bug#68214: Completion sorting customization by category

[Juri Linkov <juri@linkov.net>]

>> What do you think about doing the same for
>> `completion-extra-properties'?
>
> Yes, makes sense. The documentation of `completion-extra-properties'
> would have to be extended accordingly, mentioning all the newly
> supported properties. It bothers me a little bit that the
> `completion-metadata-get' lookup wouldn't be allocation-free anymore
> (because of keywords versus plain symbols), but one could cache the
> keyword symbol.
>
> (defun completion-metadata-get (metadata prop)
>   "Get property PROP from completion METADATA.
> If the metadata specifies a completion category, the variables
> `completion-category-overrides' and
> `completion-category-defaults' take precedence for
> category-specific overrides.  If the completion metadata does not
> specify the property, the `completion-extra-properties' plist is
> consulted.  Note that the keys of the
> `completion-extra-properties' plist are keyword symbols, not
> plain symbols."
>   (if-let (((not (eq prop 'category)))
>            (cat (alist-get 'category metadata))
>            (over (completion--category-override cat prop)))
>       (cdr over)
>     (or (alist-get prop metadata)
>         (plist-get completion-extra-properties
>                    ;; Cache the keyword
>                    (or (get prop 'completion-extra-properties--keyword)
>                        (put prop 'completion-extra-properties--keyword
>                             (intern (concat ":" (symbol-name prop)))))))))

Thanks, this is now pushed as well in the commit aff1d53cd46.

Now I tried to use a more user-friendly let-binding of
'completion-extra-properties' like below, but then discovered
that it doesn't work for the category.

Maybe 'completion-metadata-get' could try to get a category
from 'completion-extra-properties' as well?

diff --git a/lisp/calendar/calendar.el b/lisp/calendar/calendar.el
index e01d5d792a6..2c3e7d28301 100644
--- a/lisp/calendar/calendar.el
+++ b/lisp/calendar/calendar.el
@@ -2337,14 +2337,12 @@ calendar-read-date
          (defmon (aref month-array (1- (calendar-extract-month default-date))))
          (completion-ignore-case t)
          (month (cdr (assoc-string
-                      (completing-read
-                       (format-prompt "Month name" defmon)
-                       (lambda (string pred action)
-	                 (if (eq action 'metadata)
-		             '(metadata (category . calendar-month))
-	                   (complete-with-action
-                            action (append month-array nil) string pred)))
-                       nil t nil nil defmon)
+                      (let ((completion-extra-properties
+                             '(:category calendar-month)))
+                        (completing-read
+                         (format-prompt "Month name" defmon)
+                         (append month-array nil)
+                         nil t nil nil defmon))
                       (calendar-make-alist month-array 1) t)))
          (defday (calendar-extract-day default-date))
          (last (calendar-last-day-of-month month year)))