Handle MockExpectationError in aggregate_failures (#1392)

When using mocks with aggregated_failures the internal failure list
can leak out and be modified, prevent that by returning a magic value.

lib/rspec/expectations/failure_aggregator.rb

@@ -4,6 +4,21 @@ module Expectations
     class FailureAggregator
       attr_reader :block_label, :metadata
 
+      # @private
+      class AggregatedFailure
+        # @private
+        MESSAGE =
+          'AggregatedFailure: This method caused a failure which has been ' \
+          'supressed to be aggregated into our failure report by returning ' \
+          'this value, further errors can be ignored.'
+
+        def inspect
+          MESSAGE
+        end
+      end
+
+      AGGREGATED_FAILURE = AggregatedFailure.new
+
       def aggregate
         RSpec::Support.with_failure_notifier(self) do
           begin
@@ -48,6 +63,8 @@ def call(failure, options)
         @seen_source_ids[source_id] = true
         assign_backtrace(failure) unless failure.backtrace
         failures << failure
+
+        AGGREGATED_FAILURE
       end
 
     private

spec/rspec/expectations/failure_aggregator_spec.rb

@@ -265,6 +265,31 @@ def expect_error_included_in_aggregated_failure(error)
       end
     end
 
+    describe 'with MockExpectationError' do
+      it 'does not allow modifications of the failure list in the test example' do
+        expect {
+          aggregate_failures do
+            dbl = double
+            array = dbl.get(2)
+            array << :foo
+          end
+        }.to raise_error(
+          be_a(MultipleExpectationsNotMetError).and have_attributes(
+            :failures => [
+              be_a(RSpec::Mocks::MockExpectationError)
+            ],
+            :other_errors => [
+              # Checking only the class name, not the full message, because the hehaviour differes bettern Ruby 2 and 3.
+              # Ruby 3 uses #inspect in NoMethodError, while Ruby 2.x uses format that I can't override:
+              # NoMethodError (undefined method `<<' for #<RSpec::Expectations::FailureAggregator::AggregatedFailure:0x000055e4bac69ba8>)
+              # even if I override #to_s
+              be_a(NoMethodError).and(have_attributes(:message => /AggregatedFailure/))
+            ]
+          )
+        )
+      end
+    end
+
     describe "message formatting" do
       it "enumerates the failures with an index label, the path of each failure and a blank line in between" do
         expect {