Fix display of minibuffer prompt in ido.el

* lisp/minibuffer.el (minibuffer--message-overlay-pos): New
function.
(set-minibuffer-message): Use it to determine where to show the
overlay with the temporary message.
* lisp/ido.el (ido-exhibit): Revert "Render Ido suggestions using
an overlay"; this restores the original code which inserted the
match-status information into the minibuffer, instead of
displaying it in an overlay with an after-string.  Put the special
'minibuffer-message' text property at the beginning of the
inserted text.  (Bug#39379)

* etc/NEWS:
* doc/lispref/display.texi (Displaying Messages):
* doc/lispref/text.texi (Special Properties): Document the
'minibuffer-message' text property and its effect.

doc/lispref/display.texi

@@ -319,7 +319,10 @@ the echo area.  See also @code{clear-message-function} that can be
 used to clear the message displayed by this function.
 
 The default value is the function that displays the message at the end
-of the minibuffer when the minibuffer is active.
+of the minibuffer when the minibuffer is active.  However, if the text
+shown in the active minibuffer has the @code{minibuffer-message} text
+property (@pxref{Special Properties}) on some character, the message
+will be displayed before the first character having that property.
 @end defvar
 
 @defvar clear-message-function
@@ -332,8 +335,8 @@ after displaying an echo-area message.  The function is expected to
 clear the message displayed by its counterpart function specified by
 @code{set-message-function}.
 
-The default value is the function that clears the message displayed at
-the end of the minibuffer when the minibuffer is active.
+The default value is the function that clears the message displayed in
+an active minibuffer.
 @end defvar
 
 @defvar inhibit-message

doc/lispref/text.texi

@@ -3741,6 +3741,16 @@ single glyph composed from components.  But the value of the property
 itself is completely internal to Emacs and should not be manipulated
 directly by, for instance, @code{put-text-property}.
 
+@item minibuffer-message
+@kindex minibuffer-message @r{(text property)}
+This text property tells where to display temporary messages in an
+active minibuffer.  Specifically, the first character of the
+minibuffer text which has this property will have the temporary
+message displayed before it.  The default is to display temporary
+messages at the end of the minibuffer text.  This text property is
+used by the function that is the default value of
+@code{set-message-function} (@pxref{Displaying Messages}).
+
 @end table
 
 @defvar inhibit-point-motion-hooks

etc/NEWS

@@ -3590,6 +3590,12 @@ in other packages are now obsolete aliases of 'xor'.
 Setting this on the first character of a help string disables
 conversions via 'substitute-command-keys'.
 
++++
+** New text property 'minibuffer-message'.
+Setting this on a character of the minibuffer text will display the
+temporary echo messages before that character, when messages need to
+be displayed while minibuffer is active.
+
 +++
 ** 'undo' can be made to ignore the active region for a command
 by setting 'undo-inhibit-region' symbol property of that command to

lisp/ido.el

@@ -4492,8 +4492,6 @@ For details of keybindings, see `ido-find-file'."
 	(ido-tidy))
       (throw 'ido contents))))
 
-(defvar ido--overlay nil)
-
 (defun ido-exhibit ()
   "Post command hook for Ido."
   ;; Find matching files and display a list in the minibuffer.
@@ -4728,16 +4726,13 @@ For details of keybindings, see `ido-find-file'."
 	(let ((inf (ido-completions contents)))
 	  (setq ido-show-confirm-message nil)
 	  (ido-trace "inf" inf)
-          (when ido--overlay
-            (delete-overlay ido--overlay))
-          (let ((o (make-overlay (point-max) (point-max) nil t t)))
-            (when (> (length inf) 0)
-              ;; For hacks that redefine ido-completions function (bug#39379)
-              (when (eq (aref inf 0) ?\n)
-                (setq inf (concat " " inf)))
-              (put-text-property 0 1 'cursor t inf))
-            (overlay-put o 'after-string inf)
-            (setq ido--overlay o)))
+          (let ((pos (point)))
+            (insert inf)
+            (if (< pos (point-max))
+                ;; Tell set-minibuffer-message where to display the
+                ;; overlay with temporary messages.
+                (put-text-property pos (1+ pos) 'minibuffer-message t)))
+          )
 	))))
 
 (defun ido-completions (name)

lisp/minibuffer.el

@@ -763,8 +763,21 @@ and `clear-minibuffer-message' called automatically via
 (defvar minibuffer-message-timer nil)
 (defvar minibuffer-message-overlay nil)
 
+(defun minibuffer--message-overlay-pos ()
+  "Return position where `set-minibuffer-message' shall put message overlay."
+  ;; Starting from point, look for non-nil 'minibuffer-message'
+  ;; property, and return its position.  If none found, return the EOB
+  ;; position.
+  (let* ((pt (point))
+         (propval (get-text-property pt 'minibuffer-message)))
+    (if propval pt
+      (next-single-property-change pt 'minibuffer-message nil (point-max)))))
+
 (defun set-minibuffer-message (message)
   "Temporarily display MESSAGE at the end of the minibuffer.
+If some part of the minibuffer text has the `minibuffer-message' property,
+the message will be displayed before the first such character, instead of
+at the end of the minibuffer.
 The text is displayed for `minibuffer-message-clear-timeout' seconds
 \(if the value is a number), or until the next input event arrives,
 whichever comes first.
@@ -784,8 +797,9 @@ via `set-message-function'."
 
       (clear-minibuffer-message)
 
-      (setq minibuffer-message-overlay
-            (make-overlay (point-max) (point-max) nil t t))
+      (let ((ovpos (minibuffer--message-overlay-pos)))
+        (setq minibuffer-message-overlay
+              (make-overlay ovpos ovpos nil t t)))
       (unless (zerop (length message))
         ;; The current C cursor code doesn't know to use the overlay's
         ;; marker's stickiness to figure out whether to place the cursor