failed rdoc building in gentoo emerge

rdoc.log

-[12122]% sudo emerge -uDN rdoc

 * IMPORTANT: 5 news items need reading for repository 'gentoo'.
 * Use eselect news read to view new items.

Calculating dependencies... done!

>>> Verifying ebuild manifests

>>> Emerging (1 of 7) dev-ruby/rdoc-6.0.1::gentoo
 * rdoc-6.0.1.tar.gz BLAKE2B SHA512 size ;-) ...                                                                                 [ ok ]
>>> Unpacking source...
 * Running unpack phase for all ...
>>> Unpacking rdoc-6.0.1.tar.gz to /var/tmp/portage/dev-ruby/rdoc-6.0.1/work/all
>>> Source unpacked in /var/tmp/portage/dev-ruby/rdoc-6.0.1/work
>>> Preparing source in /var/tmp/portage/dev-ruby/rdoc-6.0.1/work ...
 * Running prepare phase for all ...
 * Running source copy phase for ruby25 ...
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/dev-ruby/rdoc-6.0.1/work ...
>>> Source configured.
>>> Compiling source in /var/tmp/portage/dev-ruby/rdoc-6.0.1/work ...
 * Running compile phase for ruby25 ...
Traceback (most recent call last):
        2: from /usr/local/bin/rake:23:in `<main>'
        1: from /usr/lib64/ruby/site_ruby/2.5.0/rubygems.rb:308:in `activate_bin_path'
/usr/lib64/ruby/site_ruby/2.5.0/rubygems.rb:289:in `find_spec_for_exe': can't find gem rake (>= 0.a) with executable rake (Gem::GemNotFoundException)
 * ERROR: dev-ruby/rdoc-6.0.1::gentoo failed (compile phase):
 *   (no error message)
 *
 * Call stack:
 *     ebuild.sh, line  124:  Called src_compile
 *   environment, line 4366:  Called ruby-ng_src_compile
 *   environment, line 4058:  Called _ruby_each_implementation 'each_ruby_compile'
 *   environment, line  477:  Called _ruby_invoke_environment 'ruby25' 'each_ruby_compile'
 *   environment, line  602:  Called each_ruby_compile
 *   environment, line  839:  Called die
 * The specific snippet of code:
 *       ${RUBY} -S rake generate || die
 *
 * If you need support, post the output of `emerge --info '=dev-ruby/rdoc-6.0.1::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=dev-ruby/rdoc-6.0.1::gentoo'`.
!!! When you file a bug report, please include the following information:
GENTOO_VM=  CLASSPATH="" JAVA_HOME=""
JAVACFLAGS="" COMPILER=""
and of course, the output of emerge --info =rdoc-6.0.1
 * The complete build log is located at '/var/tmp/portage/dev-ruby/rdoc-6.0.1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-ruby/rdoc-6.0.1/temp/environment'.
 * Working directory: '/var/tmp/portage/dev-ruby/rdoc-6.0.1/work/ruby25/rdoc-6.0.1'
 * S: '/var/tmp/portage/dev-ruby/rdoc-6.0.1/work/ruby25/rdoc-6.0.1'

>>> Failed to emerge dev-ruby/rdoc-6.0.1, Log file:

>>>  '/var/tmp/portage/dev-ruby/rdoc-6.0.1/temp/build.log'

 * Messages for package dev-ruby/rdoc-6.0.1:

 * ERROR: dev-ruby/rdoc-6.0.1::gentoo failed (compile phase):
 *   (no error message)
 *
 * Call stack:
 *     ebuild.sh, line  124:  Called src_compile
 *   environment, line 4366:  Called ruby-ng_src_compile
 *   environment, line 4058:  Called _ruby_each_implementation 'each_ruby_compile'
 *   environment, line  477:  Called _ruby_invoke_environment 'ruby25' 'each_ruby_compile'
 *   environment, line  602:  Called each_ruby_compile
 *   environment, line  839:  Called die
 * The specific snippet of code:
 *       ${RUBY} -S rake generate || die
 *
 * If you need support, post the output of `emerge --info '=dev-ruby/rdoc-6.0.1::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=dev-ruby/rdoc-6.0.1::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/dev-ruby/rdoc-6.0.1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-ruby/rdoc-6.0.1/temp/environment'.
 * Working directory: '/var/tmp/portage/dev-ruby/rdoc-6.0.1/work/ruby25/rdoc-6.0.1'
 * S: '/var/tmp/portage/dev-ruby/rdoc-6.0.1/work/ruby25/rdoc-6.0.1'