eval.c: New functions `defvar-1` and `defconst-1` (bug#55156)

The bytecode interpreter can't directly call special forms, so
the byte-compiler usually converts special forms into some sequence of
byte codes (basically, providing a duplicate definition of the special
form).  There are still two exceptions to this: `defconst` and `defvar`,
where the compiler instead generates a convoluted chunk of code like:

    (funcall '(lambda (x) (defvar <sym> x <doc>)) <value>)

where the quote makes sure we keep the function non-compiled, so as
to end up running the special form at run time.

Get rid of this workaround by introducing `defvar-1` and `defconst-1`
which provide a *functional* interface to the functionality of the
corresponding special form.

* src/eval.c (defvar, Fdefvar_1, Fdefconst_1): New functions, extracted from
`Fdefvar` and `Fdefconst`.
(Fdefvar, Fdefconst): Use them.
(syms_of_eval): `defsubr` the new functions.

* lisp/emacs-lisp/bytecomp.el (byte-compile-tmp-var): Delete const.
(byte-compile-defvar): Simplify using the new functions.

* doc/lispref/variables.texi (Defining Variables): Adjust the doc of
`defvar` to reflect the actual semantics implemented.

1 files changed, 10 insertions, 13 deletions

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))