[pbrannan@zaphod md]$ cat extconf.rb 
require 'mkmf'
create_makefile('foo')

[pbrannan@zaphod md]$ ruby1.9 -v extconf.rb 
ruby 1.9.0 (2008-01-24 revision 0) [i686-linux]
/usr/local/lib/ruby/1.9.0/mkmf.rb:1090: warning: empty expression
/usr/local/lib/ruby/1.9.0/mkmf.rb:1128: warning: empty expression
creating Makefile

Seems this warning may have been added in 1.9; is it intentional?

[pbrannan@zaphod md]$ ruby1.9 -w -e 'x = true; p((42 unless x))'
-e:1: warning: empty expression
nil
[pbrannan@zaphod md]$ ruby1.9 -w -e 'x = false; p((42 unless x))'
-e:1: warning: empty expression
42
[pbrannan@zaphod md]$ ruby -w -e 'x = true; p((42 unless x))'
nil
[pbrannan@zaphod md]$ ruby -w -e 'x = false; p((42 unless x))'
42

Paul