Pardon me if I'm wrong, but as far as I can tell the WASM multi-memory proposal doesn't exactly use fat pointers. There's simply a new immediate added to the memory instructions which indicates which memory to operate on - so the compiler has to know which ops go to which memory during compilation, which complicates things. That's why I was talking about special syntaxes.
One use-case I was hoping to explore was to expose shared memory between modules while also giving each their own private heap. I think there's also interesting potential for exposing host-side memory as readonly to modules.
Yeah, I really hope so. Would be a shame for Rust to miss out on such a useful feature