ol.el: add description format parameter to org-link-parameters

* ol.el (org-link-parameters): Add parameter `:insert-description', a
string or a function.
* (org-insert-link): If no description is provided (pre-existing or as
an argument), next option is to use the `:insert-description' (if
non-nil) parameter to generate one.
* (org-link-make-description-function): Add documentation to describe
behaviour of nil return value, like that of `:insert-description'.

Default descriptions are predictable within a link type, but because
link types are quite diverse, are NOT predictable across many types.
A type-parameter is thus a good place to store information on the
default description.

etc/ORG-NEWS

@@ -400,6 +400,13 @@ purpose of the variable.  The replacement variable
 accepts =listings= and =verbatim= in place of =t= and =nil= (which
 still work, but are no longer listed as valid options).
 
+*** ~org-link-parameters~ has a new ~:insert-description~ parameter
+
+The value of ~:insert-description~ is used as the initial input when
+prompting for a link description.  It can be a string (used as-is) or
+a function (called with the same arguments as
+~org-make-link-description-function~ to return a string to use).
+
 * Version 9.5
 
 ** Important announcements and breaking changes

lisp/ol.el

@@ -141,6 +141,19 @@ link.
   Function that inserts a link with completion.  The function
   takes one optional prefix argument.
 
+`:insert-description'
+
+  String or function used as a default when prompting users for a
+  link's description.  A string is used as-is, a function is
+  called with two arguments: the link location (a string such as
+  \"~/foobar\", \"id:some-org-id\" or \"https://www.foo.com\")
+  and the description generated by `org-insert-link'.  It should
+  return the description to use (this reflects the behaviour of
+  `org-link-make-description-function').  If it returns nil, no
+  default description is used, but no error is thrown (from the
+  user's perspective, this is equivalent to a default description
+  of \"\").
+
 `:display'
 
   Value for `invisible' text property on the hidden parts of the
@@ -200,7 +213,9 @@ You can interactively set the value of this variable by calling
 This function must take two parameters: the first one is the
 link, the second one is the description generated by
 `org-insert-link'.  The function should return the description to
-use."
+use.  If it returns nil, no default description is used, but no
+error is thrown (from the user’s perspective, this is equivalent
+to a default description of \"\")."
   :group 'org-link
   :type '(choice (const nil) (function))
   :safe #'null)
@@ -1802,11 +1817,14 @@ prefix negates `org-link-keep-stored-after-insertion'.
 If the LINK-LOCATION parameter is non-nil, this value will be used as
 the link location instead of reading one interactively.
 
-If the DESCRIPTION parameter is non-nil, this value will be used as the
-default description.  Otherwise, if `org-link-make-description-function'
-is non-nil, this function will be called with the link target, and the
-result will be the default link description.  When called non-interactively,
-don't allow to edit the default description."
+If the DESCRIPTION parameter is non-nil, this value will be used
+as the default description.  If not, and the chosen link type has
+a non-nil `:insert-description' parameter, that is used to
+generate a description as described in `org-link-parameters'
+docstring. Otherwise, if `org-link-make-description-function' is
+non-nil, this function will be called with the link target, and
+the result will be the default link description.  When called
+non-interactively, don't allow to edit the default description."
   (interactive "P")
   (let* ((wcf (current-window-configuration))
 	 (origbuf (current-buffer))
@@ -1816,7 +1834,10 @@ don't allow to edit the default description."
 	 (desc region)
 	 (link link-location)
 	 (abbrevs org-link-abbrev-alist-local)
-	 entry all-prefixes auto-desc)
+	 (all-prefixes (append (mapcar #'car abbrevs)
+			       (mapcar #'car org-link-abbrev-alist)
+			       (org-link-types)))
+         entry auto-desc)
     (cond
      (link-location)		      ; specified by arg, just use it.
      ((org-in-regexp org-link-bracket-re 1)
@@ -1857,9 +1878,6 @@ Use TAB to complete link prefixes, then RET for type-specific completion support
 	  (unless (pos-visible-in-window-p (point-max))
 	    (org-fit-window-to-buffer))
 	  (and (window-live-p cw) (select-window cw))))
-      (setq all-prefixes (append (mapcar #'car abbrevs)
-				 (mapcar #'car org-link-abbrev-alist)
-				 (org-link-types)))
       (unwind-protect
 	  ;; Fake a link history, containing the stored links.
 	  (let ((org-link--history
@@ -1956,17 +1974,36 @@ Use TAB to complete link prefixes, then RET for type-specific completion support
 	    (setq desc path)))))
 
     (unless auto-desc
-      (let ((initial-input
-	     (cond
-	      (description)
-	      ((not org-link-make-description-function) desc)
-	      (t (condition-case nil
-		     (funcall org-link-make-description-function link desc)
-		   (error
-		    (message "Can't get link description from %S"
-			     (symbol-name org-link-make-description-function))
-		    (sit-for 2)
-		    nil))))))
+      (let* ((type
+              (cond
+               ((string-match (rx-to-string `(: string-start (submatch (or ,@all-prefixes)) ":")) link)
+                (match-string 1 link))
+               ((file-name-absolute-p link) "file")
+               ((string-match "\\`\\.\\.?/" link) "file")))
+             (initial-input
+	      (cond
+	       (description)
+               (desc)
+               ((org-link-get-parameter type :insert-description)
+                (let ((def (org-link-get-parameter type :insert-description)))
+                  (condition-case nil
+                      (cond
+                       ((stringp def) def)
+                       ((functionp def)
+                        (funcall def link desc)))
+                    (error
+                     (message "Can't get link description from org link parameter `:insert-description': %S"
+			      def)
+		     (sit-for 2)
+                     nil))))
+	       (org-link-make-description-function
+                (condition-case nil
+		    (funcall org-link-make-description-function link desc)
+		  (error
+		   (message "Can't get link description from %S"
+		            org-link-make-description-function)
+		   (sit-for 2)
+		   nil))))))
 	(setq desc (if (called-interactively-p 'any)
 		       (read-string "Description: " initial-input)
 		     initial-input))))