Change (apply 'nconc (mapcar ...)) to (mapcan ...); warn about first form. lisp/ChangeLog addition: 2010-09-16 Aidan Kehoe <kehoea@parhasard.net> * byte-optimize.el (byte-optimize-apply): Transform (apply 'nconc (mapcar ...)) to (mapcan ...); warn about use of the first idiom. * update-elc.el (do-autoload-commands): * packages.el (packages-find-package-library-path): * frame.el (frame-list): * extents.el (extent-descendants): * etags.el (buffer-tag-table-files): * dumped-lisp.el (preloaded-file-list): * device.el (device-list): * bytecomp-runtime.el (proclaim-inline, proclaim-notinline) Use #'mapcan, not (apply #'nconc (mapcar ...) in all these files. * bytecomp-runtime.el (eval-when-compile, eval-and-compile): In passing, mention that these macros also evaluate the body when interpreted.

--- a/lisp/dumped-lisp.el	Thu Sep 16 13:36:03 2010 +0100
+++ b/lisp/dumped-lisp.el	Thu Sep 16 13:51:49 2010 +0100
@@ -300,7 +300,4 @@
 	))
 
 (setq preloaded-file-list
-      (apply #'nconc
-	     (mapcar #'(lambda (x)
-			 (if (listp x) x (list x)))
-		     preloaded-file-list)))
+      (mapcan #'(lambda (x) (if (listp x) x (list x))) preloaded-file-list))