Steve Purcell's Ruby Flymake support, with stricter warnings.

flymake-ruby.el

;;; A flymake handler for ruby-mode files
;;;
;;; Author: Steve Purcell
;;; Homepage: http://github.com/purcell/emacs.d/blob/master/site-lisp/flymake-ruby/flymake-ruby.el
;;;
;;; Usage:
;;;   (require 'flymake-ruby)
;;;   (add-hook 'ruby-mode-hook 'flymake-ruby-load)


(defvar flymake-ruby-err-line-patterns '(("^\\(.*\\):\\([0-9]+\\): \\(.*\\)$" 1 2 nil 3)))
(defvar flymake-ruby-allowed-file-name-masks '((".+\\.\\(rb\\|rake\\)$" flymake-ruby-init)
                                               ("Rakefile$" flymake-ruby-init)))

;; Not provided by flymake itself, curiously
(defun flymake-create-temp-in-system-tempdir (filename prefix)
  (make-temp-file (or prefix "flymake-ruby")))

;; Invoke ruby with '-c' to get syntax checking
(defun flymake-ruby-init ()
  (list "./rubycheck.sh" (list (flymake-init-create-temp-buffer-copy 'flymake-create-temp-in-system-tempdir))))

(defun flymake-ruby-load ()
  (interactive)
  (unless (eq buffer-file-name nil)
    (set (make-local-variable 'flymake-allowed-file-name-masks) flymake-ruby-allowed-file-name-masks)
    (set (make-local-variable 'flymake-err-line-patterns) flymake-ruby-err-line-patterns)
    (flymake-mode t)))


(provide 'flymake-ruby)

rubycheck.sh

#!/bin/sh

RUBY=`which ruby`
COMMAND="$RUBY -c -w -W2 $*"
OUT=`$COMMAND 2>/dev/null`
ERR=`$COMMAND 2>&1 1>/dev/null`

echo $ERR >&2

[ "$OUT" = "Syntax OK" ] && [ -z "$ERR" ]