+;; Line numbers for `rectangle-number-line-callback'.
+(defvar rectangle-number-line-counter)
+
+(defun rectangle-number-line-callback (start _end format-string)
+ (move-to-column start t)
+ (insert (format format-string rectangle-number-line-counter))
+ (setq rectangle-number-line-counter
+ (1+ rectangle-number-line-counter)))
+
+(defun rectange--default-line-number-format (start end start-at)
+ (concat "%"
+ (int-to-string (length (int-to-string (+ (count-lines start end)
+ start-at))))
+ "d "))
+
+;;;###autoload
+(defun rectangle-number-lines (start end start-at &optional format)
+ "Insert numbers in front of the region-rectangle.
+
+START-AT, if non-nil, should be a number from which to begin
+counting. FORMAT, if non-nil, should be a format string to pass
+to `format' along with the line count. When called interactively
+with a prefix argument, prompt for START-AT and FORMAT."
+ (interactive
+ (if current-prefix-arg
+ (let* ((start (region-beginning))
+ (end (region-end))
+ (start-at (read-number "Number to count from: " 1)))
+ (list start end start-at
+ (read-string "Format string: "
+ (rectange--default-line-number-format
+ start end start-at))))
+ (list (region-beginning) (region-end) 1 nil)))
+ (unless format
+ (setq format (rectange--default-line-number-format start end start-at)))
+ (let ((rectangle-number-line-counter start-at))
+ (apply-on-rectangle 'rectangle-number-line-callback
+ start end format)))
+