rm -rf rdoc; cvs up -dPA; make ruby; ./ruby -Ilib ./bin/rdoc -Y -o rdoc

generates:

[...]
                              zlib.c: [...]
Generating RI...
/Users/ryan/Work/cvs/ruby-lang/ruby/lib/yaml.rb:9:in `require': no  
such file to load -- stringio (LoadError)
         from /Users/ryan/Work/cvs/ruby-lang/ruby/lib/yaml.rb:9
         from /Users/ryan/Work/cvs/ruby-lang/ruby/lib/rdoc/ri/ 
ri_descriptions.rb:1
         from /Users/ryan/Work/cvs/ruby-lang/ruby/lib/rdoc/ri/ 
ri_reader.rb:1
         from /Users/ryan/Work/cvs/ruby-lang/ruby/lib/rdoc/generators/ 
ri_generator.rb:46
         from /Users/ryan/Work/cvs/ruby-lang/ruby/lib/rdoc/rdoc.rb: 
250:in `document'
         from ./bin/rdoc:63