For more up2date information: http://www.hellorust.com/emscripten/
Emscripten is the LLVM-based project to compile stuff into JavaScript. Luckily, Rust already uses LLVM and just recently got an upgrade to the used version of LLVM. Now emscripten uses a fork of LLVM, which is on the way to use more or less the same base LLVM as Rust. See the next-merge branch.