[duped]

I don't really understand the questions. Have you looked at any prior art to see how C++ and Rust handle different string representations? C++ is probably the best influence due to type coercion since it sounds like you care about ergonomics over correctness.

For FFI there's nothing a compiler can do. That's why FFI is unsafe and restricted to rudimentary types in most languages - it's up to the caller to ensure the data is laid out as the callee expects.

I also don't know what interface types have to do with anything. Wasm is far lower level than interfaces, and nothing is stopping you from implementing interfaces in your language and doing automatic type conversion through them to handle string representations as required.

Look past the web for a moment - wasm is a competitor with the JVM, GraalVM, and LLVM as a platform and implementation independent byte code. Think about how your language would be implemented on those targets before the web.