From f9354f99d1d27212659cbee0420b35e883373a5d Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Wed, 3 Nov 2021 19:22:07 +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 | 41 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/prog-mode.el b/lisp/progmodes/prog-mode.el
index db350a5f70..7fcdc7b5e3 100644
--- a/lisp/progmodes/prog-mode.el
+++ b/lisp/progmodes/prog-mode.el
@@ -293,6 +293,44 @@ turn-on-prettify-symbols-mode
 (define-globalized-minor-mode global-prettify-symbols-mode
   prettify-symbols-mode turn-on-prettify-symbols-mode)
 
+(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
+   '(("\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 ()
+  "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))
+  (let ((v bidi-reordering-characters-visible))
+    (aset buffer-display-table ?\N{LEFT-TO-RIGHT EMBEDDING} (if v [?→] nil))
+    (aset buffer-display-table ?\N{RIGHT-TO-LEFT EMBEDDING} (if v [?←] nil))
+    (aset buffer-display-table ?\N{LEFT-TO-RIGHT OVERRIDE} (if v [?→] nil))
+    (aset buffer-display-table ?\N{RIGHT-TO-LEFT OVERRIDE} (if v [?←] nil))
+    (aset buffer-display-table ?\N{LEFT-TO-RIGHT ISOLATE} (if v [?→] nil))
+    (aset buffer-display-table ?\N{RIGHT-TO-LEFT ISOLATE} (if v [?←] nil))
+    (aset buffer-display-table ?\N{FIRST STRONG ISOLATE} (if v [?↓] nil))
+    (aset buffer-display-table ?\N{POP DIRECTIONAL FORMATTING} (if v [?↑] nil))
+    (aset buffer-display-table ?\N{POP DIRECTIONAL ISOLATE} (if v [?↑] nil))))
+
 ;;;###autoload
 (define-derived-mode prog-mode fundamental-mode "Prog"
   "Major mode for editing programming language source code."
@@ -300,7 +338,8 @@ 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