-put ov 'face 'helm-match))
+                                  (with-selected-window (minibuffer-window)
+                                    (minibuffer-message
+                                     "Register `%s' is available" pat))))))))))
+             (setq result (read-from-minibuffer prompt)))
+           (string-to-char result))
+      (when timer (cancel-timer timer))
       (let ((w (get-buffer-window buffer)))
         (and (window-live-p w) (delete-window w)))
       (and (get-buffer buffer) (kill-buffer buffer)))))



In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
version 1.16.0, Xaw3d scroll bars) of 2023-10-01 built on IPad-S340
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Linux Mint 21.2

Configured using:
 'configure CFLAGS=-O8 --bindir=/usr/local/sbin/emacs-29.1 --with-cairo
--with-x-toolkit=lucid --with-modules --without-tree-sitter
--without-native-compilation'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
X11 XAW3D XDBE XIM XINPUT2 XPM LUCID ZLIB

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

Major mode: 

Minor modes in effect:
  emms-mode-line-mode: t
  emms-playing-time-display-mode: t
  emms-playing-time-mode: t
  bug-reference-prog-mode: t
  server-mode: t
  psession-mode: t
  psession-savehist-mode: t
  global-undo-tree-mode: t
  undo-tree-mode: t
  global-git-gutter-mode: t
  git-gutter-mode: t
  display-time-mode: t
  winner-mode: t
  tv-save-place-mode: t
  helm-epa-mode: t
  helm-descbinds-mode: t
  helm-top-poll-mode: t
  helm-adaptive-mode: t
  helm-mode: t
  helm-minibuffer-history-mode: t
  helm-ff-icon-mode: t
  shell-dirtrack-mode: t
  helm-popup-tip-mode: t
  async-bytecomp-package-mode: t
  dired-async-mode: t
  minibuffer-depth-indicate-mode: t
  gcmh-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  column-number-mode: t
  line-number-mode: t
  auto-fill-function: do-auto-fill
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(epa-mail face-remap addressbook-bookmark tv-mu4e-config config-w3m
mu4e-contrib eshell esh-cmd generator esh-ext esh-opt esh-proc esh-io
esh-arg esh-module esh-groups esh-util mu4e-patch mu4e mu4e-org
org-config ob-gnuplot org-crypt org-protocol org ob ob-tangle ob-ref
ob-lob ob-table ob-exp org-macro org-src ob-comint org-pcomplete
org-list org-footnote org-faces org-entities noutline outline
ob-emacs-lisp ob-core ob-eval org-cycle org-table ol org-fold
org-fold-core org-keys oc org-loaddefs org-version org-compat org-macs
mu4e-notification notifications mu4e-main mu4e-view mu4e-mime-parts
gnus-art mm-uu mml2015 mm-view mml-smime smime gnutls dig gnus-sum
gnus-group gnus-undo gnus-start gnus-dbus gnus-cloud nnimap nnmail
mail-source utf7 nnoo gnus-spec gnus-int gnus-range gnus-win gnus
nnheader range appt diary-lib diary-loaddefs cal-menu calendar
cal-loaddefs mu4e-headers mu4e-thread mu4e-compose mu4e-draft
mu4e-actions smtpmail mu4e-search mu4e-lists mu4e-bookmarks mu4e-mark
mu4e-message shr pixel-fill kinsoku url-file svg dom flow-fill hl-line
mu4e-contacts mu4e-update mu4e-folders mu4e-context mu4e-query-items
mu4e-server mu4e-modeline mu4e-vars mu4e-helpers mu4e-config mu4e-window
ido mu4e-obsolete mailalias mailclient textsec uni-scripts idna-mapping
ucs-normalize uni-confusable textsec-check qp helm-dabbrev shadow
mail-extr emacsbug message yank-media puny rfc822 mml mml-sec 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 smerge-mode whitespace cl-extra helm-command helm-x-files
helm-for-files helm-bookmark bookmark emms-config emms-mpris
emms-librefm-stream emms-librefm-scrobbler emms-playlist-limit emms-i18n
emms-history emms-score emms-stream-info emms-metaplaylist-mode
emms-bookmarks emms-cue emms-mode-line-icon emms-browser sort
emms-volume emms-volume-sndioctl emms-volume-mixerctl emms-volume-pulse
emms-volume-amixer emms-playlist-sort emms-last-played emms-player-xine
emms-player-mpd tq emms-lyrics emms-url emms-streams emms-show-all
emms-tag-editor emms-tag-tracktag emms-mark emms-mode-line emms-cache
emms-info-native emms-info-spc bindat emms-info-exiftool
emms-info-tinytag emms-info-metaflac emms-info-opusinfo
emms-info-ogginfo emms-info-mp3info emms-playlist-mode emms-player-vlc
emms-player-mpv emms-playing-time emms-info emms-later-do
emms-player-mplayer emms-player-simple emms-source-playlist
emms-source-file locate emms-setup emms emms-compat emms-auto
helm-external helm-net tramp-archive tramp-gvfs tramp-cache time-stamp
zeroconf dbus xml helm-ring helm-elisp helm-eval edebug debug backtrace
find-func helm-info cl-indent helm-ls-git vc-git diff-mode vc
vc-dispatcher jka-compr make-mode flymake-shellcheck cus-start
flymake-proc flymake project warnings thingatpt sh-script smie treesit
executable bug-reference naquadah-theme server imenu psession frameset
undo-tree diff queue pcase git-gutter mule-util dired-extension time
winner describe-variable help-fns radix-tree help-mode tv-utils
tv-save-place.el advice init-helm epa derived epg rfc6068 epg-config
helm-epa isl helm-descbinds cus-edit pp icons wid-edit helm-sys popup
helm-adaptive helm-mode helm-misc helm-files image-dired
image-dired-tags image-dired-external image-dired-util xdg image-mode
exif filenotify tramp tramp-loaddefs trampver tramp-integration files-x
tramp-compat rx shell pcomplete parse-time iso8601 time-date
helm-buffers all-the-icons all-the-icons-faces data-material
data-weathericons data-octicons data-fileicons data-faicons
data-alltheicons helm-occur helm-tags helm-locate helm-grep wgrep-helm
wgrep grep compile text-property-search comint ansi-osc ring helm-regexp
format-spec ansi-color helm-utils helm-help helm-types
helm-extensions-autoloads helm-autoloads helm helm-global-bindings
helm-easymenu edmacro kmacro helm-core easy-mmode async-bytecomp
helm-source helm-multi-match helm-lib dired-async async dired-aux dired
dired-loaddefs mb-depth avoid cus-load gcmh all-the-icons-autoloads
gcmh-autoloads info ledger-mode-autoloads markdown-mode-autoloads
nerd-icons-autoloads w3m-load w3m-autoloads yaml-mode-autoloads package
browse-url url url-proxy url-privacy url-expand url-methods url-history
url-cookie generate-lisp-file url-domsuf url-util mailcap url-handlers
url-parse auth-source cl-seq eieio eieio-core cl-macs password-cache
json subr-x map byte-opt gv bytecomp byte-compile url-vars cl-loaddefs
cl-lib 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 x-toolkit xinput2 x multi-tty
make-network-process emacs)

Memory information:
((conses 16 854462 334476)
 (symbols 48 35404 26)
 (strings 32 219034 39971)
 (string-bytes 1 6357489)
 (vectors 16 99232)
 (vector-slots 8 2128904 396764)
 (floats 8 1791 2270)
 (intervals 56 37515 32129)
 (buffers 976 138))
<#secure method=pgpmime mode=sign>

-- 
Thierry