Generalize the prefix-command machinery of C-u

* lisp/simple.el (prefix-command-echo-keystrokes-functions)
(prefix-command-preserve-state-hook): New hooks.
(internal-echo-keystrokes-prefix): New function.
(prefix-command--needs-update, prefix-command--last-echo): New vars.
(prefix-command-update, prefix-command-preserve): New functions.
(reset-this-command-lengths): New compatibility definition.
(universal-argument--mode): Call prefix-command-update.
(universal-argument, universal-argument-more, negative-argument)
(digit-argument): Call prefix-command-preserve-state.

* src/keyboard.c: Call internal-echo-keystrokes-prefix to build
the "prefix argument" to echo.
(this_command_key_count_reset, before_command_key_count)
(before_command_echo_length): Delete variables.
(echo_add_key): Always add a space.
(echo_char): Remove.
(echo_dash): Don't give up when this_command_key_count is 0, since that
is now the case after a prefix command.
(echo_update): New function, extracted from echo_now.
(echo_now): Use it.
(add_command_key, read_char, record_menu_key): Remove old disabled code.
(command_loop_1): Don't refrain from pushing an undo boundary when
prefix-arg is set.  Remove other prefix-arg special case, now handled
directly in the prefix commands instead.  But call echo_now if there's
a prefix state to echo.
(read_char, record_menu_key): Use echo_update instead of echo_char.
(read_key_sequence): Use echo_now rather than echo_dash/echo_char.
(Freset_this_command_lengths): Delete function.
(syms_of_keyboard): Define Qinternal_echo_keystrokes_prefix.
(syms_of_keyboard): Don't defsubr Sreset_this_command_lengths.

* lisp/simple.el: Use those new hooks for C-u.
(universal-argument--description): New function.
(prefix-command-echo-keystrokes-functions): Use it.
(universal-argument--preserve): New function.
(prefix-command-preserve-state-hook): Use it.
(command-execute): Call prefix-command-update if needed.

* lisp/kmacro.el (kmacro-step-edit-prefix-commands)
(kmacro-step-edit-prefix-index): Delete variables.
(kmacro-step-edit-query, kmacro-step-edit-insert): Remove ad-hoc
support for prefix arg commands.
(kmacro-step-edit-macro): Don't bind kmacro-step-edit-prefix-index.

* lisp/emulation/cua-base.el (cua--prefix-override-replay)
(cua--shift-control-prefix): Use prefix-command-preserve-state.
Remove now unused arg `arg'.
(cua--prefix-override-handler, cua--prefix-repeat-handler)
(cua--shift-control-c-prefix, cua--shift-control-x-prefix):
Update accordingly.
(cua--prefix-override-timeout): Don't call reset-this-command-lengths
any more.
(cua--keep-active, cua-exchange-point-and-mark): Don't set mark-active
if the mark is not set.

5 files changed, 188 insertions, 277 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 3832ffae1f4..e50e7a79173 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -965,6 +965,10 @@ be updated accordingly.
 
 * Lisp Changes in Emacs 25.1
 
+** New hooks prefix-command-echo-keystrokes-functions and
+prefix-command-preserve-state-hook, to allow the definition of prefix
+commands other than the predefined C-u.
+
 ** New functions `filepos-to-bufferpos' and `bufferpos-to-filepos'.
 
 ** The default value of `load-read-function' is now `read'.
diff --git a/lisp/emulation/cua-base.el b/lisp/emulation/cua-base.el
index e91ce80bbe2..52e1647ede7 100644
--- a/lisp/emulation/cua-base.el
+++ b/lisp/emulation/cua-base.el
@@ -685,7 +685,7 @@ a cons (TYPE . COLOR), then both properties are affected."
 (defvar cua--prefix-override-timer nil)
 (defvar cua--prefix-override-length nil)
 
-(defun cua--prefix-override-replay (arg repeat)
+(defun cua--prefix-override-replay (repeat)
   (let* ((keys (this-command-keys))
          (i (length keys))
          (key (aref keys (1- i))))
@@ -705,21 +705,23 @@ a cons (TYPE . COLOR), then both properties are affected."
     ;; Don't record this command
     (setq this-command last-command)
     ;; Restore the prefix arg
-    (setq prefix-arg arg)
-    (reset-this-command-lengths)
+    ;; This should make it so that exchange-point-and-mark gets the prefix when
+    ;; you do C-u C-x C-x C-x work (where the C-u is properly passed to the C-x
+    ;; C-x binding after the first C-x C-x was rewritten to just C-x).
+    (prefix-command-preserve-state)
     ;; Push the key back on the event queue
     (setq unread-command-events (cons key unread-command-events))))
 
-(defun cua--prefix-override-handler (arg)
+(defun cua--prefix-override-handler ()
   "Start timer waiting for prefix key to be followed by another key.
 Repeating prefix key when region is active works as a single prefix key."
-  (interactive "P")
-  (cua--prefix-override-replay arg 0))
+  (interactive)
+  (cua--prefix-override-replay 0))
 
-(defun cua--prefix-repeat-handler (arg)
+(defun cua--prefix-repeat-handler ()
   "Repeating prefix key when region is active works as a single prefix key."
-  (interactive "P")
-  (cua--prefix-override-replay arg 1))
+  (interactive)
+  (cua--prefix-override-replay 1))
 
 (defun cua--prefix-copy-handler (arg)
   "Copy region/rectangle, then replay last key."
@@ -742,7 +744,8 @@ Repeating prefix key when region is active works as a single prefix key."
   (when (= (length (this-command-keys)) cua--prefix-override-length)
     (setq unread-command-events (cons 'timeout unread-command-events))
     (if prefix-arg
-      (reset-this-command-lengths)
+        nil
+      ;; FIXME: Why?
       (setq overriding-terminal-local-map nil))
     (cua--select-keymaps)))
 
@@ -755,8 +758,9 @@ Repeating prefix key when region is active works as a single prefix key."
   (call-interactively this-command))
 
 (defun cua--keep-active ()
-  (setq mark-active t
-        deactivate-mark nil))
+  (when (mark t)
+    (setq mark-active t
+          deactivate-mark nil)))
 
 (defun cua--deactivate (&optional now)
   (if (not now)
@@ -944,7 +948,7 @@ See also `exchange-point-and-mark'."
   (cond ((null cua-enable-cua-keys)
          (exchange-point-and-mark arg))
         (arg
-         (setq mark-active t))
+         (when (mark t) (setq mark-active t)))
         (t
          (let (mark-active)
            (exchange-point-and-mark)
@@ -1212,25 +1216,28 @@ If ARG is the atom `-', scroll upward by nearly full screen."
 
 (defvar cua--keymaps-initialized nil)
 
-(defun cua--shift-control-prefix (prefix arg)
+(defun cua--shift-control-prefix (prefix)
   ;; handle S-C-x and S-C-c by emulating the fast double prefix function.
   ;; Don't record this command
   (setq this-command last-command)
   ;; Restore the prefix arg
-  (setq prefix-arg arg)
-  (reset-this-command-lengths)
+  ;; This should make it so that exchange-point-and-mark gets the prefix when
+  ;; you do C-u S-C-x C-x work (where the C-u is properly passed to the C-x
+  ;; C-x binding after the first S-C-x was rewritten to just C-x).
+  (prefix-command-preserve-state)
   ;; Activate the cua--prefix-repeat-keymap
   (setq cua--prefix-override-timer 'shift)
   ;; Push duplicate keys back on the event queue
-  (setq unread-command-events (cons prefix (cons prefix unread-command-events))))
+  (setq unread-command-events
+        (cons prefix (cons prefix unread-command-events))))
 
-(defun cua--shift-control-c-prefix (arg)
-  (interactive "P")
-  (cua--shift-control-prefix ?\C-c arg))
+(defun cua--shift-control-c-prefix ()
+  (interactive)
+  (cua--shift-control-prefix ?\C-c))
 
-(defun cua--shift-control-x-prefix (arg)
-  (interactive "P")
-  (cua--shift-control-prefix ?\C-x arg))
+(defun cua--shift-control-x-prefix ()
+  (interactive)
+  (cua--shift-control-prefix ?\C-x))
 
 (defun cua--init-keymaps ()
   ;; Cache actual rectangle modifier key.
diff --git a/lisp/kmacro.el b/lisp/kmacro.el
index 9636a36b1e2..ddf3005bab5 100644
--- a/lisp/kmacro.el
+++ b/lisp/kmacro.el
@@ -941,7 +941,6 @@ without repeating the prefix."
 (defvar kmacro-step-edit-inserting)      ;; inserting into macro
 (defvar kmacro-step-edit-appending)      ;; append to end of macro
 (defvar kmacro-step-edit-replace)        ;; replace orig macro when done
-(defvar kmacro-step-edit-prefix-index)   ;; index of first prefix arg key
 (defvar kmacro-step-edit-key-index)      ;; index of current key
 (defvar kmacro-step-edit-action)         ;; automatic action on next pre-command hook
 (defvar kmacro-step-edit-help)           ;; kmacro step edit help enabled
@@ -976,11 +975,6 @@ This keymap is an extension to the `query-replace-map', allowing the
 following additional answers: `insert', `insert-1', `replace', `replace-1',
 `append', `append-end', `act-repeat', `skip-end', `skip-keep'.")
 
-(defvar kmacro-step-edit-prefix-commands
-  '(universal-argument universal-argument-more universal-argument-minus
-                       digit-argument negative-argument)
-  "Commands which build up a prefix arg for the current command.")
-
 (defun kmacro-step-edit-prompt (macro index)
   ;; Show step-edit prompt
   (let ((keys (and (not kmacro-step-edit-appending)
@@ -1084,21 +1078,13 @@ following additional answers: `insert', `insert-1', `replace', `replace-1',
       ;; Handle prefix arg, or query user
       (cond
        (act act) ;; set above
-       ((memq this-command kmacro-step-edit-prefix-commands)
-        (unless kmacro-step-edit-prefix-index
-          (setq kmacro-step-edit-prefix-index kmacro-step-edit-key-index))
-        (setq act 'universal-argument))
-       ((eq this-command 'universal-argument-other-key)
-        (setq act 'universal-argument))
        (t
-        (kmacro-step-edit-prompt macro (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index))
+        (kmacro-step-edit-prompt macro kmacro-step-edit-key-index)
         (setq act (lookup-key kmacro-step-edit-map
                               (vector (with-current-buffer (current-buffer) (read-event))))))))
 
     ;; Resume macro execution and perform the action
     (cond
-     ((eq act 'universal-argument)
-      nil)
      ((cond
        ((eq act 'act)
         t)
@@ -1110,7 +1096,6 @@ following additional answers: `insert', `insert-1', `replace', `replace-1',
         (setq kmacro-step-edit-active 'ignore)
         nil)
        ((eq act 'skip)
-        (setq kmacro-step-edit-prefix-index nil)
         nil)
        ((eq act 'skip-keep)
         (setq this-command 'ignore)
@@ -1123,12 +1108,11 @@ following additional answers: `insert', `insert-1', `replace', `replace-1',
         (setq act t)
         t)
        ((member act '(insert-1 insert))
-        (setq executing-kbd-macro-index (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index))
+        (setq executing-kbd-macro-index kmacro-step-edit-key-index)
         (setq kmacro-step-edit-inserting (if (eq act 'insert-1) 1 t))
         nil)
        ((member act '(replace-1 replace))
         (setq kmacro-step-edit-inserting (if (eq act 'replace-1) 1 t))
-        (setq kmacro-step-edit-prefix-index nil)
         (if (= executing-kbd-macro-index (length executing-kbd-macro))
             (setq executing-kbd-macro (vconcat executing-kbd-macro [nil])
                   kmacro-step-edit-appending t))
@@ -1148,19 +1132,19 @@ following additional answers: `insert', `insert-1', `replace', `replace-1',
         (setq act t)
         t)
        ((eq act 'help)
-        (setq executing-kbd-macro-index (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index))
+        (setq executing-kbd-macro-index kmacro-step-edit-key-index)
         (setq kmacro-step-edit-help (not kmacro-step-edit-help))
         nil)
        (t ;; Ignore unknown responses
-        (setq executing-kbd-macro-index (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index))
+        (setq executing-kbd-macro-index kmacro-step-edit-key-index)
         nil))
-      (if (> executing-kbd-macro-index (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index))
+      (if (> executing-kbd-macro-index kmacro-step-edit-key-index)
           (setq kmacro-step-edit-new-macro
                 (vconcat kmacro-step-edit-new-macro
                          (substring executing-kbd-macro
-                                    (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index)
-                                    (if (eq act t) nil executing-kbd-macro-index)))
-                kmacro-step-edit-prefix-index nil))
+                                    kmacro-step-edit-key-index
+                                    (if (eq act t) nil
+                                      executing-kbd-macro-index)))))
       (if restore-index
           (setq executing-kbd-macro-index restore-index)))
      (t
@@ -1175,12 +1159,10 @@ following additional answers: `insert', `insert-1', `replace', `replace-1',
         (executing-kbd-macro nil)
         (defining-kbd-macro nil)
         cmd keys next-index)
-    (setq executing-kbd-macro-index (or kmacro-step-edit-prefix-index kmacro-step-edit-key-index)
-          kmacro-step-edit-prefix-index nil)
+    (setq executing-kbd-macro-index kmacro-step-edit-key-index)
     (kmacro-step-edit-prompt macro nil)
     ;; Now, we have read a key sequence from the macro, but we don't want
     ;; to execute it yet.  So push it back and read another sequence.
-    (reset-this-command-lengths)
     (setq keys (read-key-sequence nil nil nil nil t))
     (setq cmd (key-binding keys t nil))
     (if (cond
@@ -1201,25 +1183,12 @@ following additional answers: `insert', `insert-1', `replace', `replace-1',
                     unread-command-events nil)))
           (setq cmd 'ignore)
           nil)
-         ((memq cmd kmacro-step-edit-prefix-commands)
-          (reset-this-command-lengths)
-          nil)
-         ((eq cmd 'universal-argument-other-key)
-          (setq kmacro-step-edit-action t)
-          (reset-this-command-lengths)
-          (if (numberp kmacro-step-edit-inserting)
-              (setq kmacro-step-edit-inserting nil))
-          nil)
          ((numberp kmacro-step-edit-inserting)
           (setq kmacro-step-edit-inserting nil)
           nil)
          ((equal keys "\C-j")
           (setq kmacro-step-edit-inserting nil)
           (setq kmacro-step-edit-action nil)
-          ;; Forget any (partial) prefix arg from next command
-          (setq kmacro-step-edit-prefix-index nil)
-          (reset-this-command-lengths)
-          (setq overriding-terminal-local-map nil)
           (setq next-index kmacro-step-edit-key-index)
           t)
          (t nil))
@@ -1278,7 +1247,6 @@ To customize possible responses, change the \"bindings\" in `kmacro-step-edit-ma
         (kmacro-step-edit-inserting nil)
         (kmacro-step-edit-appending nil)
         (kmacro-step-edit-replace t)
-        (kmacro-step-edit-prefix-index nil)
         (kmacro-step-edit-key-index 0)
         (kmacro-step-edit-action nil)
         (kmacro-step-edit-help nil)
diff --git a/lisp/simple.el b/lisp/simple.el
index 6f76d755292..b8d4e741775 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -1711,9 +1711,13 @@ The argument SPECIAL, if non-nil, means that this command is executing
 a special event, so ignore the prefix argument and don't clear it."
   (setq debug-on-next-call nil)
   (let ((prefixarg (unless special
+                     ;; FIXME: This should probably be done around
+                     ;; pre-command-hook rather than here!
                      (prog1 prefix-arg
                        (setq current-prefix-arg prefix-arg)
-                       (setq prefix-arg nil)))))
+                       (setq prefix-arg nil)
+                       (when current-prefix-arg
+                         (prefix-command-update))))))
     (if (and (symbolp cmd)
              (get cmd 'disabled)
              disabled-command-function)
@@ -3626,6 +3630,73 @@ see other processes running on the system, use `list-system-processes'."
   (display-buffer buffer)
   nil)
 
+;;;; Prefix commands
+
+(setq prefix-command--needs-update nil)
+(setq prefix-command--last-echo nil)
+
+(defun internal-echo-keystrokes-prefix ()
+  ;; BEWARE: Called directly from the C code.
+  (if (not prefix-command--needs-update)
+      prefix-command--last-echo
+    (setq prefix-command--last-echo
+          (let ((strs nil))
+            (run-hook-wrapped 'prefix-command-echo-keystrokes-functions
+                              (lambda (fun) (push (funcall fun) strs)))
+            (setq strs (delq nil strs))
+            (when strs (mapconcat #'identity strs " "))))))
+
+(defvar prefix-command-echo-keystrokes-functions nil
+  "Abnormal hook which constructs the description of the current prefix state.
+Each function is called with no argument, should return a string or nil.")
+
+(defun prefix-command-update ()
+  "Update state of prefix commands.
+Call it whenever you change the \"prefix command state\"."
+  (setq prefix-command--needs-update t))
+
+(defvar prefix-command-preserve-state-hook nil
+  "Normal hook run when a command needs to preserve the prefix.")
+
+(defun prefix-command-preserve-state ()
+  "Pass the current prefix command state to the next command.
+Should be called by all prefix commands.
+Runs `prefix-command-preserve-state-hook'."
+  (run-hooks 'prefix-command-preserve-state-hook)
+  ;; If the current command is a prefix command, we don't want the next (real)
+  ;; command to have `last-command' set to, say, `universal-argument'.
+  (setq this-command last-command)
+  (setq real-this-command real-last-command)
+  (prefix-command-update))
+
+(defun reset-this-command-lengths ()
+  (declare (obsolete prefix-command-preserve-state "25.1"))
+  nil)
+
+;;;;; The main prefix command.
+
+;; FIXME: Declaration of `prefix-arg' should be moved here!?
+
+(add-hook 'prefix-command-echo-keystrokes-functions
+          #'universal-argument--description)
+(defun universal-argument--description ()
+  (when prefix-arg
+    (concat "C-u"
+            (pcase prefix-arg
+              (`(-) " -")
+              (`(,(and (pred integerp) n))
+               (let ((str ""))
+                 (while (and (> n 4) (= (mod n 4) 0))
+                   (setq str (concat str " C-u"))
+                   (setq n (/ n 4)))
+                 (if (= n 4) str (format " %s" prefix-arg))))
+              (_ (format " %s" prefix-arg))))))
+
+(add-hook 'prefix-command-preserve-state-hook
+          #'universal-argument--preserve)
+(defun universal-argument--preserve ()
+  (setq prefix-arg current-prefix-arg))
+
 (defvar universal-argument-map
   (let ((map (make-sparse-keymap))
         (universal-argument-minus
@@ -3664,7 +3735,8 @@ see other processes running on the system, use `list-system-processes'."
   "Keymap used while processing \\[universal-argument].")
 
 (defun universal-argument--mode ()
-  (set-transient-map universal-argument-map))
+  (prefix-command-update)
+  (set-transient-map universal-argument-map nil))
 
 (defun universal-argument ()
   "Begin a numeric argument for the following command.
@@ -3677,6 +3749,7 @@ For some commands, just \\[universal-argument] by itself serves as a flag
 which is different in effect from any particular numeric argument.
 These commands include \\[set-mark-command] and \\[start-kbd-macro]."
   (interactive)
+  (prefix-command-preserve-state)
   (setq prefix-arg (list 4))
   (universal-argument--mode))
 
@@ -3684,6 +3757,7 @@ These commands include \\[set-mark-command] and \\[start-kbd-macro]."
   ;; A subsequent C-u means to multiply the factor by 4 if we've typed
   ;; nothing but C-u's; otherwise it means to terminate the prefix arg.
   (interactive "P")
+  (prefix-command-preserve-state)
   (setq prefix-arg (if (consp arg)
                        (list (* 4 (car arg)))
                      (if (eq arg '-)
@@ -3695,6 +3769,7 @@ These commands include \\[set-mark-command] and \\[start-kbd-macro]."
   "Begin a negative numeric argument for the next command.
 \\[universal-argument] following digits or minus sign ends the argument."
   (interactive "P")
+  (prefix-command-preserve-state)
   (setq prefix-arg (cond ((integerp arg) (- arg))
                          ((eq arg '-) nil)
                          (t '-)))
@@ -3704,6 +3779,7 @@ These commands include \\[set-mark-command] and \\[start-kbd-macro]."
   "Part of the numeric argument for the next command.
 \\[universal-argument] following digits or minus sign ends the argument."
   (interactive "P")
+  (prefix-command-preserve-state)
   (let* ((char (if (integerp last-command-event)
                    last-command-event
                  (get last-command-event 'ascii-character)))
diff --git a/src/keyboard.c b/src/keyboard.c
index d7a533b80b7..a8b1e9828bf 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -107,10 +107,6 @@ static Lisp_Object recent_keys;
 Lisp_Object this_command_keys;
 ptrdiff_t this_command_key_count;
 
-/* True after calling Freset_this_command_lengths.
-   Usually it is false.  */
-static bool this_command_key_count_reset;
-
 /* This vector is used as a buffer to record the events that were actually read
    by read_key_sequence.  */
 static Lisp_Object raw_keybuf;
@@ -124,11 +120,6 @@ static int raw_keybuf_count;
    that precede this key sequence.  */
 static ptrdiff_t this_single_command_key_start;
 
-/* Record values of this_command_key_count and echo_length ()
-   before this command was read.  */
-static ptrdiff_t before_command_key_count;
-static ptrdiff_t before_command_echo_length;
-
 #ifdef HAVE_STACK_OVERFLOW_HANDLING
 
 /* For longjmp to recover from C stack overflow.  */
@@ -441,10 +432,12 @@ echo_add_key (Lisp_Object c)
   ptrdiff_t size = sizeof initbuf;
   char *buffer = initbuf;
   char *ptr = buffer;
-  Lisp_Object echo_string;
+  Lisp_Object echo_string = KVAR (current_kboard, echo_string);
   USE_SAFE_ALLOCA;
 
-  echo_string = KVAR (current_kboard, echo_string);
+  if (STRINGP (echo_string) && SCHARS (echo_string) > 0)
+    /* Add a space at the end as a separator between keys.  */
+    ptr++[0] = ' ';
 
   /* If someone has passed us a composite event, use its head symbol.  */
   c = EVENT_HEAD (c);
@@ -486,48 +479,12 @@ echo_add_key (Lisp_Object c)
       ptr += len;
     }
 
-  /* Replace a dash from echo_dash with a space, otherwise add a space
-     at the end as a separator between keys.  */
-  AUTO_STRING (space, " ");
-  if (STRINGP (echo_string) && SCHARS (echo_string) > 1)
-    {
-      Lisp_Object last_char, prev_char, idx;
-
-      idx = make_number (SCHARS (echo_string) - 2);
-      prev_char = Faref (echo_string, idx);
-
-      idx = make_number (SCHARS (echo_string) - 1);
-      last_char = Faref (echo_string, idx);
-
-      /* We test PREV_CHAR to make sure this isn't the echoing of a
-         minus-sign.  */
-      if (XINT (last_char) == '-' && XINT (prev_char) != ' ')
-        Faset (echo_string, idx, make_number (' '));
-      else
-        echo_string = concat2 (echo_string, space);
-    }
-  else if (STRINGP (echo_string) && SCHARS (echo_string) > 0)
-    echo_string = concat2 (echo_string, space);
-
   kset_echo_string
     (current_kboard,
      concat2 (echo_string, make_string (buffer, ptr - buffer)));
   SAFE_FREE ();
 }
 
-/* Add C to the echo string, if echoing is going on.  C can be a
-   character or a symbol.  */
-
-static void
-echo_char (Lisp_Object c)
-{
-  if (current_kboard->immediate_echo)
-    {
-      echo_add_key (c);
-      echo_now ();
-    }
-}
-
 /* Temporarily add a dash to the end of the echo string if it's not
    empty, so that it serves as a mini-prompt for the very next
    character.  */
@@ -539,9 +496,6 @@ echo_dash (void)
   if (NILP (KVAR (current_kboard, echo_string)))
     return;
 
-  if (this_command_key_count == 0)
-    return;
-
   if (!current_kboard->immediate_echo
       && SCHARS (KVAR (current_kboard, echo_string)) == 0)
     return;
@@ -574,39 +528,39 @@ echo_dash (void)
   echo_now ();
 }
 
-/* Display the current echo string, and begin echoing if not already
-   doing so.  */
-
 static void
-echo_now (void)
+echo_update (void)
 {
-  if (!current_kboard->immediate_echo)
+  if (current_kboard->immediate_echo)
     {
       ptrdiff_t i;
-      current_kboard->immediate_echo = true;
+      kset_echo_string (current_kboard,
+                        call0 (Qinternal_echo_keystrokes_prefix));
 
       for (i = 0; i < this_command_key_count; i++)
         {
           Lisp_Object c;
 
-          /* Set before_command_echo_length to the value that would
-             have been saved before the start of this subcommand in
-             command_loop_1, if we had already been echoing then.  */
-          if (i == this_single_command_key_start)
-            before_command_echo_length = echo_length ();
-
           c = AREF (this_command_keys, i);
           if (! (EVENT_HAS_PARAMETERS (c)
                  && EQ (EVENT_HEAD_KIND (EVENT_HEAD (c)), Qmouse_movement)))
-            echo_char (c);
+            echo_add_key (c);
         }
 
-      /* Set before_command_echo_length to the value that would
-         have been saved before the start of this subcommand in
-         command_loop_1, if we had already been echoing then.  */
-      if (this_command_key_count == this_single_command_key_start)
-        before_command_echo_length = echo_length ();
+      echo_now ();
+    }
+}
+
+/* Display the current echo string, and begin echoing if not already
+   doing so.  */
 
+static void
+echo_now (void)
+{
+  if (!current_kboard->immediate_echo)
+    {
+      current_kboard->immediate_echo = true;
+      echo_update ();
       /* Put a dash at the end to invite the user to type more.  */
       echo_dash ();
     }
@@ -666,20 +620,6 @@ echo_truncate (ptrdiff_t nchars)
 static void
 add_command_key (Lisp_Object key)
 {
-#if 0 /* Not needed after we made Freset_this_command_lengths
-         do the job immediately.  */
-  /* If reset-this-command-length was called recently, obey it now.
-     See the doc string of that function for an explanation of why.  */
-  if (before_command_restore_flag)
-    {
-      this_command_key_count = before_command_key_count_1;
-      if (this_command_key_count < this_single_command_key_start)
-        this_single_command_key_start = this_command_key_count;
-      echo_truncate (before_command_echo_length_1);
-      before_command_restore_flag = 0;
-    }
-#endif
-
   if (this_command_key_count >= ASIZE (this_command_keys))
     this_command_keys = larger_vector (this_command_keys, 1, -1);
 
@@ -1285,10 +1225,6 @@ static void adjust_point_for_property (ptrdiff_t, bool);
 /* The last boundary auto-added to buffer-undo-list.  */
 Lisp_Object last_undo_boundary;
 
-/* FIXME: This is wrong rather than test window-system, we should call
-   a new set-selection, which will then dispatch to x-set-selection, or
-   tty-set-selection, or w32-set-selection, ...  */
-
 Lisp_Object
 command_loop_1 (void)
 {
@@ -1306,7 +1242,6 @@ command_loop_1 (void)
   cancel_echoing ();
 
   this_command_key_count = 0;
-  this_command_key_count_reset = false;
   this_single_command_key_start = 0;
 
   if (NILP (Vmemory_full))
@@ -1394,9 +1329,6 @@ command_loop_1 (void)
           && !NILP (Ffboundp (Qrecompute_lucid_menubar)))
         call0 (Qrecompute_lucid_menubar);
 
-      before_command_key_count = this_command_key_count;
-      before_command_echo_length = echo_length ();
-
       Vthis_command = Qnil;
       Vreal_this_command = Qnil;
       Vthis_original_command = Qnil;
@@ -1424,7 +1356,6 @@ command_loop_1 (void)
         {
           cancel_echoing ();
           this_command_key_count = 0;
-          this_command_key_count_reset = false;
           this_single_command_key_start = 0;
           goto finalize;
         }
@@ -1509,14 +1440,13 @@ command_loop_1 (void)
               }
 #endif
 
-            if (NILP (KVAR (current_kboard, Vprefix_arg))) /* FIXME: Why?  --Stef  */
-              {
-                Lisp_Object undo = BVAR (current_buffer, undo_list);
-                Fundo_boundary ();
-                last_undo_boundary
-                  = (EQ (undo, BVAR (current_buffer, undo_list))
-                     ? Qnil : BVAR (current_buffer, undo_list));
-              }
+            {
+              Lisp_Object undo = BVAR (current_buffer, undo_list);
+              Fundo_boundary ();
+              last_undo_boundary
+                = (EQ (undo, BVAR (current_buffer, undo_list))
+                   ? Qnil : BVAR (current_buffer, undo_list));
+            }
             call1 (Qcommand_execute, Vthis_command);
 
 #ifdef HAVE_WINDOW_SYSTEM
@@ -1544,31 +1474,23 @@ command_loop_1 (void)
 
       safe_run_hooks (Qdeferred_action_function);
 
-      /* If there is a prefix argument,
-         1) We don't want Vlast_command to be ``universal-argument''
-         (that would be dumb), so don't set Vlast_command,
-         2) we want to leave echoing on so that the prefix will be
-         echoed as part of this key sequence, so don't call
-         cancel_echoing, and
-         3) we want to leave this_command_key_count non-zero, so that
-         read_char will realize that it is re-reading a character, and
-         not echo it a second time.
-
-         If the command didn't actually create a prefix arg,
-         but is merely a frame event that is transparent to prefix args,
-         then the above doesn't apply.  */
-      if (NILP (KVAR (current_kboard, Vprefix_arg))
-          || CONSP (last_command_event))
+      kset_last_command (current_kboard, Vthis_command);
+      kset_real_last_command (current_kboard, Vreal_this_command);
+      if (!CONSP (last_command_event))
+        kset_last_repeatable_command (current_kboard, Vreal_this_command);
+
+      this_command_key_count = 0;
+      this_single_command_key_start = 0;
+
+      if (current_kboard->immediate_echo
+          && !NILP (call0 (Qinternal_echo_keystrokes_prefix)))
         {
-          kset_last_command (current_kboard, Vthis_command);
-          kset_real_last_command (current_kboard, Vreal_this_command);
-          if (!CONSP (last_command_event))
-            kset_last_repeatable_command (current_kboard, Vreal_this_command);
-          cancel_echoing ();
-          this_command_key_count = 0;
-          this_command_key_count_reset = false;
-          this_single_command_key_start = 0;
+          current_kboard->immediate_echo = false;
+          /* Refresh the echo message.  */
+          echo_now ();
         }
+      else
+        cancel_echoing ();
 
       if (!NILP (BVAR (current_buffer, mark_active))
           && !NILP (Vrun_hooks))
@@ -2389,10 +2311,6 @@ read_char (int commandflag, Lisp_Object map,
 
   also_record = Qnil;
 
-#if 0  /* This was commented out as part of fixing echo for C-u left.  */
-  before_command_key_count = this_command_key_count;
-  before_command_echo_length = echo_length ();
-#endif
   c = Qnil;
   previous_echo_area_message = Qnil;
 
@@ -2471,8 +2389,6 @@ read_char (int commandflag, Lisp_Object map,
       goto reread_for_input_method;
     }
 
-  this_command_key_count_reset = false;
-
   if (!NILP (Vexecuting_kbd_macro))
     {
       /* We set this to Qmacro; since that's not a frame, nobody will
@@ -2570,7 +2486,7 @@ read_char (int commandflag, Lisp_Object map,
 
      (3) There's only one place in 20.x where ok_to_echo_at_next_pause
      is set to a non-null value.  This is done in read_char and it is
-     set to echo_area_glyphs after a call to echo_char.  That means
+     set to echo_area_glyphs.  That means
      ok_to_echo_at_next_pause is either null or
      current_kboard->echobuf with the appropriate current_kboard at
      that time.
@@ -2674,7 +2590,8 @@ read_char (int commandflag, Lisp_Object map,
   if (minibuf_level == 0
       && !end_time
       && !current_kboard->immediate_echo
-      && this_command_key_count > 0
+      && (this_command_key_count > 0
+          || !NILP (call0 (Qinternal_echo_keystrokes_prefix)))
       && ! noninteractive
       && echo_keystrokes_p ()
       && (/* No message.  */
@@ -3018,7 +2935,6 @@ read_char (int commandflag, Lisp_Object map,
     {
       Lisp_Object keys;
       ptrdiff_t key_count;
-      bool key_count_reset;
       ptrdiff_t command_key_start;
       ptrdiff_t count = SPECPDL_INDEX ();
 
@@ -3028,20 +2944,8 @@ read_char (int commandflag, Lisp_Object map,
       Lisp_Object saved_echo_string = KVAR (current_kboard, echo_string);
       ptrdiff_t saved_echo_after_prompt = current_kboard->echo_after_prompt;
 
-#if 0
-      if (before_command_restore_flag)
-        {
-          this_command_key_count = before_command_key_count_1;
-          if (this_command_key_count < this_single_command_key_start)
-            this_single_command_key_start = this_command_key_count;
-          echo_truncate (before_command_echo_length_1);
-          before_command_restore_flag = 0;
-        }
-#endif
-
       /* Save the this_command_keys status.  */
       key_count = this_command_key_count;
-      key_count_reset = this_command_key_count_reset;
       command_key_start = this_single_command_key_start;
 
       if (key_count > 0)
@@ -3051,7 +2955,6 @@ read_char (int commandflag, Lisp_Object map,
 
       /* Clear out this_command_keys.  */
       this_command_key_count = 0;
-      this_command_key_count_reset = false;
       this_single_command_key_start = 0;
 
       /* Now wipe the echo area.  */
@@ -3075,7 +2978,6 @@ read_char (int commandflag, Lisp_Object map,
       /* Restore the saved echoing state
          and this_command_keys state.  */
       this_command_key_count = key_count;
-      this_command_key_count_reset = key_count_reset;
       this_single_command_key_start = command_key_start;
       if (key_count > 0)
         this_command_keys = keys;
@@ -3141,28 +3043,23 @@ read_char (int commandflag, Lisp_Object map,
       goto retry;
     }
 
-  if ((! reread || this_command_key_count == 0
-       || this_command_key_count_reset)
+  if ((! reread || this_command_key_count == 0)
       && !end_time)
     {
 
       /* Don't echo mouse motion events.  */
-      if (echo_keystrokes_p ()
-          && ! (EVENT_HAS_PARAMETERS (c)
-                && EQ (EVENT_HEAD_KIND (EVENT_HEAD (c)), Qmouse_movement)))
-        {
-          echo_char (c);
-          if (! NILP (also_record))
-            echo_char (also_record);
-          /* Once we reread a character, echoing can happen
-             the next time we pause to read a new one.  */
-          ok_to_echo_at_next_pause = current_kboard;
-        }
+      if (! (EVENT_HAS_PARAMETERS (c)
+             && EQ (EVENT_HEAD_KIND (EVENT_HEAD (c)), Qmouse_movement)))
+        /* Once we reread a character, echoing can happen
+           the next time we pause to read a new one.  */
+        ok_to_echo_at_next_pause = current_kboard;
 
       /* Record this character as part of the current key.  */
       add_command_key (c);
       if (! NILP (also_record))
         add_command_key (also_record);
+
+      echo_update ();
     }
 
   last_input_event = c;
@@ -3218,23 +3115,13 @@ record_menu_key (Lisp_Object c)
 
   record_char (c);
 
-#if 0
-  before_command_key_count = this_command_key_count;
-  before_command_echo_length = echo_length ();
-#endif
-
-  /* Don't echo mouse motion events.  */
-  if (echo_keystrokes_p ())
-    {
-      echo_char (c);
-
-      /* Once we reread a character, echoing can happen
-         the next time we pause to read a new one.  */
-      ok_to_echo_at_next_pause = 0;
-    }
+  /* Once we reread a character, echoing can happen
+     the next time we pause to read a new one.  */
+  ok_to_echo_at_next_pause = NULL;
 
   /* Record this character as part of the current key.  */
   add_command_key (c);
+  echo_update ();
 
   /* Re-reading in the middle of a command.  */
   last_input_event = c;
@@ -9120,11 +9007,12 @@ read_key_sequence (Lisp_Object *keybuf, int bufsize, Lisp_Object prompt,
         {
           key = keybuf[t];
           add_command_key (key);
-          if (echo_keystrokes_p ()
-              && current_kboard->immediate_echo)
+          if (current_kboard->immediate_echo)
             {
-              echo_add_key (key);
-              echo_dash ();
+              /* Set immediate_echo to false so as to force echo_now to
+                 redisplay (it will set immediate_echo right back to true).  */
+              current_kboard->immediate_echo = false;
+              echo_now ();
             }
         }
 
@@ -9788,11 +9676,8 @@ read_key_sequence (Lisp_Object *keybuf, int bufsize, Lisp_Object prompt,
 
      Better ideas?  */
   for (; t < mock_input; t++)
-    {
-      if (echo_keystrokes_p ())
-        echo_char (keybuf[t]);
-      add_command_key (keybuf[t]);
-    }
+    add_command_key (keybuf[t]);
+  echo_update ();
 
   return t;
 }
@@ -9819,7 +9704,6 @@ read_key_sequence_vs (Lisp_Object prompt, Lisp_Object continue_echo,
   if (NILP (continue_echo))
     {
       this_command_key_count = 0;
-      this_command_key_count_reset = false;
       this_single_command_key_start = 0;
     }
 
@@ -10076,33 +9960,6 @@ The value is always a vector.  */)
   return Fvector (raw_keybuf_count, XVECTOR (raw_keybuf)->contents);
 }
 
-DEFUN ("reset-this-command-lengths", Freset_this_command_lengths,
-       Sreset_this_command_lengths, 0, 0, 0,
-       doc: /* Make the unread events replace the last command and echo.
-Used in `universal-argument-other-key'.
-
-`universal-argument-other-key' rereads the event just typed.
-It then gets translated through `function-key-map'.
-The translated event has to replace the real events,
-both in the value of (this-command-keys) and in echoing.
-To achieve this, `universal-argument-other-key' calls
-`reset-this-command-lengths', which discards the record of reading
-these events the first time.  */)
-  (void)
-{
-  this_command_key_count = before_command_key_count;
-  if (this_command_key_count < this_single_command_key_start)
-    this_single_command_key_start = this_command_key_count;
-
-  echo_truncate (before_command_echo_length);
-
-  /* Cause whatever we put into unread-command-events
-     to echo as if it were being freshly read from the keyboard.  */
-  this_command_key_count_reset = true;
-
-  return Qnil;
-}
-
 DEFUN ("clear-this-command-keys", Fclear_this_command_keys,
        Sclear_this_command_keys, 0, 1, 0,
        doc: /* Clear out the vector that `this-command-keys' returns.
@@ -10113,7 +9970,6 @@ KEEP-RECORD is non-nil.  */)
   int i;
 
   this_command_key_count = 0;
-  this_command_key_count_reset = false;
 
   if (NILP (keep_record))
     {
@@ -11210,6 +11066,7 @@ syms_of_keyboard (void)
   staticpro (&raw_keybuf);
 
   DEFSYM (Qcommand_execute, "command-execute");
+  DEFSYM (Qinternal_echo_keystrokes_prefix, "internal-echo-keystrokes-prefix");
 
   accent_key_syms = Qnil;
   staticpro (&accent_key_syms);
@@ -11253,7 +11110,6 @@ syms_of_keyboard (void)
   defsubr (&Sthis_command_keys_vector);
   defsubr (&Sthis_single_command_keys);
   defsubr (&Sthis_single_command_raw_keys);
-  defsubr (&Sreset_this_command_lengths);
   defsubr (&Sclear_this_command_keys);
   defsubr (&Ssuspend_emacs);
   defsubr (&Sabort_recursive_edit);