"the value of the associated type must be specified" when storing a struct with a type argument in another struct

Question

I would like to know how to have the struct Interface as a field in another struct when there is a type argument.

pub struct Interface<'a, 'b, 'c, DeviceT: Device + 'a> {}

pub struct Foo {
    iface: Interface<'static, 'static, 'static, Device + 'static>,
}

pub trait Device {
    type RxBuffer: AsRef<[u8]>;
    type TxBuffer: AsRef<[u8]> + AsMut<[u8]>;
}

This results in the error:

error[E0191]: the value of the associated type `TxBuffer` (from the trait `Device`) must be specified
 --> src/main.rs:4:49
  |
4 |     iface: Interface<'static, 'static, 'static, Device + 'static>,
  |                                                 ^^^^^^^^^^^^^^^^ missing associated type `TxBuffer` value

error[E0191]: the value of the associated type `RxBuffer` (from the trait `Device`) must be specified
 --> src/main.rs:4:49
  |
4 |     iface: Interface<'static, 'static, 'static, Device + 'static>,
  |                                                 ^^^^^^^^^^^^^^^^ missing associated type `RxBuffer` value

What is the correct way of having interface inside Foo?

Answer 1

Your issue is that Device + 'static is not a type, but a bound, so you can't use it as a type argument. You can parameterise the struct with a type parameter that fulfills the bound:

pub struct Foo<D : Device + 'static> {
    iface: Interface<'static, 'static, 'static, D>,  
}

I don't think the compiler will allow for the kind of erasure you might be looking for - consider if we had similar code:

trait Foo<A> {
    fn get_a() -> A;
}

struct Bar {
    foo: Box<Foo<A>>,
}

// How would the compiler describe this type?
let a: ??? = some_bar.foo.get_a();