New customization variable `completion-eager-display'

The customization option can be set to t or nil, to respectively
always or never show the *Completions* buffer eagerly at the
beginning of a completion session.  Furthermore the option can
be set to the value auto.  In this case the *Completions* buffer
will only be shown if requested by the completion table.
Completion tables can use the `eager-display' completion
metadata to do so.  (Bug#74616, Bug#74617)

* lisp/minibuffer.el (completion-eager-display): New
customization variable.
(completion-metadata): Update docstring, document the
new `eager-display' completion metadata.
(completion-extra-properties): Update docstring, document the
new `:eager-display' completion metadata.
(completion-category-overrides): Add `eager-display' to the
custom type specification.
(completing-read-default): Handle the `completion-eager-display'
customization variable and the `eager-display' completion
metadata.
(completion-table-with-metadata): New function to create
a completion table with metadata.
(minibuffer-complete-defaults, minibuffer-complete-history):
Use it.
* lisp/ffap.el (ffap-menu-ask): Add `ffap-menu' completion
category and `eager-display' completion metadata.  Use
`completion-table-with-metadata'.
* lisp/imenu.el (imenu-eager-completion-buffer): Correct
docstring, which had been inverted.
(imenu--completion-buffer): Add `eager-display' completion
metadata.  Use `completion-table-with-metadata'.
* lisp/tmm.el (tmm-prompt): Add `tmm' completion category and
`eager-display' completion metadata.  Use
`completion-table-with-metadata'.  Add keymap setup.
(tmm-add-prompt): Remove keymap setup.
(tmm-goto-completions): Call `tmm-add-prompt' to ensure that a
*Completions* buffer is shown.
(tmm--completion-table): Remove unused internal function.
* etc/NEWS: Announce the change.

etc/NEWS

@@ -102,6 +102,14 @@ files and features.
 
 ** Minibuffer and Completions
 
++++
+*** New user option 'completion-eager-display'.
+This option configures whether completion commands should display
+the *Completions* buffer immediately.  When the variable is set to t all
+completion commands show *Completions* immediately, respectively nil
+disables the eager display for all commands.  The default setting auto
+enables eager completion only if requested by the command.
+
 +++
 *** New user option 'completion-pcm-leading-wildcard'.
 This option configures how the partial-completion style does completion.

lisp/ffap.el

@@ -1739,14 +1739,15 @@ Function CONT is applied to the entry chosen by the user."
 				    alist))))))
      ;; minibuffer with completion buffer:
      (t
-      (let ((minibuffer-setup-hook 'minibuffer-completion-help))
-	;; Bug: prompting may assume unique strings, no "".
-	(setq choice
-	      (completing-read
-	       (format-prompt title (car (car alist)))
-	       alist nil t
-	       ;; (cons (car (car alist)) 0)
-	       nil)))
+      ;; Bug: prompting may assume unique strings, no "".
+      (setq choice
+	    (completing-read
+	     (format-prompt title (car (car alist)))
+             (completion-table-with-metadata
+	      alist '((category . ffap-menu) (eager-display . t)))
+             nil t
+	     ;; (cons (car (car alist)) 0)
+	     nil))
       (sit-for 0)			; redraw original screen
       ;; Convert string to its entry, or else the default:
       (setq choice (or (assoc choice alist) (car alist)))))

lisp/imenu.el

@@ -100,7 +100,7 @@ If `on-mouse' use a popup menu when `imenu' was invoked with the mouse."
                  (other :tag "Always" t)))
 
 (defcustom imenu-eager-completion-buffer t
-  "If non-nil, eagerly pop up the completion buffer."
+  "If nil, eagerly pop up the completion buffer."
   :type 'boolean
   :version "22.1")
 
@@ -767,27 +767,25 @@ Return one of the entries in index-alist or nil."
                          (imenu--in-alist name prepared-index-alist)
                          ;; Default to `name' if it's in the alist.
                          name))))
-    ;; Display the completion buffer.
     (minibuffer-with-setup-hook
-        (lambda ()
-          (setq-local minibuffer-allow-text-properties t)
-          (setq-local completion-extra-properties
-                      `( :category imenu
-                         ,@(when (eq imenu-flatten 'annotation)
-                             `(:annotation-function
-                               ,(lambda (s) (get-text-property
-                                             0 'imenu-section s))))
-                         ,@(when (eq imenu-flatten 'group)
-                             `(:group-function
-                               ,(lambda (s transform)
-                                  (if transform s
-                                    (get-text-property
-                                     0 'imenu-section s)))))))
-          (unless imenu-eager-completion-buffer
-            (minibuffer-completion-help)))
-      (setq name (completing-read prompt
-				  prepared-index-alist
-				  nil t nil 'imenu--history-list name)))
+        (lambda () (setq-local minibuffer-allow-text-properties t))
+      (setq name (completing-read
+                  prompt
+                  (completion-table-with-metadata
+		   prepared-index-alist
+                   `((category . imenu)
+                     (eager-display . ,(not imenu-eager-completion-buffer))
+                     ,@(when (eq imenu-flatten 'annotation)
+                         `((annotation-function
+                            . ,(lambda (s) (get-text-property
+                                            0 'imenu-section s)))))
+                     ,@(when (eq imenu-flatten 'group)
+                         `((group-function
+                            . ,(lambda (s transform)
+                                 (if transform s
+                                   (get-text-property
+                                    0 'imenu-section s))))))))
+		  nil t nil 'imenu--history-list name)))
 
     (when (stringp name)
       (or (get-text-property 0 'imenu-choice name)

lisp/minibuffer.el

@@ -138,6 +138,9 @@ This metadata is an alist.  Currently understood keys are:
    of completions.  Can operate destructively.
 - `cycle-sort-function': function to sort entries when cycling.
    Works like `display-sort-function'.
+- `eager-display': non-nil to request eager display of the
+  completion candidates.  Can also be a function which is invoked
+  after minibuffer setup.
 The metadata of a completion table should be constant between two boundaries."
   (let ((metadata (if (functionp table)
                       (funcall table string pred 'metadata))))
@@ -277,6 +280,15 @@ case sensitive instead."
     (let ((completion-ignore-case (not dont-fold)))
       (complete-with-action action table string pred))))
 
+(defun completion-table-with-metadata (table metadata)
+  "Return new completion TABLE with METADATA.
+METADATA should be an alist of completion metadata.  See
+`completion-metadata' for a list of supported metadata."
+  (lambda (string pred action)
+    (if (eq action 'metadata)
+        `(metadata . ,metadata)
+      (complete-with-action action table string pred))))
+
 (defun completion-table-subvert (table s1 s2)
   "Return a completion table from TABLE with S1 replaced by S2.
 The result is a completion table which completes strings of the
@@ -1031,6 +1043,25 @@ If the current buffer is not a minibuffer, erase its entire contents."
 (defvar completion-show-inline-help t
   "If non-nil, print helpful inline messages during completion.")
 
+(defcustom completion-eager-display 'auto
+  "Whether completion commands should display *Completions* buffer eagerly.
+
+If the variable is set to t, completion commands show the *Completions*
+buffer always immediately.  Setting the variable to nil disables the
+eager *Completions* display for all commands.
+
+For the value `auto', completion commands show the *Completions* buffer
+immediately only if requested by the completion command.  Completion
+tables can request eager display via the `eager-display' metadata.
+
+See also the variables `completion-category-overrides' and
+`completion-extra-properties' for the `eager-display' completion
+metadata."
+  :type '(choice (const :tag "Never show *Completions* eagerly" nil)
+                 (const :tag "Always show *Completions* eagerly" t)
+                 (const :tag "If requested by the completion command" auto))
+  :version "31.1")
+
 (defcustom completion-auto-help t
   "Non-nil means automatically provide help for invalid completion input.
 If the value is t, the *Completions* buffer is displayed whenever completion
@@ -1247,7 +1278,11 @@ overrides the default specified in `completion-category-defaults'."
            (cons :tag "Completion Affixation"
                  (const :tag "Select one value from the menu."
                         affixation-function)
-                 (choice (function :tag "Custom function"))))))
+                 (choice (function :tag "Custom function")))
+           (cons :tag "Eager display"
+                 (const :tag "Select one value from the menu."
+                        eager-display)
+                 boolean))))
 
 (defun completion--category-override (category tag)
   (or (assq tag (cdr (assq category completion-category-overrides)))
@@ -2499,6 +2534,8 @@ These include:
 
 `:cycle-sort-function': Function to sort entries when cycling.
 
+`:eager-display': Show the *Completions* buffer eagerly.
+
 See more information about these functions above
 in `completion-metadata'.
 
@@ -4815,7 +4852,17 @@ See `completing-read' for the meaning of the arguments."
                 (setq-local minibuffer--require-match require-match)
                 (setq-local minibuffer--original-buffer buffer)
                 ;; Copy the value from original buffer to the minibuffer.
-                (setq-local completion-ignore-case c-i-c))
+                (setq-local completion-ignore-case c-i-c)
+                ;; Show the completion help eagerly if
+                ;; `completion-eager-display' is t or if eager display
+                ;; has been requested by the completion table.
+                (when completion-eager-display
+                  (let* ((md (completion-metadata (or initial-input "")
+                                                  collection predicate))
+                         (fun (completion-metadata-get md 'eager-display)))
+                    (when (or fun (eq completion-eager-display t))
+                      (funcall (if (functionp fun)
+                                   fun #'minibuffer-completion-help))))))
             (read-from-minibuffer prompt initial-input keymap
                                   nil hist def inherit-input-method))))
     (when (and (equal result "") def)
@@ -5005,11 +5052,9 @@ instead of the default completion table."
            (lambda () (get-buffer-window "*Completions*" 0))))
       (completion-in-region
        (minibuffer--completion-prompt-end) (point-max)
-       (lambda (string pred action)
-         (if (eq action 'metadata)
-             '(metadata (display-sort-function . identity)
-                        (cycle-sort-function . identity))
-           (complete-with-action action completions string pred)))))))
+       (completion-table-with-metadata
+        completions '((display-sort-function . identity)
+                      (cycle-sort-function . identity)))))))
 
 (defun minibuffer-complete-defaults ()
   "Complete minibuffer defaults as far as possible.
@@ -5025,11 +5070,9 @@ instead of the completion table."
          (lambda () (get-buffer-window "*Completions*" 0))))
     (completion-in-region
      (minibuffer--completion-prompt-end) (point-max)
-     (lambda (string pred action)
-       (if (eq action 'metadata)
-           '(metadata (display-sort-function . identity)
-                      (cycle-sort-function . identity))
-         (complete-with-action action completions string pred))))))
+     (completion-table-with-metadata
+      completions '((display-sort-function . identity)
+                    (cycle-sort-function . identity))))))
 
 (define-key minibuffer-local-map [?\C-x up] 'minibuffer-complete-history)
 (define-key minibuffer-local-map [?\C-x down] 'minibuffer-complete-defaults)

lisp/tmm.el

@@ -119,12 +119,6 @@ is displayed with the `highlight' face to help identify it.  The
   '((t :inherit shadow))
   "Face used for inactive menu items.")
 
-(defun tmm--completion-table (items)
-  (lambda (string pred action)
-    (if (eq action 'metadata)
-	'(metadata (display-sort-function . identity))
-      (complete-with-action action items string pred))))
-
 (defvar tmm--history nil)
 
 ;;;###autoload
@@ -222,7 +216,9 @@ is used to go back through those sub-menus."
              (setq out
                    (if default-item
                        (car (nth index-of-default tmm-km-list))
-                     (minibuffer-with-setup-hook #'tmm-add-prompt
+                     (minibuffer-with-setup-hook
+                         (lambda ()
+                           (setq tmm-old-mb-map (tmm-define-keys t)))
                        ;; tmm-km-list is reversed, because history
                        ;; needs it in LIFO order.  But default list
                        ;; needs it in non-reverse order, so that the
@@ -233,7 +229,12 @@ is used to go back through those sub-menus."
                        (completing-read-default
                         (concat gl-str
                                 " (up/down to change, PgUp to menu): ")
-                        (tmm--completion-table tmm-km-list) nil t nil
+                        (completion-table-with-metadata
+                         tmm-km-list '((category . tmm)
+                                       (eager-display . tmm-add-prompt)
+                                       (display-sort-function . identity)
+                                       (cycle-sort-function . identity)))
+                        nil t nil
                         'tmm--history (reverse tmm--history)))))))
       (if (and (stringp out) (string= "^" out))
           ;; A fake choice to please the destructuring later.
@@ -402,7 +403,6 @@ Stores a list of all the shortcuts in the free variable `tmm-short-cuts'."
 (defun tmm-add-prompt ()
   (unless tmm-c-prompt
     (error "No active menu entries"))
-  (setq tmm-old-mb-map (tmm-define-keys t))
   (or tmm-completion-prompt
       (add-hook 'completion-setup-hook
                 #'tmm-completion-delete-prompt 'append))
@@ -458,6 +458,7 @@ Stores a list of all the shortcuts in the free variable `tmm-short-cuts'."
 (defun tmm-goto-completions ()
   "Jump to the completions buffer."
   (interactive)
+  (tmm-add-prompt)
   (setq tmm-c-prompt (buffer-substring (minibuffer-prompt-end) (point-max)))
   ;; Clear minibuffer old contents before using *Completions* buffer for
   ;; selection.