emacs delete-trailing-whitespace except current line

Since delete-trailing-whitespace respects narrowing, one solution is to narrow the buffer to the portion before the current line and call it, then narrow to the portion after the current line and call it again:

(defun delete-trailing-whitespace-except-current-line ()
  (interactive)
  (let ((begin (line-beginning-position))
        (end (line-end-position)))
    (save-excursion
      (when (< (point-min) begin)
        (save-restriction
          (narrow-to-region (point-min) (1- begin))
          (delete-trailing-whitespace)))
      (when (> (point-max) end)
        (save-restriction
          (narrow-to-region (1+ end) (point-max))
          (delete-trailing-whitespace))))))

Put this function on your before-save-hook instead of delete-trailing-whitespace.