org-element: Use internal version of `org-list-struct'

* lisp/org-element.el (org-element--list-struct): New function.
(org-element-plain-list-parser, org-element--current-element): Use new
function.

This patch removes dependency on org-list.el to parsing lists. For
now, it leads to code duplication, but, ultimately (i.e., when parsing
will be faster), org-list.el will delegate the parsing job to
org-element. The new implementation is also faster than the previous one.

lisp/org-element.el

@@ -1146,6 +1146,89 @@ CONTENTS is the contents of the element."
 
 ;;;; Plain List
 
+(defun org-element--list-struct (limit)
+;; Return structure of list at point.  Internal function.  See
+;; `org-list-struct' for details.
+  (let ((case-fold-search t)
+	(item-re (org-item-re))
+	(drawers-re (concat ":\\("
+			    (mapconcat 'regexp-quote org-drawers "\\|")
+			    "\\):[ \t]*$"))
+	(inlinetask-re (and (featurep 'org-inlinetask) "^\\*+ "))
+	items struct)
+    (save-excursion
+      (catch 'exit
+	(while t
+	  (cond
+	   ;; At limit: end all items.
+	   ((>= (point) limit)
+	    (throw 'exit
+		   (let ((end (progn (skip-chars-backward " \r\t\n")
+				     (forward-line)
+				     (point))))
+		     (dolist (item items (sort (nconc items struct)
+					       'car-less-than-car))
+		       (setcar (nthcdr 6 item) end)))))
+	   ;; At list end: end all items.
+	   ((looking-at org-list-end-re)
+	    (throw 'exit (dolist (item items (sort (nconc items struct)
+						   'car-less-than-car))
+			   (setcar (nthcdr 6 item) (point)))))
+	   ;; At a new item: end previous sibling.
+	   ((looking-at item-re)
+	    (let ((ind (save-excursion (skip-chars-forward " \t")
+				       (current-column))))
+	      (while (and items (<= ind (nth 1 (car items))))
+		(let ((item (pop items)))
+		  (setcar (nthcdr 6 item) (point))
+		  (push item struct)))
+	      (push (progn (looking-at org-list-full-item-re)
+			   (let ((bullet (match-string-no-properties 1)))
+			     (list (point)
+				   ind
+				   bullet
+				   (match-string-no-properties 2) ; counter
+				   (match-string-no-properties 3) ; checkbox
+				   ;; Description tag.
+				   (and (save-match-data
+					  (string-match "[-+*]" bullet))
+					(match-string-no-properties 4))
+				   ;; Ending position, unknown so far.
+				   nil)))
+		    items))
+	    (forward-line 1))
+	   ;; Skip empty lines.
+	   ((looking-at "^[ \t]*$") (forward-line))
+	   ;; Skip inline tasks and blank lines along the way.
+	   ((and inlinetask-re (looking-at inlinetask-re))
+	    (forward-line)
+	    (let ((origin (point)))
+	      (when (re-search-forward inlinetask-re limit t)
+		(if (looking-at "^\\*+ END[ \t]*$") (forward-line)
+		  (goto-char origin)))))
+	   ;; At some text line.  Check if it ends any previous item.
+	   (t
+	    (let ((ind (progn (skip-chars-forward " \t") (current-column))))
+	      (while (<= ind (nth 1 (car items)))
+		(let ((item (pop items)))
+		  (setcar (nthcdr 6 item)
+			  (if items (line-beginning-position)
+			    (skip-chars-backward " \r\t\n")
+			    (forward-line)
+			    (point)))
+		  (push item struct)
+		  (unless items
+		    (throw 'exit (sort struct 'car-less-than-car))))))
+	    ;; Skip blocks (any type) and drawers contents.
+	    (cond
+	     ((and (looking-at "#\\+BEGIN\\(:[ \t]*$\\|_\\S-\\)+")
+		   (re-search-forward
+		    (format "^[ \t]*#\\+END%s[ \t]*$" (match-string 1))
+		    limit t)))
+	     ((and (looking-at drawers-re)
+		   (re-search-forward "^[ \t]*:END:[ \t]*$" limit t))))
+	    (forward-line))))))))
+
 (defun org-element-plain-list-parser (limit affiliated structure)
   "Parse a plain list.
 
@@ -1162,9 +1245,8 @@ containing `:type', `:begin', `:end', `:contents-begin' and
 
 Assume point is at the beginning of the list."
   (save-excursion
-    (let* ((struct (or structure (org-list-struct)))
+    (let* ((struct (or structure (org-element--list-struct limit)))
 	   (prevs (org-list-prevs-alist struct))
-	   (parents (org-list-parents-alist struct))
 	   (type (org-list-get-list-type (point) struct prevs))
 	   (contents-begin (point))
 	   (begin (car affiliated))
@@ -3870,7 +3952,8 @@ element it has to parse."
 	     ;; List.
 	     ((looking-at (org-item-re))
 	      (org-element-plain-list-parser
-	       limit affiliated (or structure (org-list-struct))))
+	       limit affiliated
+	       (or structure (org-element--list-struct limit))))
 	     ;; Default element: Paragraph.
 	     (t (org-element-paragraph-parser limit affiliated)))))))))