Function relative 

pub fn relative(from: &str, to: &str) -> String

method returns the relative path from from to to based on the current working directory. If from and to each resolve to the same path (after calling resolve() on each), a zero-length string is returned.

assert_eq!(nodejs_path::posix::relative("/data/orandea/test/aaa", "/data/orandea/impl/bbb"), "../../impl/bbb");