Clear let/subject memoization when referenced in before(:all).

lib/rspec/core/example_group.rb

@@ -299,11 +299,26 @@ def self.before_all_ivars
       # @private
       def self.store_before_all_ivars(example_group_instance)
         return if example_group_instance.instance_variables.empty?
+
         example_group_instance.instance_variables.each { |ivar|
-          before_all_ivars[ivar] = example_group_instance.instance_variable_get(ivar)
+          value = example_group_instance.instance_variable_get(ivar)
+
+          if ivar.to_sym == :@__memoized
+            warn_about_unsound_let_usage(value)
+          else
+            before_all_ivars[ivar] = value
+          end
         }
       end
 
+      def self.warn_about_unsound_let_usage(let_hash)
+        called_lets = let_hash.keys.map { |l| "`#{l}`" }.join(' and ')
+
+        ::RSpec.warn_deprecation "WARNING: let declaration(s) #{called_lets} referenced " +
+          "in a `before(:all)` hook. The memoized value(s) will be discarded since this " +
+          "is outside the scope of an example."
+      end
+
       # @private
       def self.assign_before_all_ivars(ivars, example_group_instance)
         ivars.each { |ivar, val| example_group_instance.instance_variable_set(ivar, val) }

spec/rspec/core/memoized_helpers_spec.rb

@@ -120,6 +120,47 @@ def subject_value_for(describe_arg, &block)
 
           expect(subject_value).to eq([4, 5, 6, :override])
         end
+
+        context 'when referenced in a `before(:all)` hook' do
+          before do
+            expect(::RSpec).to respond_to(:warn_deprecation)
+            ::RSpec.stub(:warn_deprecation)
+          end
+
+          def define_and_run_group
+            final_subject_value_in_before_all = subject_value_in_example = nil
+
+            ExampleGroup.describe do
+              subject { [1, 2] }
+
+              before(:all) do
+                subject << 3
+                final_subject_value_in_before_all = subject
+              end
+
+              example { subject_value_in_example = subject }
+            end.run
+
+            return final_subject_value_in_before_all, subject_value_in_example
+          end
+
+          it 'memoizes the value within the before(:all) hook' do
+            value, _ = define_and_run_group
+            expect(value).to eq([1, 2, 3])
+          end
+
+          it 'clears the memoization before the first example' do
+            _, value = define_and_run_group
+            expect(value).to eq([1, 2])
+          end
+
+          it 'prints a warning since `subject` declarations are not intended to be used in :all hooks' do
+            ::RSpec.should_receive(:warn_deprecation).
+               with(/let declaration\(s\) `subject` referenced in a `before\(:all\)`/)
+
+            define_and_run_group
+          end
+        end
       end
 
       describe "with a name" do
@@ -466,6 +507,47 @@ def count
         expect(value).to eq(:late_exit)
       end
     end
+
+    context 'when referenced in a `before(:all)` hook' do
+      before do
+        expect(::RSpec).to respond_to(:warn_deprecation)
+        ::RSpec.stub(:warn_deprecation)
+      end
+
+      def define_and_run_group
+        final_list_value_in_before_all = list_value_in_example = nil
+
+        ExampleGroup.describe do
+          let(:list) { [1, 2] }
+
+          before(:all) do
+            list << 3
+            final_list_value_in_before_all = list
+          end
+
+          example { list_value_in_example = list }
+        end.run
+
+        return final_list_value_in_before_all, list_value_in_example
+      end
+
+      it 'memoizes the value within the before(:all) hook' do
+        value, _ = define_and_run_group
+        expect(value).to eq([1, 2, 3])
+      end
+
+      it 'clears the memoization before the first example' do
+        _, value = define_and_run_group
+        expect(value).to eq([1, 2])
+      end
+
+      it 'prints a warning since `let` declarations are not intended to be used in :all hooks' do
+        ::RSpec.should_receive(:warn_deprecation).
+           with(/let declaration\(s\) `list` referenced in a `before\(:all\)`/)
+
+        define_and_run_group
+      end
+    end
   end
 
   describe "#let!" do
@@ -510,3 +592,4 @@ def count
     end
   end
 end
+