Using my fork [1] (you'll have to build it yourself, sorry), you can create Emscripten bins. After building your program, run `$NACL_SDK_ROOT/toolchain/linux_pnacl/bin/pnacl-thaw $RUSTC_OUT` on the output (rustc emits stable pexe's by default) and then run `$EMSCRIPTEN/emcc -o $RUSTC_OUT.html $RUSTC_OUT`. Volia.

I make no guarantees that it'll all work however; I don't test Rust->JS like I do Rust->PNaCl.

[1]: https://github.com/DiamondLovesYou/rust.git

EDIT: Grammar.