Add a new minor mode `glyphless-display-mode'

* doc/emacs/display.texi (Text Display): Mention it.

* doc/lispref/display.texi (Glyphless Chars): Document it.
* lisp/textmodes/glyphless-mode.el: New minor mode (bug#27544).

1 files changed, 67 insertions, 0 deletions

diff --git a/lisp/textmodes/glyphless-mode.el b/lisp/textmodes/glyphless-mode.el
new file mode 100644
index 00000000000..3aeb360084a
--- /dev/null
+++ b/lisp/textmodes/glyphless-mode.el
@@ -0,0 +1,67 @@
+;;; glyphless-mode.el --- minor mode for displaying glyphless characters  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Maintainer: emacs-devel@gnu.org
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(defcustom glyphless-mode-types '(all)
+  "Which glyphless characters to display.
+The value can be any of the groups supported by
+`glyphless-char-display-control' (which see), and in addition
+`all', for all glyphless characters."
+  :version "29.1"
+  :type '(repeat (choice (const :tag "All" all)
+                         (const :tag "No font" no-font)
+                         (const :tag "C0 Control" c0-control)
+                         (const :tag "C1 Control" c1-control)
+                         (const :tag "Format Control" format-control)
+                         (const :tag "Variation Selectors" variation-selectors)
+                         (const :tag "No Font" no-font)))
+  :group 'display)
+
+;;;###autoload
+(define-minor-mode glyphless-display-mode
+  "Minor mode for displaying glyphless characters in the current buffer.
+If enabled, all glyphless characters will be displayed as boxes
+that display their acronyms."
+  :lighter " Glyphless"
+  (if glyphless-display-mode
+      (progn
+        (setq-local glyphless-char-display
+                    (let ((table (make-display-table)))
+                      (set-char-table-parent table glyphless-char-display)
+                      table))
+        (glyphless-mode--setup))
+    (kill-local-variable 'glyphless-char-display)))
+
+(defun glyphless-mode--setup ()
+  (let ((types (if (memq 'all glyphless-mode-types)
+                   '(c0-control c1-control format-control
+                                variation-selectors no-font)
+                 glyphless-mode-types)))
+    (when types
+      (update-glyphless-char-display
+       nil (mapcar (lambda (e) (cons e 'acronym)) types)))))
+
+(provide 'glyphless-mode)
+
+;;; glyphless-mode.el ends here