If you mean producing a wasm module without touching third-party tooling, then this can already be done with Rust today.

The standard rust compiler is capable of cross-compiling out of the box, so the only thing you need in order to produce a working wasm module are the standard libraries which can be obtained from rust-lang.org.

I wrote a somewhat detailed guide about the process here: https://www.reddit.com/r/rust/comments/9t95fd/howto_setting_...