(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`.

doc/lispintro/emacs-lisp-intro.texi

@@ -3845,7 +3845,9 @@ well.  When executing @code{getx}, the current global value of
 If we use dynamic binding instead, the behavior is different:
 
 @example
-;;; -*- lexical-binding: nil -*-
+;;; -*- lexical-binding: t -*-
+
+(defvar x) ;; Use dynamic binding for 'x'.
 
 (setq x 0)
 
@@ -3867,8 +3869,9 @@ binding.  This time, @code{getx} doesn't see the global value for
 @code{x}, since its binding is below the one from our @code{let}
 expression in the stack of bindings.
 
-(Some variables are also ``special'', and they are always dynamically
-bound even when @code{lexical-binding} is @code{t}.  @xref{defvar, ,
+(The @code{defvar} declaration above is said to make the variable
+``special'', which causes it to obey the dynamic binding rules instead of
+the default binding rules.  @xref{defvar, ,
 Initializing a Variable with @code{defvar}}.)
 
 @node if

doc/lispref/edebug.texi

@@ -722,13 +722,6 @@ lists).  Any other prefix will result in the value being
 pretty-printed in a separate buffer.
 @end table
 
-@cindex lexical binding (Edebug)
-  Edebug supports evaluation of expressions containing references to
-lexically bound symbols created by the following constructs in
-@file{cl.el}: @code{lexical-let}, @code{macrolet}, and
-@code{symbol-macrolet}.
-@c FIXME?  What about lexical-binding = t?
-
 @node Eval List
 @subsection Evaluation List Buffer
 

doc/lispref/variables.texi

@@ -285,7 +285,7 @@ previous example is equivalent to using nested @code{let} bindings:
 @defspec letrec (bindings@dots{}) forms@dots{}
 This special form is like @code{let*}, but all the variables are bound
 before any of the local values are computed.  The values are then
-assigned to the locally bound variables.  This is only useful when
+assigned to the locally bound variables.  This is useful only when
 lexical binding is in effect, and you want to create closures that
 refer to bindings that would otherwise not yet be in effect when using
 @code{let*}.
@@ -351,7 +351,7 @@ A function call is in the tail position if it's the very last thing
 done so that the value returned by the call is the value of @var{body}
 itself, as is the case in the recursive call to @code{sum} above.
 
-@code{named-let} can only be used when lexical-binding is enabled.
+@code{named-let} can be used only when lexical-binding is enabled.
 @xref{Lexical Binding}.
 @end defspec
 
@@ -392,7 +392,7 @@ object can be; but it is still a value.  If a variable is void, trying
 to evaluate the variable signals a @code{void-variable} error, instead
 of returning a value.
 
-  Under the optional lexical scoping rule, the value cell only holds
+  Under the optional lexical scoping rule, the value cell holds only
 the variable's global value---the value outside of any lexical binding
 construct.  When a variable is lexically bound, the local value is
 determined by the lexical environment; hence, variables can have local
@@ -710,8 +710,8 @@ completely.
   The usual way to reference a variable is to write the symbol which
 names it.  @xref{Symbol Forms}.
 
-  Occasionally, you may want to reference a variable which is only
-determined at run time.  In that case, you cannot specify the variable
+  Occasionally, you may want to reference a variable which is determined
+only at run time.  In that case, you cannot specify the variable
 name in the text of the program.  You can use the @code{symbol-value}
 function to extract the value.
 
@@ -1991,8 +1991,8 @@ to use this, @pxref{Auto Major Mode}.
 
 @defvar permanently-enabled-local-variables
 Some local variable settings will, by default, be heeded even if
-@code{enable-local-variables} is @code{nil}.  By default, this is only
-the case for the @code{lexical-binding} local variable setting, but
+@code{enable-local-variables} is @code{nil}.  By default, this is the
+case only for the @code{lexical-binding} local variable setting, but
 this can be controlled by using this variable, which is a list of
 symbols.
 @end defvar

etc/NEWS

@@ -1293,6 +1293,14 @@ Previously, its argument was always evaluated using dynamic binding.
 
 * Lisp Changes in Emacs 31.1
 
+---
+** You can change the default value of 'lexical-binding'.
+While the default is still the use dynamic binding dialect of ELisp
+in those places that don't explicitly set 'lexical-binding' you can
+change it globally with:
+
+    (set-default-toplevel-value 'lexical-binding t)
+
 +++
 ** New functions 'plusp' and 'minusp'.
 They return non-nil if a number is positive or negative, respectively,

lisp/loadup.el

@@ -617,9 +617,9 @@ directory got moved.  This is set to be a pair in the form of:
           (unwind-protect
               (let ((tmp-dump-mode dump-mode)
                     (dump-mode nil)
-                    ;; Set `lexical-binding' to nil by default
+                    ;; Set `lexical-binding' to its default value
                     ;; in the dumped Emacs.
-                    (lexical-binding nil))
+                    (lexical-binding (default-toplevel-value 'lexical-binding)))
                 (if (member tmp-dump-mode '("pdump" "pbootstrap"))
                     (dump-emacs-portable (expand-file-name output invocation-directory))
                   (dump-emacs output (if (eq system-type 'ms-dos)

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);