bug#70846: Imenu flatten

[Juri Linkov]

This patch revives the feature existed in the initial version of imenu.el,
but that was unfortunately removed later.

When comparing it with the version in breadcrumb-jump that was discussed in
https://lists.gnu.org/archive/html/emacs-devel/2023-12/msg00278.html
the advantage of this version is that it doesn't use 'cl-loop':

diff --git a/lisp/imenu.el b/lisp/imenu.el
index f628936cedc..ccc73fe42d2 100644
--- a/lisp/imenu.el
+++ b/lisp/imenu.el
@@ -66,6 +66,13 @@ imenu
   :group 'convenience
   :link '(custom-manual "(elisp)Imenu"))
 
+(defcustom imenu-flatten nil
+  "If non-nil, popup the completion buffer with a flattened menu.
+The string from `imenu-level-separator' is used to separate names of
+nested levels while flattening nested indexes with name concatenation."
+  :type 'boolean
+  :version "30.1")
+
 (defcustom imenu-use-markers t
   "Non-nil means use markers instead of integers for Imenu buffer positions.
 
@@ -142,8 +149,7 @@ imenu-space-replacement
 
 (defcustom imenu-level-separator ":"
   "The separator between index names of different levels.
-Used for making mouse-menu titles and for flattening nested indexes
-with name concatenation."
+Used for flattening nested indexes with name concatenation."
   :type 'string)
 
 (defcustom imenu-generic-skip-comments-and-strings t
@@ -763,6 +770,26 @@ imenu--mouse-menu
                                            menu)))))
     (popup-menu map event)))
 
+(defun imenu--flatten-index-alist (index-alist &optional concat-names prefix)
+  ;; Takes a nested INDEX-ALIST and returns a flat index alist.
+  ;; If optional CONCAT-NAMES is non-nil, then a nested index has its
+  ;; name and a space concatenated to the names of the children.
+  ;; Third argument PREFIX is for internal use only.
+  (mapcan
+   (lambda (item)
+     (let* ((name (car item))
+           (pos (cdr item))
+           (new-prefix (and concat-names
+                            (if prefix
+                                (concat prefix imenu-level-separator name)
+                              name))))
+       (cond
+       ((or (markerp pos) (numberp pos))
+        (list (cons new-prefix pos)))
+       (t
+        (imenu--flatten-index-alist pos concat-names new-prefix)))))
+   index-alist))
+
 (defun imenu-choose-buffer-index (&optional prompt alist)
   "Let the user select from a buffer index and return the chosen index.
 
@@ -792,6 +819,8 @@ imenu-choose-buffer-index
     ;; Create a list for this buffer only when needed.
     (while (eq result t)
       (setq index-alist (if alist alist (imenu--make-index-alist)))
+      (when imenu-flatten
+        (setq index-alist (imenu--flatten-index-alist index-alist t)))
       (setq result
            (if (and imenu-use-popup-menu
                     (or (eq imenu-use-popup-menu t) mouse-triggered))