Speed up partial rendering by caching "variable" calculation

This commit speeds up rendering partials by caching the variable name
calculation on the template.  The variable name is based on the "virtual
path" used for looking up the template.  The same virtual path
information lives on the template, so we can just ask the cached
template object for the variable.

This benchmark takes a couple files, so I'll cat them below:

```
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ cat render_benchmark.rb
require "benchmark/ips"
require "action_view"
require "action_pack"
require "action_controller"

class TestController < ActionController::Base
end

TestController.view_paths = [File.expand_path("test/benchmarks")]
controller_view = TestController.new.view_context

result = Benchmark.ips do |x|
  x.report("render") do
    controller_view.render("many_partials")
  end
end
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ cat test/benchmarks/test/_many_partials.html.erb
Looping:
<ul>
<% 100.times do |i| %>
  <%= render partial: "list_item", locals: { i: i } %>
<% end %>
</ul>
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ cat test/benchmarks/test/_list_item.html.erb
<li>Number: <%= i %></li>
```

Benchmark results (master):

```
[aaron@TC ~/g/r/actionview (master)]$ be ruby render_benchmark.rb
Warming up --------------------------------------
              render    41.000  i/100ms
Calculating -------------------------------------
              render    424.269  (± 3.5%) i/s -      2.132k in   5.031455s
```

Benchmark results (this branch):

```
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ be ruby render_benchmark.rb
Warming up --------------------------------------
              render    50.000  i/100ms
Calculating -------------------------------------
              render    521.862  (± 3.8%) i/s -      2.650k in   5.085885s
```

actionview/lib/action_view/renderer/partial_renderer.rb

@@ -295,8 +295,21 @@ def initialize(*)
     end
 
     def render(context, options, block)
-      setup(context, options, block)
-      template = find_partial
+      as = as_variable(options)
+      setup(context, options, as, block)
+
+      if @path
+        if @has_object || @collection
+          @variable, @variable_counter, @variable_iteration = retrieve_variable(@path, as)
+          @template_keys = retrieve_template_keys(@variable)
+        else
+          @template_keys = @locals.keys
+        end
+        template = find_partial(@path, @template_keys)
+        @variable ||= template.variable
+      else
+        template = nil
+      end
 
       @lookup_context.rendered_format ||= begin
         if template && template.formats.first
@@ -359,7 +372,7 @@ def render_partial(view, template)
       # If +options[:partial]+ is a string, then the <tt>@path</tt> instance variable is
       # set to that string. Otherwise, the +options[:partial]+ object must
       # respond to +to_partial_path+ in order to setup the path.
-      def setup(context, options, block)
+      def setup(context, options, as, block)
         @options = options
         @block   = block
 
@@ -382,25 +395,25 @@ def setup(context, options, block)
 
           if @collection
             paths = @collection_data = @collection.map { |o| partial_path(o, context) }
-            @path = paths.uniq.one? ? paths.first : nil
+            if paths.uniq.length == 1
+              @path = paths.first
+            else
+              paths.map! { |path| retrieve_variable(path, as).unshift(path) }
+              @path = nil
+            end
           else
             @path = partial_path(@object, context)
           end
         end
 
+        self
+      end
+
+      def as_variable(options)
         if as = options[:as]
           raise_invalid_option_as(as) unless /\A[a-z_]\w*\z/.match?(as.to_s)
-          as = as.to_sym
+          as.to_sym
         end
-
-        if @path
-          @variable, @variable_counter, @variable_iteration = retrieve_variable(@path, as)
-          @template_keys = retrieve_template_keys
-        else
-          paths.map! { |path| retrieve_variable(path, as).unshift(path) }
-        end
-
-        self
       end
 
       def collection_from_options
@@ -414,8 +427,8 @@ def collection_from_object
         @object.to_ary if @object.respond_to?(:to_ary)
       end
 
-      def find_partial
-        find_template(@path, @template_keys) if @path
+      def find_partial(path, template_keys)
+        find_template(path, template_keys)
       end
 
       def find_template(path, locals)
@@ -511,9 +524,9 @@ def merge_prefix_into_object_path(prefix, object_path)
         end
       end
 
-      def retrieve_template_keys
+      def retrieve_template_keys(variable)
         keys = @locals.keys
-        keys << @variable if @has_object || @collection
+        keys << variable
         if @collection
           keys << @variable_counter
           keys << @variable_iteration

actionview/lib/action_view/template.rb

@@ -128,6 +128,8 @@ class Template
       end
     end
 
+    attr_reader :variable
+
     def initialize(source, identifier, handler, details)
       format = details[:format] || (handler.default_format if handler.respond_to?(:default_format))
 
@@ -138,6 +140,13 @@ def initialize(source, identifier, handler, details)
       @original_encoding = nil
       @locals            = details[:locals] || []
       @virtual_path      = details[:virtual_path]
+
+      @variable = if @virtual_path
+        base = @virtual_path[-1] == "/" ? "" : File.basename(@virtual_path)
+        base =~ /\A_?(.*?)(?:\.\w+)*\z/
+        $1.to_sym
+      end
+
       @updated_at        = details[:updated_at] || Time.now
       @formats           = Array(format).map { |f| f.respond_to?(:ref) ? f.ref : f  }
       @variants          = [details[:variant]]

activerecord/lib/active_record/railties/collection_cache_association_loading.rb

@@ -3,7 +3,7 @@
 module ActiveRecord
   module Railties # :nodoc:
     module CollectionCacheAssociationLoading #:nodoc:
-      def setup(context, options, block)
+      def setup(context, options, as, block)
         @relation = relation_from_options(options)
 
         super