Don't propagate lexical variables into inlined functions

Functions compiled when inlined (thus from inside the optimiser)
mustn't retain the lexical environment of the caller or there will be
tears.  See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg01227.html .

Bug found by Stefan Monnier.

* lisp/emacs-lisp/byte-opt.el (byte-compile-inline-expand):
Bind byte-optimize--lexvars to nil when re-entering the compiler
recursively.
* test/lisp/emacs-lisp/bytecomp-resources/bc-test-alpha.el:
* test/lisp/emacs-lisp/bytecomp-resources/bc-test-beta.el: New files.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-defsubst): New test.

3 files changed, 33 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/bytecomp-resources/bc-test-alpha.el b/test/lisp/emacs-lisp/bytecomp-resources/bc-test-alpha.el
new file mode 100644
index 00000000000..6997d91b26a
--- /dev/null
+++ b/test/lisp/emacs-lisp/bytecomp-resources/bc-test-alpha.el
@@ -0,0 +1,9 @@
+;;; -*- lexical-binding: t -*-
+
+(require 'bc-test-beta)
+
+(defun bc-test-alpha-f (x)
+  (let ((y nil))
+    (list y (bc-test-beta-f x))))
+
+(provide 'bc-test-alpha)
diff --git a/test/lisp/emacs-lisp/bytecomp-resources/bc-test-beta.el b/test/lisp/emacs-lisp/bytecomp-resources/bc-test-beta.el
new file mode 100644
index 00000000000..9205a13d7d5
--- /dev/null
+++ b/test/lisp/emacs-lisp/bytecomp-resources/bc-test-beta.el
@@ -0,0 +1,6 @@
+;;; -*- lexical-binding: t -*-
+
+(defsubst bc-test-beta-f (y)
+  y)
+
+(provide 'bc-test-beta)
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index c9ab3ec1f1b..33413f5a002 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -1312,6 +1312,24 @@ compiled correctly."
                    (funcall f 3))
                  4)))
 
+(declare-function bc-test-alpha-f (ert-resource-file "bc-test-alpha.el"))
+
+(ert-deftest bytecomp-defsubst ()
+  ;; Check that lexical variables don't leak into inlined code.  See
+  ;; https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg01227.html
+
+  ;; First, remove any trace of the functions and package defined:
+  (fmakunbound 'bc-test-alpha-f)
+  (fmakunbound 'bc-test-beta-f)
+  (setq features (delq 'bc-test-beta features))
+  ;; Byte-compile one file that uses a function from another file that isn't
+  ;; compiled.
+  (let ((file (ert-resource-file "bc-test-alpha.el"))
+        (load-path (cons (ert-resource-directory) load-path)))
+    (byte-compile-file file)
+    (load-file (concat file "c"))
+    (should (equal (bc-test-alpha-f 'a) '(nil a)))))
+
 ;; Local Variables:
 ;; no-byte-compile: t
 ;; End: