/usr/lib/ruby/ is in ruby-rspec-mocks 2.5.0-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/spec/
/usr/lib/ruby/vendor_ruby/spec/mocks.rb
/usr/lib/ruby/vendor_ruby/rspec/
/usr/lib/ruby/vendor_ruby/rspec/mocks.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/
/usr/lib/ruby/vendor_ruby/rspec/mocks/argument_matchers.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/standalone.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/method_double.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/extensions/
/usr/lib/ruby/vendor_ruby/rspec/mocks/extensions/instance_exec.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/extensions/marshal.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/errors.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/framework.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/space.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/mock.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/error_generator.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/methods.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/spec_methods.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/serialization.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/argument_expectation.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/proxy.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/message_expectation.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/version.rb
/usr/lib/ruby/vendor_ruby/rspec/mocks/order_group.rb