bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> > > Try `M-: (use-global-map (make-keymap)) RET`
> > >
> > > Should we prevent users from doing that?
> >
> > It's a misleading question.  No "user" would ever do that.  Sure, it's 
> > a nice example, but only an Elisp hacker would do that, in the middle 
> > of a debugging session, and they would do that on purpose (although 
> > perhaps without knowing the effect in advance).  Which has nothing to 
> > do with a regular user who just opens a file.
>
> FWIW, the above is my standard example because I ended up doing exactly 
> that by accident, locking myself out of the session I was trying to 
> debug, so in a sense, I'd have been happy (that one time) if Emacs had 
> prevented me from doing it.

So... what do you think of the following? 😉

diff --git a/src/keymap.c b/src/keymap.c
index 506b755e5d..e42bc64717 100644
--- a/src/keymap.c
+++ b/src/keymap.c
@@ -1881,6 +1881,17 @@ DEFUN ("use-global-map", Fuse_global_map, 
Suse_global_map, 1, 1, 0,
   (Lisp_Object keymap)
 {
   keymap = get_keymap (keymap, 1, 1);
+
+  /* Prevent locking Emacs if someone inadvertently evaluates
+     (use-global-map (make-keymap)) */
+  if (EQ (Fequal (keymap, Fmake_keymap (Qnil)), Qt))
+    {
+      Lisp_Object meta_colon_key = CALLN (Fvector, make_fixnum (134217786));
+      Lisp_Object default_key = CALLN (Fvector, Qt);
+      Fdefine_key (keymap, meta_colon_key, intern ("eval-expression"), Qnil);
+      Fdefine_key (keymap, default_key, intern ("self-insert-command"), Qnil);
+    }
+
   current_global_map = keymap;

   return Qnil;