Add ExcerptDrop and remove excerpt's ability to refer to itself in Liquid

lib/jekyll/drops/drop.rb

@@ -3,7 +3,7 @@
 module Jekyll
   module Drops
     class Drop < Liquid::Drop
-      NON_CONTENT_METHODS = [:[], :[]=, :inspect, :to_h, :fallback_data].freeze
+      NON_CONTENT_METHODS = [:[], :[]=, :inspect, :to_h, :fallback_data, :to_s].freeze
 
       # Get or set whether the drop class is mutable.
       # Mutability determines whether or not pre-defined fields may be

lib/jekyll/drops/excerpt_drop.rb

@@ -0,0 +1,11 @@
+# encoding: UTF-8
+
+module Jekyll
+  module Drops
+    class ExcerptDrop < DocumentDrop
+      def excerpt
+        nil
+      end
+    end
+  end
+end

lib/jekyll/excerpt.rb

@@ -7,7 +7,7 @@ module Jekyll
     attr_writer   :output
 
     def_delegators :@doc, :site, :name, :ext, :relative_path, :extname,
-                          :render_with_liquid?, :collection, :related_posts
+                          :render_with_liquid?, :collection, :related_posts, :url
 
     # Initialize this Excerpt instance.
     #
@@ -59,10 +59,7 @@ module Jekyll
     end
 
     def to_liquid
-      doc.data['excerpt'] = nil
+      Jekyll::Drops::ExcerptDrop.new(self)
-      @to_liquid ||= doc.to_liquid
-      doc.data['excerpt'] = self
-      @to_liquid
     end
 
     # Returns the shorthand String identifier of this doc.

test/test_excerpt_drop.rb

@@ -0,0 +1,31 @@
+require 'helper'
+
+class TestExcerptDrop < JekyllUnitTest
+  context "an excerpt drop" do
+    setup do
+      @site = fixture_site
+      @site.read
+      @doc = @site.docs_to_write.first
+      @doc_drop = @doc.to_liquid
+      @excerpt = @doc.data['excerpt']
+      @excerpt_drop = @excerpt.to_liquid
+    end
+
+    should "have the right thing" do
+      assert @doc.is_a? Jekyll::Document
+      assert @doc_drop.is_a? Jekyll::Drops::DocumentDrop
+      assert @excerpt.is_a? Jekyll::Excerpt
+      assert @excerpt_drop.is_a? Jekyll::Drops::ExcerptDrop
+    end
+
+    should "not have an excerpt" do
+      assert_nil @excerpt.data['excerpt']
+      assert @excerpt_drop.class.invokable? 'excerpt'
+      assert_nil @excerpt_drop['excerpt']
+    end
+
+    should "inherit values from the document" do
+      assert_equal @excerpt_drop.keys, @doc_drop.keys
+    end
+  end
+end