I think this is actually most interesting as a demonstration of Cargo's support for "features" (http://doc.crates.io/manifest.html#the-%5Bfeatures%5D-sectio...), which is a concept I haven't encountered in other package managers before. Is it something that Cargo inherited from Bundler?
Though it's not a package manager, configure scripts have had this for a while -- e.g. ./configure --no-git. Still, it's certainly cool to have it built into Cargo!
