3 files changed, 68 insertions, 42 deletions

diff --git a/doc/lispref/variables.texi b/doc/lispref/variables.texi
index f0e3f337a69..c29547d00db 100644
--- a/doc/lispref/variables.texi
+++ b/doc/lispref/variables.texi
@@ -527,10 +527,11 @@ If @var{symbol} has a buffer-local binding in the current buffer,
 rather than the buffer-local binding.  It sets the default value if
 the default value is void.  @xref{Buffer-Local Variables}.
 
-If @var{symbol} is already lexically bound (e.g., if the @code{defvar}
-form occurs in a @code{let} form with lexical binding enabled), then
-@code{defvar} sets the dynamic value.  The lexical binding remains in
-effect until its binding construct exits.  @xref{Variable Scoping}.
+If @var{symbol} is already let bound (e.g., if the @code{defvar}
+form occurs in a @code{let} form), then @code{defvar} sets the toplevel
+default value, like @code{set-default-toplevel-value}.
+The let binding remains in effect until its binding construct exits.
+@xref{Variable Scoping}.
 
 @cindex @code{eval-defun}, and @code{defvar} forms
 @cindex @code{eval-last-sexp}, and @code{defvar} forms
diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index d7140ad9e63..ee530f95d09 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -4943,8 +4943,6 @@ binding slots have been popped."
     (push (nth 1 (nth 1 form)) byte-compile-global-not-obsolete-vars))
   (byte-compile-normal-call form))
 
-(defconst byte-compile-tmp-var (make-symbol "def-tmp-var"))
-
 (defun byte-compile-defvar (form)
   ;; This is not used for file-level defvar/consts.
   (when (and (symbolp (nth 1 form))
@@ -4977,18 +4975,17 @@ binding slots have been popped."
        string
        "third arg to `%s %s' is not a string: %s"
        fun var string))
+    ;; Delegate the actual work to the function version of the
+    ;; special form, named with a "-1" suffix.
     (byte-compile-form-do-effect
-     (if (cddr form)  ; `value' provided
-         ;; Quote with `quote' to prevent byte-compiling the body,
-         ;; which would lead to an inf-loop.
-         `(funcall '(lambda (,byte-compile-tmp-var)
-                      (,fun ,var ,byte-compile-tmp-var ,@(nthcdr 3 form)))
-                   ,value)
-        (if (eq fun 'defconst)
-            ;; This will signal an appropriate error at runtime.
-            `(eval ',form)
-          ;; A simple (defvar foo) just returns foo.
-          `',var)))))
+     (cond
+      ((eq fun 'defconst) `(defconst-1 ',var ,@(nthcdr 2 form)))
+      ((not (cddr form)) `',var) ; A simple (defvar foo) just returns foo.
+      (t `(defvar-1 ',var
+                    ;; Don't eval `value' if `defvar' wouldn't eval it either.
+                    ,(if (macroexp-const-p value) value
+                       `(if (boundp ',var) nil ,value))
+                    ,@(nthcdr 3 form)))))))
 
 (defun byte-compile-autoload (form)
   (and (macroexp-const-p (nth 1 form))
diff --git a/src/eval.c b/src/eval.c
index 08e2dce61e4..c3be1dc12c8 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -756,6 +756,33 @@ value.  */)
   return Qnil;
 }
 
+static Lisp_Object
+defvar (Lisp_Object sym, Lisp_Object initvalue, Lisp_Object docstring, bool eval)
+{
+  Lisp_Object tem;
+
+  CHECK_SYMBOL (sym);
+
+  tem = Fdefault_boundp (sym);
+
+  /* Do it before evaluating the initial value, for self-references.  */
+  Finternal__define_uninitialized_variable (sym, docstring);
+
+  if (NILP (tem))
+    Fset_default (sym, eval ? eval_sub (initvalue) : initvalue);
+  else
+    { /* Check if there is really a global binding rather than just a let
+             binding that shadows the global unboundness of the var.  */
+      union specbinding *binding = default_toplevel_binding (sym);
+      if (binding && EQ (specpdl_old_value (binding), Qunbound))
+        {
+          set_specpdl_old_value (binding,
+                                 eval ? eval_sub (initvalue) : initvalue);
+        }
+    }
+  return sym;
+}
+
 DEFUN ("defvar", Fdefvar, Sdefvar, 1, UNEVALLED, 0,
        doc: /* Define SYMBOL as a variable, and return SYMBOL.
 You are not required to define a variable in order to use it, but
@@ -770,12 +797,10 @@ value.  If SYMBOL is buffer-local, its default value is what is set;
 buffer-local values are not affected.  If INITVALUE is missing,
 SYMBOL's value is not set.
 
-If SYMBOL has a local binding, then this form affects the local
-binding.  This is usually not what you want.  Thus, if you need to
-load a file defining variables, with this form or with `defconst' or
-`defcustom', you should always load that file _outside_ any bindings
-for these variables.  (`defconst' and `defcustom' behave similarly in
-this respect.)
+If SYMBOL is let-bound, then this form does not affect the local let
+binding but the toplevel default binding instead, like
+`set-toplevel-default-binding`.
+(`defcustom' behaves similarly in this respect.)
 
 The optional argument DOCSTRING is a documentation string for the
 variable.
@@ -786,7 +811,7 @@ To define a buffer-local variable, use `defvar-local'.
 usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
   (Lisp_Object args)
 {
-  Lisp_Object sym, tem, tail;
+  Lisp_Object sym, tail;
 
   sym = XCAR (args);
   tail = XCDR (args);
@@ -798,24 +823,8 @@ usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
       if (!NILP (XCDR (tail)) && !NILP (XCDR (XCDR (tail))))
         error ("Too many arguments");
       Lisp_Object exp = XCAR (tail);
-
-      tem = Fdefault_boundp (sym);
       tail = XCDR (tail);
-
-      /* Do it before evaluating the initial value, for self-references.  */
-      Finternal__define_uninitialized_variable (sym, CAR (tail));
-
-      if (NILP (tem))
-        Fset_default (sym, eval_sub (exp));
-      else
-        { /* Check if there is really a global binding rather than just a let
-             binding that shadows the global unboundness of the var.  */
-          union specbinding *binding = default_toplevel_binding (sym);
-          if (binding && EQ (specpdl_old_value (binding), Qunbound))
-            {
-              set_specpdl_old_value (binding, eval_sub (exp));
-            }
-        }
+      return defvar (sym, exp, CAR (tail), true);
     }
   else if (!NILP (Vinternal_interpreter_environment)
            && (SYMBOLP (sym) && !XSYMBOL (sym)->u.s.declared_special))
@@ -834,6 +843,14 @@ usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
   return sym;
 }
 
+DEFUN ("defvar-1", Fdefvar_1, Sdefvar_1, 2, 3, 0,
+       doc: /* Like `defvar' but as a function.
+More specifically behaves like (defvar SYM 'INITVALUE DOCSTRING).  */)
+  (Lisp_Object sym, Lisp_Object initvalue, Lisp_Object docstring)
+{
+  return defvar (sym, initvalue, docstring, false);
+}
+
 DEFUN ("defconst", Fdefconst, Sdefconst, 2, UNEVALLED, 0,
        doc: /* Define SYMBOL as a constant variable.
 This declares that neither programs nor users should ever change the
@@ -863,9 +880,18 @@ usage: (defconst SYMBOL INITVALUE [DOCSTRING])  */)
         error ("Too many arguments");
       docstring = XCAR (XCDR (XCDR (args)));
     }
+  tem = eval_sub (XCAR (XCDR (args)));
+  return Fdefconst_1 (sym, tem, docstring);
+}
 
+DEFUN ("defconst-1", Fdefconst_1, Sdefconst_1, 2, 3, 0,
+       doc: /* Like `defconst' but as a function.
+More specifically, behaves like (defconst SYM 'INITVALUE DOCSTRING).  */)
+  (Lisp_Object sym, Lisp_Object initvalue, Lisp_Object docstring)
+{
+  CHECK_SYMBOL (sym);
+  Lisp_Object tem = initvalue;
   Finternal__define_uninitialized_variable (sym, docstring);
-  tem = eval_sub (XCAR (XCDR (args)));
   if (!NILP (Vpurify_flag))
     tem = Fpurecopy (tem);
   Fset_default (sym, tem);      /* FIXME: set-default-toplevel-value? */
@@ -4333,9 +4359,11 @@ alist of active lexical bindings.  */);
   defsubr (&Sdefault_toplevel_value);
   defsubr (&Sset_default_toplevel_value);
   defsubr (&Sdefvar);
+  defsubr (&Sdefvar_1);
   defsubr (&Sdefvaralias);
   DEFSYM (Qdefvaralias, "defvaralias");
   defsubr (&Sdefconst);
+  defsubr (&Sdefconst_1);
   defsubr (&Sinternal__define_uninitialized_variable);
   defsubr (&Smake_var_non_special);
   defsubr (&Slet);