Print a warning when users configure RSpec after defining an example …

…group.

lib/rspec/core.rb

@@ -54,12 +54,33 @@ def self.configuration
   end
 
   def self.configure
+    warn_about_deprecated_configure if RSpec.world.example_groups.any?
     yield configuration if block_given?
   end
 
   def self.clear_remaining_example_groups
     world.example_groups.clear
   end
+
+  private
+
+    def self.warn_about_deprecated_configure
+      warn <<-NOTICE
+
+*****************************************************************
+DEPRECATION WARNING: you are using deprecated behaviour that will
+be removed from RSpec 3.
+
+You have set some configuration options after an example group has
+already been defined.  In RSpec 3, this will not be allowed.  All
+configuration should happen before the first example group is
+defined.  The configuration is happening at:
+
+  #{caller[1]}
+*****************************************************************
+
+NOTICE
+    end
 end
 
 require 'rspec/core/backward_compatibility'

spec/rspec/core_spec.rb

@@ -11,13 +11,41 @@
   end
 
   describe "#configure" do
+    around(:each) do |example|
+      RSpec.allowing_configure_warning(&example)
+    end
+
+    before(:each) do
+      RSpec.stub(:warn)
+    end
 
     it "yields the current configuration" do
       RSpec.configure do |config|
         config.should == RSpec::configuration
       end
     end
 
+    context "when an example group has already been defined" do
+      before(:each) do
+        RSpec.world.stub(:example_groups).and_return([stub.as_null_object])
+      end
+
+      it "prints a deprecation warning" do
+        RSpec.should_receive(:warn).with(/configuration should happen before the first example group/)
+        RSpec.configure { |c| }
+      end
+    end
+
+    context "when no examples have been defined yet" do
+      before(:each) do
+        RSpec.world.stub(:example_groups).and_return([])
+      end
+
+      it "does not print a deprecation warning" do
+        RSpec.should_not_receive(:warn)
+        RSpec.configure { |c| }
+      end
+    end
   end
 
   describe "#world" do

spec/spec_helper.rb

@@ -50,6 +50,27 @@ def in_editor?
   ENV.has_key?('TM_MODE') || ENV.has_key?('EMACS') || ENV.has_key?('VIM')
 end
 
+class << RSpec
+  alias_method :original_warn_about_deprecated_configure, :warn_about_deprecated_configure
+
+  def warn_about_deprecated_configure
+    # no-op: in our specs we don't want to see the warning.
+  end
+
+  alias_method :null_warn_about_deprecated_configure, :warn_about_deprecated_configure
+
+  def allowing_configure_warning
+    (class << self; self; end).class_eval do
+      alias_method :warn_about_deprecated_configure, :original_warn_about_deprecated_configure
+      begin
+        yield
+      ensure
+        alias_method :warn_about_deprecated_configure, :null_warn_about_deprecated_configure
+      end
+    end
+  end
+end
+
 RSpec.configure do |c|
   c.color_enabled = !in_editor?
   c.filter_run :focus => true