Further experimentation. Was able to cut the cost of rendering 100 partials in a collection in half.

  To discuss: What are the desired semantics (if any) for layouts in a collection. There are no
  tests for it at present, and I'm not sure if it's needed at all.

  Deprecated on this branch: `object` pointing at the current object in partials. You can still
  use the partial name, or use :as to achieve the same thing. This is obviously up for discussion.

4 files changed, 65 insertions, 20 deletions

diff --git a/actionpack/examples/minimal.rb b/actionpack/examples/minimal.rb
index c4e5ffd36e..90435d0b89 100644
--- a/actionpack/examples/minimal.rb
+++ b/actionpack/examples/minimal.rb
@@ -104,22 +104,22 @@ ActionController::Base.use_accept_header = false
 unless ENV["PROFILE"]
   Runner.run(BasePostController.action(:overhead), N, 'overhead', false)
   Runner.run(BasePostController.action(:index), N, 'index', false)
+  Runner.run(BasePostController.action(:show_template), N, 'template', false)
   Runner.run(BasePostController.action(:partial), N, 'partial', false)
   Runner.run(BasePostController.action(:many_partials), N, 'many_partials', false)
   Runner.run(BasePostController.action(:partial_collection), N, 'collection', false)
   Runner.run(BasePostController.action(:hundred_partials), N, 'hundred_partials', false)
   Runner.run(BasePostController.action(:large_collection), N, 'large_collection', false)
-  Runner.run(BasePostController.action(:show_template), N, 'template', false)
 
   (ENV["M"] || 1).to_i.times do
     Runner.run(BasePostController.action(:overhead), N, 'overhead')
     Runner.run(BasePostController.action(:index), N, 'index')
+    Runner.run(BasePostController.action(:show_template), N, 'template')
     Runner.run(BasePostController.action(:partial), N, 'partial')
     Runner.run(BasePostController.action(:many_partials), N, 'many_partials')
     Runner.run(BasePostController.action(:partial_collection), N, 'collection')
     Runner.run(BasePostController.action(:hundred_partials), N, 'hundred_partials')
     Runner.run(BasePostController.action(:large_collection), N, 'large_collection')
-    Runner.run(BasePostController.action(:show_template), N, 'template')
   end
 else
   Runner.run(BasePostController.action(ENV["PROFILE"].to_sym), N, ENV["PROFILE"])
diff --git a/actionpack/examples/views/_hundred_partials.erb b/actionpack/examples/views/_hundred_partials.erb
new file mode 100644
index 0000000000..15e99c1b68
--- /dev/null
+++ b/actionpack/examples/views/_hundred_partials.erb
@@ -0,0 +1,12 @@
+<% 10.times do %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+  <%= render :partial => '/hello' %>
+<% end %>
\ No newline at end of file
diff --git a/actionpack/lib/action_view/render/partials.rb b/actionpack/lib/action_view/render/partials.rb
index 64f08c447d..2aa3bd7db8 100644
--- a/actionpack/lib/action_view/render/partials.rb
+++ b/actionpack/lib/action_view/render/partials.rb
@@ -207,9 +207,7 @@ module ActionView
       end
 
       def render_collection
-        # Even if no template is rendered, this will ensure that the MIME type
-        # for the empty response is the same as the provided template
-        @options[:_template] = default_template = find_template
+        @options[:_template] = template = find_template
 
         return nil if collection.blank?
 
@@ -217,15 +215,48 @@ module ActionView
           spacer = find_template(@options[:spacer_template]).render(@view, @locals)
         end
 
-        segments = []
+        result = template ? collection_with_template(template) : collection_without_template
+        result.join(spacer)
+      end
+
+      def collection_with_template(template)
+        options = @options
+
+        segments, locals, as = [], @locals, options[:as]
+
+        [].tap do |segments|
+          variable_name = template.variable_name
+          counter_name  = template.counter_name
+          locals[counter_name] = -1
 
-        collection.each_with_index do |object, index|
-          template = default_template || find_template(partial_path(object))
-          @locals[template.counter_name] = index
-          segments << render_template(template, object)
+          collection.each do |object|
+            locals[counter_name] += 1
+            locals[variable_name] = object
+            locals[as] = object if as
+
+            segments << template.render(@view, locals)
+          end
         end
+      end
 
-        segments.join(spacer)
+      def collection_without_template
+        options = @options
+
+        segments, locals, as = [], @locals, options[:as]
+        index, template = -1, nil
+
+        [].tap do |segments|
+          collection.each do |object|
+            template = find_template(partial_path(object))
+            locals[template.counter_name] = (index += 1)
+            locals[template.variable_name] = object
+            locals[as] = object if as
+
+            segments << template.render(@view, locals)
+          end
+
+          @options[:_template] = template
+        end
       end
 
       def render_template(template, object = @object)
diff --git a/actionpack/lib/action_view/test_case.rb b/actionpack/lib/action_view/test_case.rb
index e51744d095..b317b6dc1a 100644
--- a/actionpack/lib/action_view/test_case.rb
+++ b/actionpack/lib/action_view/test_case.rb
@@ -9,14 +9,16 @@ module ActionView
     end
 
     attr_internal :rendered
-    alias_method :_render_template_without_template_tracking, :_render_single_template
-    def _render_single_template(template, local_assigns, &block)
-      if template.respond_to?(:identifier) && template.present?
-        @_rendered[:partials][template] += 1 if template.partial?
-        @_rendered[:template] ||= []
-        @_rendered[:template] << template
-      end
-      _render_template_without_template_tracking(template, local_assigns, &block)
+  end
+
+  class Template
+    alias_method :render_without_tracking, :render
+    def render(view, locals, &blk)
+      rendered = view.rendered
+      rendered[:partials][self] += 1 if partial?
+      rendered[:template] ||= []
+      rendered[:template] << self
+      render_without_tracking(view, locals, &blk)
     end
   end
 
@@ -68,7 +70,7 @@ module ActionView
       def initialize
         @request = ActionController::TestRequest.new
         @response = ActionController::TestResponse.new
-        
+
         @params = {}
         send(:initialize_current_url)
       end