wasmtime chicken and egg

wasmtime_huh.rs

use wasmtime::{Callable, Engine, Extern, FuncType, HostRef, Module, Store, Trap, Val, ValType};

// The goal of this code is that I need a function invoked by the guest wasm module on the 
// host to be able to access the memory of that instance, and to be able to call additional
// functions on that instance in response.
//
// When I use wasmer, the call signature includes a Ctx, which allows me to manipulate the instance
// memory and call functions.

struct GuestCallable {}
impl Callable for GuestCallable {
    fn call(
        &self,
        params: &[Val],
        results: &mut [Val],
    ) -> ::std::result::Result<(), HostRef<Trap>> {
        // How does this thing access the memory of the instance that invoked it??
        Ok(())
    }
}

fn main() -> {
   let engine = HostRef::new(Engine::default());
        let store = HostRef::new(Store::new(&engine));

        let module = HostRef::new(Module::new(&store, buf).expect("wasm module"));
        let ft = FuncType::new(
            Box::new([ValType::I32, ValType::I32, ValType::I32, ValType::I32]),
            Box::new([ValType::I32]),
        );
        let callable = GuestCallable {};
        let f = wasmtime::Func::new(&store, ft, Rc::new(callable));
        let e = Extern::Func(HostRef::new(f));
        let instance = wasmtime::Instance::new(&store, &module, &[e]).expect("wasm instance");
  
        // At this point I've got a callable that's moved out of scope, so I can't
        // go back and give it a reference to the instance or the instance handle.
}

The requirements for Func::new() are that something implement Callable... would an Arc also implement Callable? Would that even work given that wasmtime's Instance type is !Send and !Sync ?

Unrelated to the chicken and the egg - this extern has no name - how does the guest module get the right import namespace and function name for dispatch?