/usr/lib/ruby/ is in ruby-fakefs 0.4.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/fakefs/
/usr/lib/ruby/vendor_ruby/fakefs.rb
/usr/lib/ruby/vendor_ruby/fakefs/base.rb
/usr/lib/ruby/vendor_ruby/fakefs/dir.rb
/usr/lib/ruby/vendor_ruby/fakefs/fake/
/usr/lib/ruby/vendor_ruby/fakefs/fake/dir.rb
/usr/lib/ruby/vendor_ruby/fakefs/fake/file.rb
/usr/lib/ruby/vendor_ruby/fakefs/fake/symlink.rb
/usr/lib/ruby/vendor_ruby/fakefs/file.rb
/usr/lib/ruby/vendor_ruby/fakefs/file_system.rb
/usr/lib/ruby/vendor_ruby/fakefs/file_test.rb
/usr/lib/ruby/vendor_ruby/fakefs/fileutils.rb
/usr/lib/ruby/vendor_ruby/fakefs/safe.rb
/usr/lib/ruby/vendor_ruby/fakefs/spec_helpers.rb
/usr/lib/ruby/vendor_ruby/fakefs/version.rb