The key people behind Rust are also on Mozilla's JavaScript team (one of them is Brendan Eich, the creator of JavaScript), and they are active in the ECMA process that's defining ES5 and ES Harmony, which are "fixed up" versions of JavaScript. Firefox 4 contains one of the first implementations of many of the improvements in ES5.

But high-performance JavaScript VMs and rendering engines are not written in JavaScript; today they are mostly written in C++. Rust is designed by and targeted at the people implementing the VMs. It's the language they hope to use to implement future JavaScript compilers.