This is brilliant. Very excited to see more applications like this making great use of the possibilities of Rust+WASM. One thing that stuck out to me was the

This is brilliant. Very excited to see more applications like this making great use of the possibilities of Rust+WASM. One thing that stuck out to me was the exploded-struct callback mechanism for reporting Mappings back to JS. I've also been struggling to handle the low-bandwidth interface between JS and WASM. That wasn't a strategy I'd considered, but it's pretty neat. It's simple enough and will work in this case, but unfortunately doesn't generalize very well. I've been exploring using well-defined binary encoding for this purpose (specifically Cap'n'Proto, but Protobuf or another binary encoding would work, too). See an example I put together: https://github.com/couchand/rust-wasm-capnproto-example https://github.com/couchand/rust-wasm-capnproto-example. I'm definitely going to go back and clean that up with some of the FFI patterns from this article.