Give a useful error when let or subject called without a block.

This is done by nil chcking the block in let before using it.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>

lib/rspec/core/memoized_helpers.rb

@@ -191,6 +191,7 @@ module ClassMethods
         def let(name, &block)
           # We have to pass the block directly to `define_method` to
           # allow it to use method constructs like `super` and `return`.
+          raise "#let or #subject called without a block" if block.nil?
           MemoizedHelpers.module_for(self).define_method(name, &block)
 
           # Apply the memoization. The method has been defined in an ancestor

spec/rspec/core/memoized_helpers_spec.rb

@@ -524,7 +524,9 @@ def count
     end
 
     it 'raises a useful error when called without a block' do
-      ExampleGroup.describe { let(:list) }
+      expect do
+        ExampleGroup.describe { let(:list) }
+      end.to raise_error(/#let or #subject called without a block/)
     end
 
     let(:a_value) { "a string" }