From 1a90d2b6f3243274af347d3cf296f32293f68cc7 Mon Sep 17 00:00:00 2001
From: Stefan Kangas <stefankangas@gmail.com>
Date: Fri, 9 Dec 2022 10:58:22 +0100
Subject: [PATCH] Make checkdoc warn if not using lexical-binding

* lisp/emacs-lisp/checkdoc.el (checkdoc-file-comments-engine):
Warn if there is no lexical-binding cookie.
---
 etc/NEWS                    |  8 ++++++++
 lisp/emacs-lisp/checkdoc.el | 39 +++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/etc/NEWS b/etc/NEWS
index 0c1fdfc454..1ed6a31197 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -76,6 +76,14 @@ using this new option.  (Or set 'display-buffer-alist' directly.)
 After manually editing 'eshell-aliases-file', you can use
 'M-x eshell-read-aliases-list' to load the edited aliases.
 
+** checkdoc
+
+---
+*** New checkdock warning if not using lexical-binding.
+Checkdoc now warns if the first line of an Emacs Lisp file does not
+end with a "-*- lexical-binding: t -*-" cookie.  Customize the user
+option 'checkdoc-lexical-binding-flag' to nil to disable this warning.
+
 
 * New Modes and Packages in Emacs 30.1
 
diff --git a/lisp/emacs-lisp/checkdoc.el b/lisp/emacs-lisp/checkdoc.el
index 3bddb93b64..0a2d9680fe 100644
--- a/lisp/emacs-lisp/checkdoc.el
+++ b/lisp/emacs-lisp/checkdoc.el
@@ -120,6 +120,14 @@
 ;; simple style rules to follow which checkdoc will auto-fix for you.
 ;; `y-or-n-p' and `yes-or-no-p' should also end in "?".
 ;;
+;; Lexical binding:
+;;
+;;   We recommend always using lexical binding in new code, and
+;; converting old code to use it.  Checkdoc warns if you don't have
+;; the recommended string "-*- lexical-binding: t -*-" at the top of
+;; the file.  You can disable this check with the user option
+;; `checkdoc-lexical-binding-flag'.
+;;
 ;; Adding your own checks:
 ;;
 ;;   You can experiment with adding your own checks by setting the
@@ -330,6 +338,12 @@ checkdoc-column-zero-backslash-before-paren
   :type 'boolean
   :version "28.1")
 
+(defcustom checkdoc-lexical-binding-flag t
+  "Non-nil means generate warnings if file is not using lexical binding.
+See Info node `(elisp) Converting to Lexical Binding' for more."
+  :type 'boolean
+  :version "29.1")
+
 ;; This is how you can use checkdoc to make mass fixes on the Emacs
 ;; source tree:
 ;;
@@ -2369,6 +2383,31 @@ checkdoc-file-comments-engine
 	      (point-min) (save-excursion (goto-char (point-min))
 					  (line-end-position))))
 	 nil))
+      (when checkdoc-lexical-binding-flag
+        (setq
+         err
+         ;; Lexical binding cookie.
+         (if (not (save-excursion
+                    (save-restriction
+                      (goto-char (point-min))
+                      (narrow-to-region (point) (pos-eol))
+                      (re-search-forward
+                       (rx "-*-" (* (* nonl) ";")
+                           (* space) "lexical-binding:" (* space) "t" (* space)
+                           (* ";" (* nonl))
+                           "-*-")
+                       nil t))))
+             (let ((pos (save-excursion (goto-char (point-min))
+                                        (goto-char (pos-eol))
+                                        (point))))
+               (if (checkdoc-y-or-n-p "There is no lexical-binding cookie!  Add one?")
+                   (progn
+                     (goto-char pos)
+                     (insert "  -*- lexical-binding: t -*-"))
+                 (checkdoc-create-error
+                  "The first line should end with \"-*- lexical-binding: t -*-\""
+                  pos (1+ pos) t)))
+           nil)))
       (setq
        err
        (or
-- 
2.35.1