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diff --git a/lisp/progmodes/erts-mode.el b/lisp/progmodes/erts-mode.el
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+;;; erts-mode.el --- major mode to edit erts files  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; Keywords: tools
+
+;; 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:
+
+(eval-when-compile (require 'cl-lib))
+(require 'ert)
+
+(defgroup erts-mode nil
+  "Major mode for editing Emacs test files."
+  :group 'lisp)
+
+(defface erts-mode-specification-name
+  '((((class color)
+      (background dark))
+     :foreground "green")
+    (((class color)
+      (background light))
+     :foreground "cornflower blue")
+    (t
+     :bold t))
+  "Face used for displaying specification names."
+  :group 'erts-mode)
+
+(defface erts-mode-specification-value
+  '((((class color)
+      (background dark))
+     :foreground "DeepSkyBlue1")
+    (((class color)
+      (background light))
+     :foreground "blue")
+    (t
+     :bold t))
+  "Face used for displaying specificaton values."
+  :group 'erts-mode)
+
+(defface erts-mode-start-test
+  '((t :inherit font-lock-keyword-face))
+  "Face used for displaying specificaton test start markers."
+  :group 'erts-mode)
+
+(defface erts-mode-end-test
+  '((t :inherit font-lock-comment-face))
+  "Face used for displaying specificaton test start markers."
+  :group 'erts-mode)
+
+(defvar erts-mode-map
+  (let ((map (make-keymap)))
+    (set-keymap-parent map prog-mode-map)
+    (define-key map "\C-c\C-r" 'erts-tag-region)
+    (define-key map "\C-c\C-c" 'erts-run-test)
+    map))
+
+(defvar erts-mode-font-lock-keywords
+  ;; Specifications.
+  `((erts-mode--match-not-in-test
+     ("^\\([^ \t\n:]+:\\)[ \t]*\\(.*\\(\n[ \t].*\\)*\\)\n?"
+      (progn (goto-char (match-beginning 0)) (match-end 0)) nil
+      (1 'erts-mode-specification-name)
+      (2 'erts-mode-specification-value)))
+    ("^=-=$" 0 'erts-mode-start-test)
+    ("^=-=-=$" 0 'erts-mode-end-test)))
+
+(defun erts-mode--match-not-in-test (_limit)
+  (when (erts-mode--in-test-p (point))
+    (erts-mode--end-of-test))
+  (let ((start (point)))
+    (goto-char
+     (if (re-search-forward "^=-=$" nil t)
+         (match-beginning 0)
+       (point-max)))
+    (if (< (point) start)
+        nil
+      ;; Here we disregard LIMIT so that we may extend the area again.
+      (set-match-data (list start (point)))
+      (point))))
+
+(defun erts-mode--end-of-test ()
+  (search-forward "^=-=-=\n" nil t))
+
+(defun erts-mode--in-test-p (point)
+  "Say whether POINT is in a test."
+  (save-excursion
+    (goto-char point)
+    (beginning-of-line)
+    (if (looking-at "=-=\\(-=\\)?$")
+        t
+      (let ((test-start (save-excursion
+                          (re-search-backward "^=-=\n" nil t))))
+        ;; Before the first test.
+        (and test-start
+             (let ((test-end (re-search-backward "^=-=-=\n" nil t)))
+               (or (null test-end)
+                   ;; Between tests.
+                   (> test-start test-end))))))))
+
+;;;###autoload
+(define-derived-mode erts-mode prog-mode "erts"
+  "Major mode for editing erts (Emacs testing) files.
+This mode mainly provides some font locking.
+
+\\{erts-mode-map}"
+  (setq-local font-lock-defaults '(erts-mode-font-lock-keywords t)))
+
+(defun erts-tag-region (start end name)
+  "Tag the region between START and END as a test.
+Interactively, this is the region.
+
+NAME should be a string appropriate for output by ert if the test fails.
+If NAME is nil or the empty string, a name will be auto-generated."
+  (interactive "r\nsTest name: " erts-mode)
+  ;; Automatically make a name.
+  (when (zerop (length name))
+    (save-excursion
+      (goto-char (point-min))
+      (let ((names nil))
+        (while (re-search-forward "^Name:[ \t]*\\(.*\\)" nil t)
+          (let ((name (match-string 1)))
+            (unless (erts-mode--in-test-p (point))
+              (push name names))))
+        (setq name
+              (cl-loop with base = (file-name-sans-extension (buffer-name))
+                       for i from 1
+                       for name = (format "%s%d" base i)
+                       unless (member name names)
+                       return name)))))
+  (save-excursion
+    (goto-char end)
+    (unless (bolp)
+      (insert "\n"))
+    (insert "=-=-=\n")
+    (goto-char start)
+    (insert "Name: " name "\n\n")
+    (insert "=-=\n")))
+
+(defun erts-mode--preceding-spec (name)
+  (save-excursion
+    ;; Find the name, but skip if it's in a test.
+    (while (and (re-search-backward (format "^%s:" name)  nil t)
+                (erts-mode--in-test-p (point))))
+    (and (not (erts-mode--in-test-p (point)))
+         (re-search-forward "^=-=$" nil t)
+         (progn
+           (goto-char (match-beginning 0))
+           (cdr (assq (intern (downcase name))
+                      (ert--erts-specifications (point))))))))
+
+(defun erts-run-test (test-function &optional verbose)
+  "Run the current test.
+If the current erts file doesn't define a test function, the user
+will be prompted for one.
+
+If VERBOSE (interactively, the prefix), display a diff of the
+expected results and the actual results in a separate buffer."
+  (interactive
+   (list (or (erts-mode--preceding-spec "Code")
+             (read-string "Transformation function: "))
+         current-prefix-arg)
+   erts-mode)
+  (save-excursion
+    (erts-mode--goto-start-of-test)
+    (condition-case arg
+        (ert-test--erts-test
+         (list (cons 'dummy t)
+               (cons 'code (car (read-from-string test-function)))
+               (cons 'point-char (erts-mode--preceding-spec "Point-Char")))
+         (buffer-file-name))
+      (:success (message "Test successful"))
+      (ert-test-failed
+       (if (not verbose)
+           (message "Test failure; result: \n%s"
+                    (substring-no-properties (cadr (cadr arg))))
+         (message "Test failure")
+         (let (expected got)
+           (unwind-protect
+               (progn
+                 (with-current-buffer
+                     (setq expected (generate-new-buffer "erts expected"))
+                   (insert (nth 1 (cadr arg))))
+                 (with-current-buffer
+                     (setq got (generate-new-buffer "erts results"))
+                   (insert (nth 2 (cadr arg))))
+                 (diff-buffers expected got))
+             (kill-buffer expected)
+             (kill-buffer got))))))))
+
+(defun erts-mode--goto-start-of-test ()
+  (if (not (erts-mode--in-test-p (point)))
+      (re-search-forward "^=-=\n" nil t)
+    (re-search-backward "^=-=\n" nil t)
+    (let ((potential-start (match-end 0)))
+      ;; See if we're in a two-clause ("before" and "after") test or not.
+      (if-let ((start (and (save-excursion (re-search-backward "^=-=\n" nil t))
+                           (match-end 0))))
+          (let ((end (save-excursion (re-search-backward "^=-=-=\n" nil t))))
+            (if (or (not end)
+                    (> start end))
+                ;; We are, so go to the real start.
+                (goto-char start)
+              (goto-char potential-start)))
+        (goto-char potential-start)))))
+
+(provide 'erts-mode)
+
+;;; erts-mode.el ends here