(lexical-binding): Allow changing the default value (bug#74145)

* lisp/loadup.el: Preserve (default-toplevel-value 'lexical-binding).

* src/lread.c (Fload, Feval_buffer): Obey (default-toplevel-value
'lexical-binding).

* doc/lispintro/emacs-lisp-intro.texi (Lexical vs Dynamic Binding
Example): Use the lexical dialect also for the dynamic
scoping example.
* doc/lispref/edebug.texi (Edebug Eval): Remove long-obsolete mention
of Edebug support for `lexical-let`.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/lread.c b/src/lread.c
index 46c705e5c76..df1caaf5732 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -1720,8 +1720,11 @@ Return t if the file exists and loads successfully.  */)
     }
   else
     {
-      if (lisp_file_lexical_cookie (Qget_file_char) == Cookie_Lex)
-        Fset (Qlexical_binding, Qt);
+      lexical_cookie_t lexc = lisp_file_lexical_cookie (Qget_file_char);
+      Fset (Qlexical_binding,
+            (lexc == Cookie_Lex ? Qt
+             : lexc == Cookie_Dyn ? Qnil
+             : Fdefault_toplevel_value (Qlexical_binding)));
 
       if (! version || version >= 22)
         readevalloop (Qget_file_char, &input, hist_file_name,
@@ -2606,8 +2609,11 @@ This function preserves the position of point.  */)
   specbind (Qstandard_output, tem);
   record_unwind_protect_excursion ();
   BUF_TEMP_SET_PT (XBUFFER (buf), BUF_BEGV (XBUFFER (buf)));
+  lexical_cookie_t lexc = lisp_file_lexical_cookie (buf);
   specbind (Qlexical_binding,
-            lisp_file_lexical_cookie (buf) == Cookie_Lex ? Qt : Qnil);
+            lexc == Cookie_Lex ? Qt
+            : lexc == Cookie_Dyn ? Qnil
+            : Fdefault_toplevel_value (Qlexical_binding));
   BUF_TEMP_SET_PT (XBUFFER (buf), BUF_BEGV (XBUFFER (buf)));
   readevalloop (buf, 0, filename,
                 !NILP (printflag), unibyte, Qnil, Qnil, Qnil);