add expect {block}.to[_not] matcher

lib/rspec/expectations/extensions.rb

@@ -1 +1,2 @@
 require 'rspec/expectations/extensions/kernel'
+require 'rspec/expectations/extensions/rspec/core/example_group'

lib/rspec/expectations/extensions/rspec/core/example_group.rb

@@ -0,0 +1,19 @@
+module Rspec
+  module Core
+    class ExampleGroup
+      module BlockAliases
+        alias_method :to,     :should
+        alias_method :to_not, :should_not
+      end
+
+      # Extends the submitted block with aliases to and to_not
+      # for should and should_not. Allows expectations like this:
+      #
+      #   expect { this_block }.to change{this.expression}.from(old_value).to(new_value)
+      #   expect { this_block }.to raise_error
+      def expect(&block)
+        block.extend BlockAliases
+      end
+    end
+  end
+end