At the current moment I would be against it. The language and library is changing too fast. Also, Rust has some other things that make it hard to use for coreutils. For example, Rust programs always call signal (SIGPIPE, SIG_IGN) or equivalent code before main(). There is no stable way to get the longstanding behavior of inheriting the signal action from the parent process [1]. This is quite annoying, but not unique to Rust [2].
[1] https://doc.rust-lang.org/beta/unstable-book/compiler-flags/... [2] https://www.pixelbeat.org/programming/sigpipe_handling.html