* progmodes/ruby-mode.el (ruby-encoding-map): Add a mapping from

`japanese-cp932' to `cp932' to fix the problem where saving a
source file written in Shift_JIS twice would end up having
`coding: japanese-cp932' which Ruby could not recognize.
(ruby-mode-set-encoding): Add support for encodings mapped to nil
in `ruby-encoding-map'.
(ruby-encoding-map): Map `us-ascii' to nil by default, meaning it
doesn't need to be explicitly declared in magic comment.
(ruby-encoding-map): Add type declaration for better customize UI.

lisp/ChangeLog

@@ -1,3 +1,15 @@
+2013-10-14  Akinori MUSHA  <knu@iDaemons.org>
+
+	* progmodes/ruby-mode.el (ruby-encoding-map): Add a mapping from
+	`japanese-cp932' to `cp932' to fix the problem where saving a
+	source file written in Shift_JIS twice would end up having
+	`coding: japanese-cp932' which Ruby could not recognize.
+	(ruby-mode-set-encoding): Add support for encodings mapped to nil
+	in `ruby-encoding-map'.
+	(ruby-encoding-map): Map `us-ascii' to nil by default, meaning it
+	doesn't need to be explicitly declared in magic comment.
+	(ruby-encoding-map): Add type declaration for better customize UI.
+
 2013-10-13  Glenn Morris  <rgm@gnu.org>
 
 	* progmodes/sh-script.el (sh-mark-line, sh-learn-buffer-indent):
@@ -54,7 +66,7 @@
 2013-10-12  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* progmodes/ruby-mode.el (ruby-smie-grammar): Add rule for paren-free
-	method calls (bug#bug#15594).
+	method calls (bug#15594).
 	(ruby-smie--args-separator-p): New function.
 	(ruby-smie--forward-token, ruby-smie--backward-token): Use it to
 	recognize paren-free method calls.

lisp/progmodes/ruby-mode.el

@@ -217,8 +217,15 @@ Also ignores spaces after parenthesis when 'space."
   "Default deep indent style."
   :options '(t nil space) :group 'ruby)
 
-(defcustom ruby-encoding-map '((shift_jis . cp932) (shift-jis . cp932))
-  "Alist to map encoding name from Emacs to Ruby."
+(defcustom ruby-encoding-map
+  '((us-ascii       . nil)       ;; Do not put coding: us-ascii
+    (shift-jis      . cp932)     ;; Emacs charset name of Shift_JIS
+    (shift_jis      . cp932)     ;; MIME charset name of Shift_JIS
+    (japanese-cp932 . cp932))    ;; Emacs charset name of CP932
+  "Alist to map encoding name from Emacs to Ruby.
+Associating an encoding name with nil means it needs not be
+explicitly declared in magic comment."
+  :type '(repeat (cons (symbol :tag "From") (symbol :tag "To")))
   :group 'ruby)
 
 (defcustom ruby-insert-encoding-magic-comment t
@@ -538,26 +545,28 @@ Also ignores spaces after parenthesis when 'space."
         (setq coding-system
               (if coding-system
                   (symbol-name
-                   (or (and ruby-use-encoding-map
-                            (cdr (assq coding-system ruby-encoding-map)))
-                       coding-system))
+                   (if ruby-use-encoding-map
+                       (let ((elt (assq coding-system ruby-encoding-map)))
+                         (if elt (cdr elt) coding-system))
+                     coding-system))
                 "ascii-8bit"))
-        (if (looking-at "^#!") (beginning-of-line 2))
-        (cond ((looking-at "\\s *#.*-\*-\\s *\\(en\\)?coding\\s *:\\s *\\([-a-z0-9_]*\\)\\s *\\(;\\|-\*-\\)")
-               (unless (string= (match-string 2) coding-system)
-                 (goto-char (match-beginning 2))
-                 (delete-region (point) (match-end 2))
-                 (and (looking-at "-\*-")
-                      (let ((n (skip-chars-backward " ")))
-                        (cond ((= n 0) (insert "  ") (backward-char))
-                              ((= n -1) (insert " "))
-                              ((forward-char)))))
-                 (insert coding-system)))
-              ((looking-at "\\s *#.*coding\\s *[:=]"))
-              (t (when ruby-insert-encoding-magic-comment
-                   (insert "# -*- coding: " coding-system " -*-\n"))))
-        (when (buffer-modified-p)
-          (basic-save-buffer-1))))))
+        (when coding-system
+          (if (looking-at "^#!") (beginning-of-line 2))
+          (cond ((looking-at "\\s *#.*-\*-\\s *\\(en\\)?coding\\s *:\\s *\\([-a-z0-9_]*\\)\\s *\\(;\\|-\*-\\)")
+                 (unless (string= (match-string 2) coding-system)
+                   (goto-char (match-beginning 2))
+                   (delete-region (point) (match-end 2))
+                   (and (looking-at "-\*-")
+                        (let ((n (skip-chars-backward " ")))
+                          (cond ((= n 0) (insert "  ") (backward-char))
+                                ((= n -1) (insert " "))
+                                ((forward-char)))))
+                   (insert coding-system)))
+                ((looking-at "\\s *#.*coding\\s *[:=]"))
+                (t (when ruby-insert-encoding-magic-comment
+                     (insert "# -*- coding: " coding-system " -*-\n"))))
+          (when (buffer-modified-p)
+            (basic-save-buffer-1)))))))
 
 (defun ruby-current-indentation ()
   "Return the indentation level of current line."