Add new major mode 'clean-mode'

* doc/lispref/modes.texi (Major Modes): Document it.

* lisp/simple.el (clean-mode): New major mode.

doc/lispref/modes.texi

@@ -266,6 +266,18 @@ This function restores the major mode recorded by
 function calls @code{normal-mode} (@pxref{Auto Major Mode,
 normal-mode}), but tries to force it not to choose any modes in
 @var{avoided-modes}, if that argument is non-@code{nil}.
+@end defun
+
+@defun clean-mode
+Changing the major mode clears out most local variables, but it
+doesn't remove all artefacts in the buffer (like text properties and
+overlays).  It's rare to change a buffer from one major mode to
+another (except from @code{fundamental-mode} to everything else), so
+this is usually not a concern.  It can sometimes be convenient (mostly
+when debugging a problem in a buffer) to do a ``full reset'' of the
+buffer, and that's what the @code{clean-mode} major mode offers.  It
+will kill all local variables (even the permanently local ones), and
+also removes all overlays and text properties.
 @end defun
 
   The easiest way to write a major mode is to use the macro

etc/NEWS

@@ -125,6 +125,11 @@ with recent versions of Firefox.
 
 * Lisp Changes in Emacs 29.1
 
++++
+** New major mode 'clean-mode'.
+This is a new major mode meant for debugging.  It kills absolutely all
+local variables and removes overlays and text properties.
+
 +++
 ** 'kill-all-local-variables' can now kill all local variables.
 If given the new optional KILL-PERMANENT argument, also kill permanent

lisp/simple.el

@@ -527,6 +527,18 @@ Other major modes are defined by comparison with this one."
   (kill-all-local-variables)
   (run-mode-hooks))
 
+(define-derived-mode clean-mode fundamental-mode "Clean"
+  "A mode that removes all overlays and text properties."
+  (kill-all-local-variables t)
+  (let ((inhibit-read-only t))
+    (dolist (overlay (overlays-in (point-min) (point-max)))
+      (delete-overlay overlay))
+    (set-text-properties (point-min) (point-max) nil)
+    (setq-local after-change-functions
+                (list
+                 (lambda (begin end _length)
+                   (set-text-properties begin end nil))))))
+
 ;; Special major modes to view specially formatted data rather than files.
 
 (defvar-keymap special-mode-map