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.

2 files changed, 16 insertions, 3 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index e4980fe4c36..2b25d6023cd 100644
--- a/doc/lispref/display.texi
+++ b/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
diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index 527057eed59..f027cdf8ede 100644
--- a/doc/lispref/text.texi
+++ b/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