variable-pitch-indent.el

;; A grown up version of https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00127.html

(defvar-local *vpi-did-full-indent?* nil)

(defun vpi-find-start ()
  (loop
   (back-to-indentation)
   (when (or (= (point-min) (point)) (zerop (current-column)))
     (return))
   (forward-line -1)))

(cl-defun variable-pitch-indent ()
  (interactive)
  (save-excursion
    (let ((spine "")
          (face `(:foreground ,(face-attribute 'default :background nil)))
          (needs-full? (not *vpi-did-full-indent?*))
          (lines 0))
      ;; Start early if we know the rest of the buffer is okay. Note
      ;; that we won't propagate anything beyond lines with no
      ;; indentation.
      (if needs-full?
          (goto-char (point-min))
        (vpi-find-start))
      (while (not (eobp))
        (back-to-indentation)
        ;; Get out early if we know the rest of the buffer is okay.
        (when (and (not needs-full?) (zerop (current-column)) (plusp lines))
          (return-from variable-pitch-indent lines))
        (unless (eolp)
          (let* ((indentation-amount (current-column))
                 (spine-cut (min (length spine) indentation-amount))
                 (spine-prefix (substring spine 0 spine-cut))
                 (spine-padding (make-string (- indentation-amount spine-cut) ?\s))
                 (spine-end (buffer-substring (point) (point-at-eol))))
            (setq spine
                  (with-temp-buffer
                    (insert spine-prefix spine-padding spine-end)
                    (untabify (point-min) (point-max))
                    (buffer-string)))
            (put-text-property (point-at-bol) (point) 'display
                               (propertize (concat spine-prefix spine-padding)
                                           'face face))))
        (forward-line 1)
        (incf lines))
      (when needs-full? (setf *vpi-did-full-indent?* t))
      lines)))

(defun variable-pitch-hook-handler (s e l)
  (variable-pitch-indent))
(push 'variable-pitch-hook-handler after-change-functions)