Things will change in Rust over the next couple of days. When you launch Rust you'll be asked if you want to play Rust or play the Experimental Version. You should probably choose Play Rust. During this transition period things might get a bit broke for a few hours. If you find yourself unable to launch Rust please try restarting Steam, try restarting your computer, and try going to Verify Contents in the Properties window for Rust in Steam.