> What about replacing clientside javascript with Rust? 

This is, in theory, possible with emscripten or, someday, WebAssembly. See http://myth.aaronlindsay.com/test/ as an example of Hematite, a Minecraft renderer, compiled with Emscripten.

Doing this isn't exactly easy at the moment, as Rust and Emscripten haven't been using the same LLVM versions for a long time, but will eventually be pretty easy to do.

I still plan on just writing JavaScript, personally.