$ bin/rbx -d -P gem --backtrace install rails
...
Exception: `EEXIST' kernel/common/errno.rb:26 - File exists - File exists - /usr
Exception: `EEXIST' kernel/common/errno.rb:26 - File exists - File exists - /usr/local
Exception: `EEXIST' kernel/common/errno.rb:26 - File exists - File exists - /usr/local/lib
Exception: `EACCES' kernel/common/errno.rb:26 - Permission denied - Permission denied - /usr/local/lib/rubinius
Exception: `ENOENT' kernel/common/errno.rb:26 - No such file or directory - No such file or directory - /usr/local/lib/rubinius
Exception: `EACCES' /home/pierre/Source/Ruby/rubinius/lib/fileutils.rb:219 - Permission denied - Permission denied - /usr/local/lib/rubinius
Exception: `NameError' kernel/common/module.rb:592 - Missing or uninitialized constant: EACCESS
ERROR:  While executing gem ... (NameError)
    Missing or uninitialized constant: EACCESS
	kernel/common/module.rb:592:in `const_missing'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/installer.rb:120:in `initialize'
	kernel/bootstrap/class.rb:18:in `new'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/dependency_installer.rb:236:in `install'
	kernel/common/array.rb:580:in `each'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/dependency_installer.rb:219:in `install'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/commands/install_command.rb:100:in `execute'
	kernel/common/array.rb:580:in `each'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/commands/install_command.rb:84:in `execute'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/command.rb:137:in `invoke'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/command_manager.rb:105:in `process_args'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/command_manager.rb:78:in `run'
	/home/pierre/Source/Ruby/rubinius/lib/rubygems/gem_runner.rb:39:in `run'
	/home/pierre/Source/Ruby/rubinius/lib/bin/gem.rb:24
	kernel/common/compiled_method.rb:253:in `activate_as_script'
	kernel/common/compiled_method.rb:246:in `as_script'
	kernel/common/compile.rb:241:in `single_load'
	kernel/common/compile.rb:103:in `unified_load'
	kernel/common/compile.rb:397:in `load'
	kernel/loader.rb:222
Exception: `Gem::SystemExitException' /home/pierre/Source/Ruby/rubinius/lib/rubygems/user_interaction.rb:237 - Exiting RubyGems with exit_code 1
Exception: `SystemExit' kernel/common/kernel.rb:183 - SystemExit
Exception: `SystemExit' kernel/common/compile.rb:247 - SystemExit