Allow completion styles to adjust completion metadata

This commit re-does the now-reverted commit with the same title.  That
version relied on generic functions, which cannot be used yet in files
such as lisp/minibuffer.el.  This version uses a symbol property
completion--adjust-metadata instead.

The new facility allows completion styles to have a say in metadata
properties such as cycle-sort-function and display-sort-function.
This is especially useful for completion styles such as 'flex', which
generally produce many matches, including some potentially "obscure"
ones.  The default sorting strategy would often bubble the latter to
the top of the list.

The sorting function for 'flex' considers pre-computed matching scores
and is thus much better than the default for this particular style.

Additionally, it overrides the completion table's cycle-sort-function
or display-sort-function properties if they exist, although it still
uses them to pre-sort the result, so that they are still relevant for
resolving ties.

* lisp/minibuffer.el (completion--nth-completion)
(completion--flex-adjust-metadata): New helper.
(flex): Put completion--adjust-metadata property.

lisp/minibuffer.el

@@ -922,31 +922,36 @@ This overrides the defaults specified in `completion-category-defaults'."
   ;; The quote/unquote function needs to come from the completion table (rather
   ;; than from completion-extra-properties) because it may apply only to some
   ;; part of the string (e.g. substitute-in-file-name).
-  (let ((requote
-         (when (and
-                (completion-metadata-get metadata 'completion--unquote-requote)
-                ;; Sometimes a table's metadata is used on another
-                ;; table (typically that other table is just a list taken
-                ;; from the output of `all-completions' or something equivalent,
-                ;; for progressive refinement).  See bug#28898 and bug#16274.
-                ;; FIXME: Rather than do nothing, we should somehow call
-                ;; the original table, in that case!
-                (functionp table))
-           (let ((new (funcall table string point 'completion--unquote)))
-             (setq string (pop new))
-             (setq table (pop new))
-             (setq point (pop new))
-	     (cl-assert (<= point (length string)))
-             (pop new))))
-        (result
-         (completion--some (lambda (style)
-                             (funcall (nth n (assq style
-                                                   completion-styles-alist))
-                                      string table pred point))
-                           (completion--styles metadata))))
+  (let* ((requote
+          (when (and
+                 (completion-metadata-get metadata 'completion--unquote-requote)
+                 ;; Sometimes a table's metadata is used on another
+                 ;; table (typically that other table is just a list taken
+                 ;; from the output of `all-completions' or something equivalent,
+                 ;; for progressive refinement).  See bug#28898 and bug#16274.
+                 ;; FIXME: Rather than do nothing, we should somehow call
+                 ;; the original table, in that case!
+                 (functionp table))
+            (let ((new (funcall table string point 'completion--unquote)))
+              (setq string (pop new))
+              (setq table (pop new))
+              (setq point (pop new))
+              (cl-assert (<= point (length string)))
+              (pop new))))
+         (result-and-style
+          (completion--some
+           (lambda (style)
+             (let ((probe (funcall (nth n (assq style
+                                                completion-styles-alist))
+                                   string table pred point)))
+               (and probe (cons probe style))))
+           (completion--styles metadata)))
+         (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
+    (when adjust-fn
+      (setcdr metadata (cdr (funcall adjust-fn metadata))))
     (if requote
-        (funcall requote result n)
-      result)))
+        (funcall requote (car result-and-style) n)
+      (car result-and-style))))
 
 (defun completion-try-completion (string table pred point &optional metadata)
   "Try to complete STRING using completion table TABLE.
@@ -3462,6 +3467,29 @@ that is non-nil."
 ;;; "flex" completion, also known as flx/fuzzy/scatter completion
 ;; Completes "foo" to "frodo" and "farfromsober"
 
+(put 'flex 'completion--adjust-metadata 'completion--flex-adjust-metadata)
+
+(defun completion--flex-adjust-metadata (metadata)
+  (cl-flet ((compose-flex-sort-fn
+             (existing-sort-fn) ; wish `cl-flet' had proper indentation...
+             (lambda (completions)
+               (let ((res
+                      (if existing-sort-fn
+                          (funcall existing-sort-fn completions)
+                        completions)))
+                 (sort
+                  res
+                  (lambda (c1 c2)
+                    (or (equal c1 minibuffer-default)
+                        (> (get-text-property 0 'completion-score c1)
+                           (get-text-property 0 'completion-score c2)))))))))
+    (let ((alist (cdr metadata)))
+      (setf (alist-get 'display-sort-function alist)
+            (compose-flex-sort-fn (alist-get 'display-sort-function alist)))
+      (setf (alist-get 'cycle-sort-function alist)
+            (compose-flex-sort-fn (alist-get 'cycle-sort-function alist)))
+      `(metadata . ,alist))))
+
 (defun completion-flex--make-flex-pattern (pattern)
   "Convert PCM-style PATTERN into PCM-style flex pattern.