Show file names in lexbind warnings when loading themes

* lisp/international/mule.el (load-with-code-conversion):
* lisp/custom.el (load-theme): Set 'files--name-of-loading-file'
buffer-locally to record the name of the file being loaded.
* lisp/files.el (files--name-of-loading-file): New buffer-local
variable.
(internal--get-default-lexical-binding): Use it to show the name
of the file from which we load or evaluate Lisp code.  (Bug#81686)

lisp/custom.el

@@ -1335,6 +1335,8 @@ This variable cannot be set in a Custom theme."
   :risky t
   :version "24.1")
 
+(defvar files--name-of-loading-file)
+
 (defun load-theme (theme &optional no-confirm no-enable)
   "Load Custom theme named THEME from its file and possibly enable it.
 The theme file is named THEME-theme.el, in one of the directories
@@ -1397,6 +1399,7 @@ Return t if THEME was successfully loaded, nil otherwise."
            (load (file-name-sans-extension file) nil t nil t))
           ((with-temp-buffer
              (insert-file-contents file)
+             (setq files--name-of-loading-file file)
              (let ((hash (secure-hash 'sha256 (current-buffer))))
                (when (or (member hash custom-safe-themes)
                          (custom-theme-load-confirm hash))

lisp/files.el

@@ -4323,19 +4323,22 @@ all the specified local variables, but ignores any settings of \"mode:\"."
               (push elem file-local-variables-alist)))
           (hack-local-variables-apply))))))
 
+(defvar-local files--name-of-loading-file nil
+  "File name of the file loaded/evaluated in the buffer.")
+
 (defun internal--get-default-lexical-binding (from)
   (let ((mib (lambda (node) (buttonize node (lambda (_) (info node))
                                   nil "mouse-2: Jump to Info node"))))
     (or (and (bufferp from) (zerop (buffer-size from)))
         (and (stringp from)
              (eql 0 (file-attribute-size (file-attributes from))))
-        (let ((source
-               (if (not (and (bufferp from)
-                             (string-match-p "\\` \\*load\\*\\(-[0-9]+\\)?\\'"
-                                             (buffer-name from))
-                             load-file-name))
-                   from
-                 (abbreviate-file-name load-file-name))))
+        (let* ((fname
+                (if (bufferp from)
+                    (buffer-local-value 'files--name-of-loading-file from)))
+               (source
+                (if fname
+                    (abbreviate-file-name fname)
+                  from)))
           (condition-case nil
               (display-warning
                `(files missing-lexbind-cookie

lisp/international/mule.el

@@ -291,6 +291,8 @@ attribute."
 (defvar hack-read-symbol-shorthands-function nil
   "Holds function to compute `read-symbol-shorthands'.")
 
+(defvar files--name-of-loading-file)
+
 (defun load-with-code-conversion (fullname file &optional noerror nomessage
                                            eval-function)
   "Execute a file of Lisp code named FILE whose absolute name is FULLNAME.
@@ -350,7 +352,11 @@ Return t if file exists."
 			       'raw-text)))
 		  (set-buffer-multibyte nil))
 	      ;; Make `kill-buffer' quiet.
-	      (set-buffer-modified-p nil))
+	      (set-buffer-modified-p nil)
+              ;; Record the name of the file in a buffer-local variable
+              ;; that 'internal--get-default-lexical-binding' can use if
+              ;; it needs to show a lexical-binding cookie warning.
+              (setq files--name-of-loading-file fullname))
 	    ;; Have the original buffer current while we eval,
             ;; but consider shorthands of the eval'ed one.
 	    (let ((read-symbol-shorthands shorthands))