https://github.com/thestinger/playpen (whitelist: https://github.com/rust-lang/rust-playpen/blob/master/whitel...)

It'd probably be feasible, but a royal pain to setup.

It would require running rustc through emscripten, which is not feasible; as ELF binaries and .so objects and .rlib files, it’s currently in the vicinity of 150MB, of which 100MB is binaries which I expect would be somewhat larger in JavaScript.