Testing indent-to with indent-tabs-mode

indent-to-test.el

(require 'cl)

(whitespace-mode t)
(setq indent-tabs-mode t
      tab-width 4)

(defun indent-to-test--run (start end)
  (interactive "r")
  (undo-boundary)
  (flet ((undo-boundary () nil))
    (let ((off 0)
          (end-m (copy-marker end)))
      (goto-char start)
      (while (< (point) (marker-position end-m))
        (beginning-of-line)
        (cond ((search-forward-regexp "!" (point-at-eol) t)
               (goto-char (match-beginning 0))
               (let ((target (incf off))
                     (returned (indent-to off))
                     (actual (current-column)))
                 (sit-for 0.2)
                 (end-of-line)
                 (indent-to 50)
               (insert (format "(indent-to %i) => %i, (current-column) => %i"
                               target returned actual))))
        (t (setq off 0)))
      (forward-line)))))

a!a
a!a
a!a
a!a
a!a
a!a
a!a
a!a
a!a

a !a
a !a
a !a
a !a
a !a
a !a
a !a
a !a

aa !a
aa !a
aa !a
aa !a
aa !a
aa !a
aa !a
aa !a