Wasm, which WebAssembly is now commonly called, is a binary format meant to complement JavaScript that Rust can be compiled to. The format is designed to run as a stack machine inside several sandboxed execution environments (such as web browsers, or the Node.js runtime) for performance-critical applications (https://blog.x5ff.xyz/blog/azure-functions-wasm-rust/). While this is—as of this writing—in its early stages, Rust and the Wasm target have been used in real-time frontend settings (such as browser games), and in 2018 there was a dedicated working group seeking to improve this integration.
Similar to other targets, such as ARM, the Wasm target is an LLVM (the compiler technology Rust is built on) backend so it has to be installed using rustup target add wasm32-unknown-unknown. Furthermore, it isn't necessary to declare the binary layout (the "C" in extern "C") and a different bindgen tool does the rest: wasm-bindgen, available at https...
