This is actually unlikely to be Rust's solution (at least in the long term), because it requires running LLVM IR through emscripten to generate asm.js to genera

This is actually unlikely to be Rust's solution (at least in the long term), because it requires running LLVM IR through emscripten to generate asm.js to generate wasm. Making Rust and emscripten use the same LLVM IR is difficult because emscripten has a large out-of-tree LLVM patch that Rust would need to share. The wasm backend though is in upstream LLVM, so rustc can use it directly, bypassing emscripten for translation of Rust.