Ensure ERB source begins with the encoding comment

actionpack/lib/action_view/template/handlers/erb.rb

@@ -16,7 +16,9 @@ module ActionView
       self.default_format = Mime::HTML
 
       def compile(template)
-        ::ERB.new("<% __in_erb_template=true %>#{template.source}", nil, erb_trim_mode, '@output_buffer').src
+        magic = $1 if template.source =~ /\A(<%#.*coding:\s*(\S+)\s*-?%>)/
+        erb = "#{magic}<% __in_erb_template=true %>#{template.source}"
+        ::ERB.new(erb, nil, erb_trim_mode, '@output_buffer').src
       end
     end
   end

actionpack/lib/action_view/template/template.rb

@@ -53,7 +53,11 @@ module ActionView
       locals_code = locals.keys.map! { |key| "#{key} = local_assigns[:#{key}];" }.join
 
       code = @handler.call(self)
-      encoding_comment = $1 if code.sub!(/\A(#.*coding.*)\n/, '')
+      if code.sub!(/\A(#.*coding.*)\n/, '')
+        encoding_comment = $1
+      elsif defined?(Encoding) && Encoding.respond_to?(:default_external)
+        encoding_comment = "#coding:#{Encoding.default_external}"
+      end
 
       source = <<-end_src
         def #{method_name}(local_assigns)

actionpack/test/fixtures/test/utf8.html.erb

@@ -1,4 +1,3 @@
-<%# encoding: utf-8 -%>
 Русский текст
 <%= "日".encoding %>
 <%= @output_buffer.encoding %>

actionpack/test/fixtures/test/utf8_magic.html.erb

@@ -0,0 +1,5 @@
+<%# encoding: utf-8 -%>
+Русский текст
+<%= "日".encoding %>
+<%= @output_buffer.encoding %>
+<%= __ENCODING__ %>

actionpack/test/template/render_test.rb

@@ -247,10 +247,27 @@ module RenderTestCases
   end
 
   if '1.9'.respond_to?(:force_encoding)
-    def test_render_utf8_template
-      result = @view.render(:file => "test/utf8.html.erb", :layouts => "layouts/yield")
-      assert_equal "Русский текст\nUTF-8\nUTF-8\nUTF-8\n", result
-      assert_equal Encoding::UTF_8, result.encoding
+    def test_render_utf8_template_with_magic_comment
+      with_external_encoding Encoding::ASCII_8BIT do
+        result = @view.render(:file => "test/utf8_magic.html.erb", :layouts => "layouts/yield")
+        assert_equal "Русский текст\nUTF-8\nUTF-8\nUTF-8\n", result
+        assert_equal Encoding::UTF_8, result.encoding
+      end
+    end
+
+    def test_render_utf8_template_with_default_external_encoding
+      with_external_encoding Encoding::UTF_8 do
+        result = @view.render(:file => "test/utf8.html.erb", :layouts => "layouts/yield")
+        assert_equal "Русский текст\nUTF-8\nUTF-8\nUTF-8\n", result
+        assert_equal Encoding::UTF_8, result.encoding
+      end
+    end
+
+    def with_external_encoding(encoding)
+      old, Encoding.default_external = Encoding.default_external, encoding
+      yield
+    ensure
+      Encoding.default_external = old
     end
   end
 end