Move lexical-binding warning from checkdoc to byte-compiler

This warning is much more appropriate for the compiler, since lexical
binding affects what it can reason and warn about, than for checkdoc
as the warning has no bearing to documentation at all.
The move also improves the reach of the warning.

* etc/NEWS: Update.
* lisp/emacs-lisp/checkdoc.el (checkdoc-lexical-binding-flag)
(checkdoc-file-comments-engine): Move warning from here....
* lisp/emacs-lisp/bytecomp.el (byte-compile-file): ...to here.
* test/lisp/emacs-lisp/bytecomp-resources/no-byte-compile.el:
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--unescaped-char-literals)
(bytecomp-tests-function-put, bytecomp-tests--not-writable-directory)
(bytecomp-tests--target-file-no-directory):
Update tests.
(bytecomp-tests--log-from-compilation)
(bytecomp-tests--lexical-binding-cookie): New test.

2 files changed, 36 insertions, 5 deletions

diff --git a/test/lisp/emacs-lisp/bytecomp-resources/no-byte-compile.el b/test/lisp/emacs-lisp/bytecomp-resources/no-byte-compile.el
index 00ad1947507..1de5cf66b66 100644
--- a/test/lisp/emacs-lisp/bytecomp-resources/no-byte-compile.el
+++ b/test/lisp/emacs-lisp/bytecomp-resources/no-byte-compile.el
@@ -1 +1 @@
-;; -*- no-byte-compile: t; -*-
+;; -*- no-byte-compile: t; lexical-binding: t; -*-
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index e644417c3d4..4aa555f1e92 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -1302,6 +1302,30 @@ byte-compiled.  Run with dynamic binding."
        (let ((elc (concat ,file-name-var ".elc")))
          (if (file-exists-p elc) (delete-file elc))))))
 
+(defun bytecomp-tests--log-from-compilation (source)
+  "Compile the string SOURCE and return the compilation log output."
+  (let ((text-quoting-style 'grave)
+        (byte-compile-log-buffer (generate-new-buffer " *Compile-Log*")))
+    (with-current-buffer byte-compile-log-buffer
+      (let ((inhibit-read-only t)) (erase-buffer)))
+    (bytecomp-tests--with-temp-file el-file
+      (write-region source nil el-file)
+      (byte-compile-file el-file))
+    (with-current-buffer byte-compile-log-buffer
+      (buffer-string))))
+
+(ert-deftest bytecomp-tests--lexical-binding-cookie ()
+  (cl-flet ((cookie-warning (source)
+              (string-search
+               "file has no `lexical-binding' directive on its first line"
+               (bytecomp-tests--log-from-compilation source))))
+    (let ((some-code "(defun my-fun () 12)\n"))
+      (should-not (cookie-warning
+                   (concat ";;; -*-lexical-binding:t-*-\n" some-code)))
+      (should-not (cookie-warning
+                   (concat ";;; -*-lexical-binding:nil-*-\n" some-code)))
+      (should (cookie-warning some-code)))))
+
 (ert-deftest bytecomp-tests--unescaped-char-literals ()
   "Check that byte compiling warns about unescaped character
 literals (Bug#20852)."
@@ -1310,7 +1334,9 @@ literals (Bug#20852)."
         (byte-compile-debug t)
         (text-quoting-style 'grave))
     (bytecomp-tests--with-temp-file source
-      (write-region "(list ?) ?( ?; ?\" ?[ ?])" nil source)
+      (write-region (concat ";;; -*-lexical-binding:t-*-\n"
+                            "(list ?) ?( ?; ?\" ?[ ?])")
+                    nil source)
       (bytecomp-tests--with-temp-file destination
         (let* ((byte-compile-dest-file-function (lambda (_) destination))
                (err (should-error (byte-compile-file source))))
@@ -1322,7 +1348,9 @@ literals (Bug#20852)."
                                     "`?\\]' expected!")))))))
     ;; But don't warn in subsequent compilations (Bug#36068).
     (bytecomp-tests--with-temp-file source
-      (write-region "(list 1 2 3)" nil source)
+      (write-region (concat ";;; -*-lexical-binding:t-*-\n"
+                            "(list 1 2 3)")
+                    nil source)
       (bytecomp-tests--with-temp-file destination
         (let ((byte-compile-dest-file-function (lambda (_) destination)))
           (should (byte-compile-file source)))))))
@@ -1330,6 +1358,7 @@ literals (Bug#20852)."
 (ert-deftest bytecomp-tests-function-put ()
   "Check `function-put' operates during compilation."
   (bytecomp-tests--with-temp-file source
+    (insert  ";;; -*-lexical-binding:t-*-\n")
     (dolist (form '((function-put 'bytecomp-tests--foo 'foo 1)
                     (function-put 'bytecomp-tests--foo 'bar 2)
                     (defmacro bytecomp-tests--foobar ()
@@ -1636,7 +1665,8 @@ writable (Bug#44631)."
            (byte-compile-error-on-warn t))
       (unwind-protect
           (progn
-            (write-region "" nil input-file nil nil nil 'excl)
+            (write-region ";;; -*-lexical-binding:t-*-\n"
+                          nil input-file nil nil nil 'excl)
             (write-region "" nil output-file nil nil nil 'excl)
             (set-file-modes input-file #o400)
             (set-file-modes output-file #o200)
@@ -1700,7 +1730,8 @@ mountpoint (Bug#44631)."
     (let* ((default-directory directory)
            (byte-compile-dest-file-function (lambda (_) "test.elc"))
            (byte-compile-error-on-warn t))
-      (write-region "" nil "test.el" nil nil nil 'excl)
+      (write-region  ";;; -*-lexical-binding:t-*-\n"
+                     nil "test.el" nil nil nil 'excl)
       (should (byte-compile-file "test.el"))
       (should (file-regular-p "test.elc"))
       (should (cl-plusp (file-attribute-size