Not necessarily an IDE but Sublime Text + Racer[0] on OS X.<p>[0]: <a href="https://github.com/phildawes/racer" rel="nofollow">https://github.com/phildawes/racer</a>
Vim on OS X with the Rust plugin [1].<p>[1]: <a href="https://github.com/rust-lang/rust.vim" rel="nofollow">https://github.com/rust-lang/rust.vim</a>