Document rendering `:renderable` and `#render_in`

Provide examples for rendering objects that respond to `render_in`. Also
highlight that the object can also define a `#format` method to control
how the rendered String should be treated.

Add test coverage for both Action View's and Action Pack's support for
`render` with `:renderable` options.

actionpack/lib/action_controller/metal/rendering.rb

@@ -39,6 +39,22 @@ module ActionController
     # If the first argument responds to +render_in+, the template will be
     # rendered by calling +render_in+ with the current view context.
     #
+    #   class Greeting
+    #     def render_in(view_context)
+    #       view_context.render html: "<h1>Hello, World</h1>"
+    #     end
+    #
+    #     def format
+    #       :html
+    #     end
+    #   end
+    #
+    #   render(Greeting.new)
+    #   # => "<h1>Hello, World</h1>"
+    #
+    #   render(renderable: Greeting.new)
+    #   # => "<h1>Hello, World</h1>"
+    #
     # ==== \Rendering Mode
     #
     # [+:partial+]
@@ -92,6 +108,14 @@ module ActionController
     #     render json: { hello: "world" }
     #     # => renders "{\"hello\":\"world\"}"
     #
+    # [+:renderable+]
+    #   Renders the provided object by calling +render_in+ with the current view
+    #   context. The response format is determined by calling +format+ on the
+    #   renderable if it responds to +format+, falling back to +text/html+ by default.
+    #
+    #     render renderable: Greeting.new
+    #     # => renders "<h1>Hello, World</h1>"
+    #
     # By default, when a rendering mode is specified, no layout template is
     # rendered.
     #

actionpack/test/controller/renderer_test.rb

@@ -73,6 +73,10 @@ class RendererTest < ActiveSupport::TestCase
       %(Hello, World!),
       renderer.render(TestRenderable.new)
     )
+    assert_equal(
+      %(Hello, World!),
+      renderer.render(renderable: TestRenderable.new)
+    )
   end
 
   test "rendering with custom env" do

actionview/lib/action_view/template/renderable.rb

@@ -17,7 +17,7 @@ module ActionView
       end
 
       def format
-        @renderable.format
+        @renderable.try(:format)
       end
     end
   end

actionview/test/template/render_test.rb

@@ -375,6 +375,11 @@ module RenderTestCases
     assert_equal "NilClass", @view.render(partial: "test/klass", object: nil)
   end
 
+  def test_render_renderable_render_in
+    assert_equal "Hello, World!", @view.render(TestRenderable.new)
+    assert_equal "Hello, World!", @view.render(renderable: TestRenderable.new)
+  end
+
   def test_render_object_different_name
     assert_equal "Hello: t.lo", @view.render(partial: "test/template_not_named_customer", object: Customer.new("t.lo"), as: "customer").chomp
   end

guides/source/layouts_and_rendering.md

@@ -283,18 +283,28 @@ TIP: `send_file` is often a faster and better option if a layout isn't required.
 
 #### Rendering Objects
 
-Rails can render objects responding to `:render_in`.
+Rails can render objects responding to `#render_in`. The format can be controlled by defining `#format` on the object.
 
 ```ruby
-render MyRenderable.new
+class Greeting
+  def render_in(view_context)
+    view_context.render html: "Hello, World"
+  end
+
+  def format
+    :html
+  end
+end
+
+render Greeting.new
+# => "Hello World"
 ```
 
-This calls `render_in` on the provided object with the current view context.
-
-You can also provide the object by using the `:renderable` option to `render`:
+This calls `render_in` on the provided object with the current view context. You can also provide the object by using the `:renderable` option to `render`:
 
 ```ruby
-render renderable: MyRenderable.new
+render renderable: Greeting.new
+# => "Hello World"
 ```
 
 #### Options for `render`