Deal with circular dependency between world and config.

`RSpec::World#initialize` references `RSpec.configuration`
and `RSpec::Configuration` references `RSpec.world` in a few
places. This circular dependency is not ideal and has triggered
infinite recursion during some recent changes I've tried to make.
It would be nice to get rid of it entirely, but for now, this is
the simplest change I can make it solve my immediate problems.

Author: myronmarston

lib/rspec/core/configuration.rb

@@ -343,7 +343,7 @@ def treat_symbols_as_metadata_keys_with_true_values=(_value)
       # @private
       attr_writer :files_to_run
       # @private
-      attr_accessor :filter_manager
+      attr_accessor :filter_manager, :world
       # @private
       attr_accessor :static_config_filter_manager
       # @private
@@ -394,6 +394,7 @@ def initialize
         @derived_metadata_blocks = FilterableItemRepository::QueryOptimized.new(:any?)
         @threadsafe = true
         @max_displayed_failure_line_count = 10
+        @world = World::Null
 
         define_built_in_hooks
       end
@@ -1256,7 +1257,7 @@ def configure_group_with(group, module_list, application_method)
 
       # @private
       def configure_existing_groups(mod, meta, application_method)
-        RSpec.world.all_example_groups.each do |group|
+        world.all_example_groups.each do |group|
           next unless meta.empty? || MetadataFilter.apply?(:any?, meta, group.metadata)
           __send__(application_method, mod, group)
         end
@@ -1352,7 +1353,7 @@ def load_spec_files
         # in that case, the spec file was loaded by `ruby` and
         # isn't loaded by us here so we only know about it because
         # of an example group being registered in it.
-        RSpec.world.registered_example_group_files.each do |f|
+        world.registered_example_group_files.each do |f|
           loaded_spec_files << f # the registered files are already expended absolute paths
         end
 
@@ -1814,7 +1815,7 @@ def define_built_in_hooks
       end
 
       def assert_no_example_groups_defined(config_option)
-        return unless RSpec.world.example_groups.any?
+        return unless world.example_groups.any?
 
         raise MustBeConfiguredBeforeExampleGroupsError.new(
           "RSpec's #{config_option} configuration option must be configured before " \

lib/rspec/core/world.rb

@@ -12,6 +12,7 @@ class World
 
       def initialize(configuration=RSpec.configuration)
         @configuration = configuration
+        configuration.world = self
         @example_groups = []
         @example_group_counts_by_spec_file = Hash.new(0)
         @filtered_examples = Hash.new do |hash, group|
@@ -208,6 +209,14 @@ def fail_if_config_and_cli_options_invalid
           1 # exit code
         )
       end
+
+      # @private
+      # Provides a null implementation for initial use by configuration.
+      module Null
+        def self.registered_example_group_files; []; end
+        def self.example_groups; []; end
+        def self.all_example_groups; []; end
+      end
     end
   end
 end

spec/rspec/core/configuration_spec.rb

@@ -10,6 +10,8 @@ module RSpec::Core
     let(:exclusion_filter) { config.exclusion_filter.rules }
     let(:inclusion_filter) { config.inclusion_filter.rules }
 
+    before { config.world = RSpec.world }
+
     shared_examples_for "warning of deprecated `:example_group` during filtering configuration" do |method, *args|
       it "issues a deprecation warning when filtering by `:example_group`" do
         args << { :example_group => { :file_location => /spec\/unit/ } }