diff --git a/doc-src/HAML_CHANGELOG.md b/doc-src/HAML_CHANGELOG.md
index bbf5d60905..036070a98a 100644
--- a/doc-src/HAML_CHANGELOG.md
+++ b/doc-src/HAML_CHANGELOG.md
@@ -8,6 +8,10 @@
 * Add a railtie so Haml and Sass will be automatically loaded in Rails 3.
   Thanks to [Daniel Neighman](http://pancakestacks.wordpress.com/).
 
+* Add a deprecation message for using `-` with methods like `form_for`
+  that return strings in Rails 3.
+  This is [the same deprecation that exists in Rails 3](http://github.com/rails/rails/commit/9de83050d3a4b260d4aeb5d09ec4eb64f913ba64).
+
 ## 2.2.21
 
 [Tagged on GitHub](http://github.com/nex3/haml/commit/2.2.21).
diff --git a/lib/haml/template/plugin.rb b/lib/haml/template/plugin.rb
index 0a8ce842ea..7a0c5d7546 100644
--- a/lib/haml/template/plugin.rb
+++ b/lib/haml/template/plugin.rb
@@ -33,6 +33,37 @@ def cache_fragment(block, name = {}, options = nil)
       end
     end
   end
+
+  # Rails 3.0 prints a deprecation warning when block helpers
+  # return strings that go unused.
+  # We want to print the same deprecation warning,
+  # so we have to compile in a method call to check for it.
+  #
+  # I don't like having this in the precompiler pipeline,
+  # and I'd like to get rid of it once Rails 3.1 is well-established.
+  if defined?(ActionView::OutputBuffer) &&
+      Haml::Util.has?(:instance_method, ActionView::OutputBuffer, :append_if_string=)
+    module Precompiler
+      def push_silent_with_haml_block_deprecation(text, *args)
+        unless block_opened? && text =~ ActionView::Template::Handlers::Erubis::BLOCK_EXPR
+          return push_silent_without_haml_block_deprecation(text, *args)
+        end
+
+        push_silent_without_haml_block_deprecation("_hamlout.append_if_string= #{text}", *args)
+      end
+      alias_method :push_silent_without_haml_block_deprecation, :push_silent
+      alias_method :push_silent, :push_silent_with_haml_block_deprecation
+    end
+
+    class Buffer
+      def append_if_string=(value)
+        if value.is_a?(String) && !value.is_a?(ActionView::NonConcattingString)
+          ActiveSupport::Deprecation.warn("- style block helpers are deprecated. Please use =", caller)
+          buffer << value
+        end
+      end
+    end
+  end
 end
 
 if defined? ActionView::Template and ActionView::Template.respond_to? :register_template_handler
diff --git a/test/haml/template_test.rb b/test/haml/template_test.rb
index d753327a8d..3fdf701fa8 100755
--- a/test/haml/template_test.rb
+++ b/test/haml/template_test.rb
@@ -250,6 +250,28 @@ def test_exceptions_should_work_correctly
     end
   end
 
+  if defined?(ActionView::OutputBuffer) &&
+      Haml::Util.has?(:instance_method, ActionView::OutputBuffer, :append_if_string=)
+    def test_av_block_deprecation_warning
+      assert_warning(/^DEPRECATION WARNING: - style block helpers are deprecated\. Please use =\./) do
+        assert_equal <<HTML, render(<<HAML, :action_view)
+<form action="" method="post">
+  Title:
+  <input id="article_title" name="article[title]" size="30" type="text" value="Hello" />
+  Body:
+  <input id="article_body" name="article[body]" size="30" type="text" value="World" />
+</form>
+HTML
+- form_for :article, @article, :url => '' do |f|
+  Title:
+  = f.text_field :title
+  Body:
+  = f.text_field :body
+HAML
+      end
+    end
+  end
+
   ## XSS Protection Tests
 
   # In order to enable these, either test against Rails 3.0
diff --git a/test/test_helper.rb b/test/test_helper.rb
index b2147310ee..bf835f8e10 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -24,7 +24,12 @@ def clean_up_sassc
   def assert_warning(message)
     the_real_stderr, $stderr = $stderr, StringIO.new
     yield
-    assert_equal message.strip, $stderr.string.strip
+
+    if message.is_a?(Regexp)
+      assert_match message, $stderr.string.strip
+    else
+      assert_equal message.strip, $stderr.string.strip
+    end
   ensure
     $stderr = the_real_stderr
   end