Edit: old title: rand does not compile on osx with rust 1.22.0
It looks like the minimum version of rust 1.22.0 no longer works with rand, due to this dependency change in libc: rust-lang/libc#1212. It's using std::mem::size_of as a constant function, but that was stablized in 1.24.0.
edit: forgot to mention I don't personally care about 1.22, so I wouldn't care if you decided to drop support for it.