Constant-propagate access to captured variables

* lisp/emacs-lisp/byte-opt.el (byte-optimize--substitutable-p):
Treat (internal-get-closed-var N) as constants for propagation
purposes, because that is effectively what such forms will be compiled
to.  This allows for the elimination of some lexical variables.

* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test case.

2 files changed, 12 insertions, 2 deletions

diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index f6db803b78e..2bdf1f55111 100644
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -342,8 +342,12 @@ for speeding up processing.")
       (numberp expr)
       (stringp expr)
       (and (consp expr)
-           (memq (car expr) '(quote function))
-           (symbolp (cadr expr)))
+           (or (and (memq (car expr) '(quote function))
+                    (symbolp (cadr expr)))
+               ;; (internal-get-closed-var N) can be considered constant for
+               ;; const-prop purposes.
+               (and (eq (car expr) 'internal-get-closed-var)
+                    (integerp (cadr expr)))))
       (keywordp expr)))
 
 (defmacro byte-optimize--pcase (exp &rest cases)
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 7e51f820b70..a442eb473be 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -686,6 +686,12 @@ inner loops respectively."
                  (let* ((x 'a))
                    (list x (funcall g) (funcall h)))))))
       (funcall (funcall f 'b)))
+
+    ;; Test constant-propagation of access to captured variables.
+    (let* ((x 2)
+           (f (lambda ()
+                (let ((y x)) (list y 3 y)))))
+      (funcall f))
     )
   "List of expressions for cross-testing interpreted and compiled code.")