$gem install webrick 
Fetching webrick-1.7.0.gem
Successfully installed webrick-1.7.0
Parsing documentation for webrick-1.7.0
Installing ri documentation for webrick-1.7.0
Done installing documentation for webrick after 1 seconds
1 gem installed

$gem install rack
Fetching rack-3.0.0.gem
Successfully installed rack-3.0.0
Parsing documentation for rack-3.0.0
Installing ri documentation for rack-3.0.0
Done installing documentation for rack after 5 seconds
1 gem installed

$gem install rackup
Successfully installed rackup-0.2.2
Parsing documentation for rackup-0.2.2
Installing ri documentation for rackup-0.2.2
Done installing documentation for rackup after 0 seconds
1 gem installed

$rackup -p 4000
C:/msys64/home/yumetodo/foo/config.ru:1:in `block in <main>': uninitialized constant Rack::Directory (NameError)
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rack-3.0.0/lib/rack/builder.rb:106:in `eval'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rack-3.0.0/lib/rack/builder.rb:106:in `new_from_string'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rack-3.0.0/lib/rack/builder.rb:97:in `load_file'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rack-3.0.0/lib/rack/builder.rb:67:in `parse_file'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:351:in `build_app_and_options_from_config'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:260:in `app'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:421:in `wrapped_app'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:323:in `block in start'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:379:in `handle_profiling'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:322:in `start'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/lib/rackup/server.rb:179:in `start'
        from C:/msys64/mingw64/lib/ruby/gems/3.1.0/gems/rackup-0.2.2/bin/rackup:5:in `<top (required)>'
        from C:/msys64/mingw64/bin/rackup:32:in `load'
        from C:/msys64/mingw64/bin/rackup:32:in `<main>'

$gem uninstall "rackup"
Remove executables:
        rackup

in addition to the gem? [Yn]  y
Removing rackup
Successfully uninstalled rackup-0.2.2

$gem install "rack" -v "2.2.4"
Successfully installed rack-2.2.4
Parsing documentation for rack-2.2.4
Installing ri documentation for rack-2.2.4
Done installing documentation for rack after 4 seconds
1 gem installed

$rackup -p 4000
[2022-10-19 16:51:31] INFO  WEBrick 1.7.0
[2022-10-19 16:51:31] INFO  ruby 3.1.2 (2022-04-12) [x64-mingw32]
[2022-10-19 16:51:31] INFO  WEBrick::HTTPServer#start: pid=25280 port=4000