I tried installing rcov on my Mac OS 10.5.6
$>sudo gem install rcov
Building native extensions. This could take a while...
ERROR: Error installing rcov:
ERROR: Failed to build gem native extension.
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb install rcov
can't find header files for ruby.
Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/rcov-0.8.1.2.0 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/rcov-0.8.1.2.0/ext/rcovrt/gem_make.out
Really? That sucks. I asked the interweb and I found this post:
OK so I installed Xcode 3 from here: http://developer.apple.com/Tools/xcode/ and then installed mac ports from here: http://www.macports.org/
Everything worked after that! Awesome!