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From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: gmane.emacs.help
Subject: Re: Displaying a string as another one
Date: Mon, 16 Mar 2020 11:58:16 -0400
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> Does anyone have a ready-made solution?

You could start with something like

    (defun my-decode-\n-\t ()
      (font-lock-add-keywords nil
        '(("\\\\[nt]"
           (0 (my-apply-decode (match-beginning 0) (match-end 0)))))))

    (defun my-apply-decode (beg end)
      (when (= 0 (mod (save-excursion (goto-char beg)
                                      (skip-chars-backward "\\\\"))
                      2))
        `(face nil
          display ,(string (if (eq (char-before end) ?n) ?\n ?\t)))))

    (add-hook 'foo-mode-hook #'my-decode-\n-\t)


-- Stefan