Xcode 11 on the Mac App Store
apps.apple.com
apps.apple.com
The only solution I could find was to restart my machine, which seemed to reset whatever mechanism is used for measuring free space.
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h
And nothing I've tried has fixed it.