Re-add the notion of echo_prompt lost in the translation

* src/keyboard.h (struct kboard): Replace echo_after_prompt with new
echo_prompt which contains the actual string.  Update all uses.

* src/keyboard.c (kset_echo_prompt): New function.
(echo_update): Add echo_prompt at the very beginning.
(read_char): Remove workaround for bug#19875, not needed any more.
(read_key_sequence): Set echo_prompt rather than echo_string (bug#21403).
(mark_kboards): Mark echo_prompt.

2 files changed, 22 insertions, 20 deletions

diff --git a/src/keyboard.c b/src/keyboard.c
index a8b1e9828bf..eefc033530a 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -380,6 +380,11 @@ kset_echo_string (struct kboard *kb, Lisp_Object val)
   kb->echo_string_ = val;
 }
 static void
+kset_echo_prompt (struct kboard *kb, Lisp_Object val)
+{
+  kb->echo_prompt_ = val;
+}
+static void
 kset_kbd_queue (struct kboard *kb, Lisp_Object val)
 {
   kb->kbd_queue_ = val;
@@ -501,8 +506,9 @@ echo_dash (void)
     return;
 
   /* Do nothing if we just printed a prompt.  */
-  if (current_kboard->echo_after_prompt
-      == SCHARS (KVAR (current_kboard, echo_string)))
+  if (STRINGP (KVAR (current_kboard, echo_prompt))
+      && (SCHARS (KVAR (current_kboard, echo_prompt))
+          == SCHARS (KVAR (current_kboard, echo_string))))
     return;
 
   /* Do nothing if we have already put a dash at the end.  */
@@ -534,8 +540,12 @@ echo_update (void)
   if (current_kboard->immediate_echo)
     {
       ptrdiff_t i;
+      Lisp_Object prompt = KVAR (current_kboard, echo_prompt);
+      Lisp_Object prefix = call0 (Qinternal_echo_keystrokes_prefix);
       kset_echo_string (current_kboard,
-                        call0 (Qinternal_echo_keystrokes_prefix));
+                        NILP (prompt) ? prefix
+                        : NILP (prefix) ? prompt
+                        : concat2 (prompt, prefix));
 
       for (i = 0; i < this_command_key_count; i++)
         {
@@ -584,7 +594,7 @@ void
 cancel_echoing (void)
 {
   current_kboard->immediate_echo = false;
-  current_kboard->echo_after_prompt = -1;
+  kset_echo_prompt (current_kboard, Qnil);
   kset_echo_string (current_kboard, Qnil);
   ok_to_echo_at_next_pause = NULL;
   echo_kboard = NULL;
@@ -2942,7 +2952,7 @@ read_char (int commandflag, Lisp_Object map,
       bool saved_immediate_echo = current_kboard->immediate_echo;
       struct kboard *saved_ok_to_echo = ok_to_echo_at_next_pause;
       Lisp_Object saved_echo_string = KVAR (current_kboard, echo_string);
-      ptrdiff_t saved_echo_after_prompt = current_kboard->echo_after_prompt;
+      Lisp_Object saved_echo_prompt = KVAR (current_kboard, echo_prompt);
 
       /* Save the this_command_keys status.  */
       key_count = this_command_key_count;
@@ -2984,15 +2994,8 @@ read_char (int commandflag, Lisp_Object map,
 
       cancel_echoing ();
       ok_to_echo_at_next_pause = saved_ok_to_echo;
-      /* Do not restore the echo area string when the user is
-         introducing a prefix argument. Otherwise we end with
-         repetitions of the partially introduced prefix
-         argument. (bug#19875) */
-      if (NILP (intern ("prefix-arg")))
-        {
-          kset_echo_string (current_kboard, saved_echo_string);
-        }
-      current_kboard->echo_after_prompt = saved_echo_after_prompt;
+      kset_echo_string (current_kboard, saved_echo_string);
+      kset_echo_prompt (current_kboard, saved_echo_prompt);
       if (saved_immediate_echo)
         echo_now ();
 
@@ -8870,8 +8873,7 @@ read_key_sequence (Lisp_Object *keybuf, int bufsize, Lisp_Object prompt,
           /* Install the string PROMPT as the beginning of the string
              of echoing, so that it serves as a prompt for the next
              character.  */
-          kset_echo_string (current_kboard, prompt);
-          current_kboard->echo_after_prompt = SCHARS (prompt);
+          kset_echo_prompt (current_kboard, prompt);
           echo_now ();
         }
       else if (cursor_in_echo_area
@@ -10695,7 +10697,7 @@ init_kboard (KBOARD *kb, Lisp_Object type)
   kb->kbd_queue_has_data = false;
   kb->immediate_echo = false;
   kset_echo_string (kb, Qnil);
-  kb->echo_after_prompt = -1;
+  kset_echo_prompt (kb, Qnil);
   kb->kbd_macro_buffer = 0;
   kb->kbd_macro_bufsize = 0;
   kset_defining_kbd_macro (kb, Qnil);
@@ -11758,6 +11760,7 @@ mark_kboards (void)
       mark_object (KVAR (kb, Vlocal_function_key_map));
       mark_object (KVAR (kb, Vdefault_minibuffer_frame));
       mark_object (KVAR (kb, echo_string));
+      mark_object (KVAR (kb, echo_prompt));
     }
   {
     union buffered_input_event *event;
diff --git a/src/keyboard.h b/src/keyboard.h
index 3e4c475d285..0402e0a582c 100644
--- a/src/keyboard.h
+++ b/src/keyboard.h
@@ -175,9 +175,8 @@ struct kboard
     /* True means echo each character as typed.  */
     bool_bf immediate_echo : 1;
 
-    /* If we have echoed a prompt string specified by the user,
-       this is its length in characters.  Otherwise this is -1.  */
-    ptrdiff_t echo_after_prompt;
+    /* If we have a prompt string specified by the user, this is it.  */
+    Lisp_Object echo_prompt_;
   };
 
 INLINE void