[Haml] Have form_for and sometimes form_tag return ErrorReturns.

doc-src/HAML_CHANGELOG.md

@@ -96,6 +96,13 @@ of the many and varied [Haml implementations](http://en.wikipedia.org/wiki/Haml#
 Haml now supports a {file:HAML_REFERENCE.md#css-filter `:css` filter}
 that surrounds the filtered text with `<style>` and CDATA tags.
 
+### Rails Support
+
+* When `form_for` is used with `=`, or `form_tag` is used with `=` and a block,
+  they will now raise errors explaining that they should be used with `-`.
+  This is similar to how {Haml::Helpers#haml\_concat} behaves,
+  and will hopefully clear up some difficult bugs for some users.
+
 ### `html2haml` Improvements
 
 * Ruby blocks within ERB are now supported.

lib/haml/helpers/action_view_mods.rb

@@ -146,7 +146,10 @@ module ActionView
             concat haml_indent
           end
           res = form_tag_without_haml(url_for_options, options, *parameters_for_url, &proc) + "\n"
-          concat "\n" if block_given?
+          if block_given?
+            concat "\n"
+            return Haml::Helpers::ErrorReturn.new("form_tag")
+          end
           res
         else
           form_tag_without_haml(url_for_options, options, *parameters_for_url, &proc)
@@ -170,6 +173,7 @@ module ActionView
         end
         form_for_without_haml(object_name, *args, &proc)
         concat "\n" if block_given? && is_haml?
+        Haml::Helpers::ErrorReturn.new("form_for") if is_haml?
       end
       alias_method :form_for_without_haml, :form_for
       alias_method :form_for, :form_for_with_haml

test/haml/template_test.rb

@@ -184,6 +184,24 @@ class TemplateTest < Test::Unit::TestCase
     assert_equal("2\n", render("= 1+1"))
   end
 
+  def test_form_for_error_return
+    assert_raise(Haml::Error) { render(<<HAML) }
+= form_for :article, @article, :url => '' do |f|
+  Title:
+  = f.text_field :title
+  Body:
+  = f.text_field :body
+HAML
+  end
+
+  def test_form_tag_error_return
+    assert_raise(Haml::Error) { render(<<HAML) }
+= form_tag '' do
+  Title:
+  Body:
+HAML
+  end
+
   def test_haml_options
     old_options = Haml::Template.options.dup
     Haml::Template.options[:suppress_eval] = true