Rust can't run without it's runtime yet, which means the runtime needs to be wrapped into JavaScript code on the Emscripten side. The runtime needs some important parallel and memory functionality and last I heard most of it isn't easily to implement in JavaScript (workers and typed arrays and other HTML5 goodies probably bring it closer to being feasible).
NaCL is a more realistic target - in theory it should be as simple as making the compiler use NaCL's GCC instead of the system's C compiler. However, I don't know about how strict the sandboxing is, there might be certain things that the generated Rust code isn't allowed to do.
NaCL is a more realistic target - in theory it should be as simple as making the compiler use NaCL's GCC instead of the system's C compiler. However, I don't know about how strict the sandboxing is, there might be certain things that the generated Rust code isn't allowed to do.