Re: Unicode confusables and reordering characters considered harmful

[Gregory Heytings <gregory@heytings.org>]

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>> The directionality characters would be highlighted, that's all (and it 
>> would be easy to add a toggle command to (un)highlight them on demand, 
>> see attached).  This functionality is orthogonal to the one you propose 
>> AFAIU, both could coexist.
>
> It could, but I don't think we should install such features.
>

Why not?  Is this not simply an improved (and built-in) version of your 
initial suggestion to use glyphless-char-display?  Except that 
glyphless-char-display is global (whereas buffer-display-table is 
buffer-local), and that a tofu is not as visible as a character 
highlighted with the font-lock-warning-face.

Here's an improved patch which also highlights these characters in strings 
and comments.

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From c15d21b833f32896138afffaa9d7c9d917d52ea7 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Wed, 3 Nov 2021 15:58:02 +0000
Subject: [PATCH] Make bidi reordering characters visible

* lisp/progmodes/prog-mode.el (bidi-reordering-characters-visible,
bidi-reordering-characters-fontify,
bidi-reordering-characters-toggle-visibility): New functions.
---
 lisp/progmodes/prog-mode.el | 47 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/prog-mode.el b/lisp/progmodes/prog-mode.el
index db350a5f70..e4f63b0645 100644
--- a/lisp/progmodes/prog-mode.el
+++ b/lisp/progmodes/prog-mode.el
@@ -289,6 +289,50 @@ turn-on-prettify-symbols-mode
              (local-variable-p 'prettify-symbols-alist))
     (prettify-symbols-mode 1)))
 
+(defvar-local bidi-reordering-characters-visible nil
+  "Internal variable used by `bidi-reordering-characters-visible'.")
+
+(defun bidi-reordering-characters-fontify ()
+  "Fontify bidi reordering characters with `font-lock-warning-face'."
+  (font-lock-add-keywords
+   nil
+   '(("⁩\\|‬\\|⁨\\|⁧\\|⁦\\|‫\\|‪\\|‮\\|‭" . (0 'font-lock-warning-face t)))))
+
+(defun bidi-reordering-characters-visible ()
+  "Display bidi reordering characters as arrows."
+  (setq buffer-display-table (or buffer-display-table
+                                 standard-display-table
+                                 (make-display-table)))
+  (bidi-reordering-character-toggle-visibility)
+  (add-hook 'font-lock-mode-hook #'bidi-reordering-characters-fontify))
+
+;;;###autoload
+(defun bidi-reordering-character-toggle-visibility ()
+  "Toggle the visibility of bidi reordering characters."
+  (interactive)
+  (setq bidi-reordering-characters-visible
+        (not bidi-reordering-characters-visible))
+  (when bidi-reordering-characters-visible
+    (aset buffer-display-table ?‪ [?→])
+    (aset buffer-display-table ?‫ [?←])
+    (aset buffer-display-table ?‭ [?→])
+    (aset buffer-display-table ?‮ [?←])
+    (aset buffer-display-table ?⁦ [?→])
+    (aset buffer-display-table ?⁧ [?←])
+    (aset buffer-display-table ?⁨ [?↓])
+    (aset buffer-display-table ?‬ [?↑])
+    (aset buffer-display-table ?⁩ [?↑]))
+  (unless bidi-reordering-characters-visible
+    (aset buffer-display-table ?‪ nil)
+    (aset buffer-display-table ?‫ nil)
+    (aset buffer-display-table ?‭ nil)
+    (aset buffer-display-table ?‮ nil)
+    (aset buffer-display-table ?⁦ nil)
+    (aset buffer-display-table ?⁧ nil)
+    (aset buffer-display-table ?⁨ nil)
+    (aset buffer-display-table ?‬ nil)
+    (aset buffer-display-table ?⁩ nil)))
+
 ;;;###autoload
 (define-globalized-minor-mode global-prettify-symbols-mode
   prettify-symbols-mode turn-on-prettify-symbols-mode)
@@ -300,7 +344,8 @@ prog-mode
   (setq-local parse-sexp-ignore-comments t)
   (add-hook 'context-menu-functions 'prog-context-menu 10 t)
   ;; Any programming language is always written left to right.
-  (setq bidi-paragraph-direction 'left-to-right))
+  (setq bidi-paragraph-direction 'left-to-right)
+  (bidi-reordering-characters-visible))
 
 (provide 'prog-mode)
 
-- 
2.33.0