From dff39cf654213122d2a926a77f14d9357d95f2ab Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Thu, 4 Nov 2021 08:47:26 +0000
Subject: [PATCH] Make bidi reordering characters visible

* lisp/progmodes/prog-mode.el (bidi-reordering-characters-visible):
New minor mode.
(bidi-reordering-characters-visible--fontify,
bidi-reordering-characters-visible--toggle): New helper functions.
(prog-mode): Enable the new minor mode.
---
 lisp/progmodes/prog-mode.el | 38 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/prog-mode.el b/lisp/progmodes/prog-mode.el
index db350a5f70..471f64ce28 100644
--- a/lisp/progmodes/prog-mode.el
+++ b/lisp/progmodes/prog-mode.el
@@ -293,6 +293,41 @@ turn-on-prettify-symbols-mode
 (define-globalized-minor-mode global-prettify-symbols-mode
   prettify-symbols-mode turn-on-prettify-symbols-mode)
 
+(defun bidi-reordering-characters-visible--fontify ()
+  "Fontify bidi reordering characters with `font-lock-warning-face'."
+  (font-lock-add-keywords
+   nil
+   '(("[\N{left-to-right embedding}\N{right-to-left embedding}\
+\N{left-to-right override}\N{right-to-left override}\
+\N{left-to-right isolate}\N{right-to-left isolate}\
+\N{first strong isolate}\N{pop directional formatting}\
+\N{pop directional isolate}]" . (0 'font-lock-warning-face t)))))
+
+(defun bidi-reordering-characters-visible--toggle ()
+  "Toggle the visibility of bidi reordering characters."
+  (let ((v bidi-reordering-characters-visible)
+        (bdt buffer-display-table))
+    (aset bdt ?\N{left-to-right embedding} (if v [?→] nil))
+    (aset bdt ?\N{right-to-left embedding} (if v [?←] nil))
+    (aset bdt ?\N{left-to-right override} (if v [?→] nil))
+    (aset bdt ?\N{right-to-left override} (if v [?←] nil))
+    (aset bdt ?\N{left-to-right isolate} (if v [?→] nil))
+    (aset bdt ?\N{right-to-left isolate} (if v [?←] nil))
+    (aset bdt ?\N{first strong isolate} (if v [?↓] nil))
+    (aset bdt ?\N{pop directional formatting} (if v [?↑] nil))
+    (aset bdt ?\N{pop directional isolate} (if v [?↑] nil))))
+
+;;;###autoload
+(define-minor-mode bidi-reordering-characters-visible
+  "Make the bidi reordering characters visible."
+  :init-value nil
+  (setq buffer-display-table (or buffer-display-table
+                                 standard-display-table
+                                 (make-display-table)))
+  (bidi-reordering-characters-visible--toggle)
+  (add-hook 'font-lock-mode-hook
+            #'bidi-reordering-characters-visible--fontify))
+
 ;;;###autoload
 (define-derived-mode prog-mode fundamental-mode "Prog"
   "Major mode for editing programming language source code."
@@ -300,7 +335,8 @@ 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