# Error code E0374

`CoerceUnsized` or `DispatchFromDyn` was implemented on a struct which does not contain a field that is being unsized.

Example of erroneous code:

    #![allow(unused)]
    #![feature(coerce_unsized)]
    fn main() {
    use std::ops::CoerceUnsized;

    struct Foo<T: ?Sized> {
        a: i32,
    }

    // error: Struct `Foo` has no unsized fields that need to be coerced.
    impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
        where T: CoerceUnsized<U> {}
    }

`CoerceUnsized` is used to coerce structs that have a field that can be unsized, like a custom `MyBox<T>` being unsized to `MyBox<dyn Trait>`. `DispatchFromDyn` is used to dispatch from `MyBox<dyn Trait>` to `MyBox<Self>` in a dyn-compatible trait.

If the struct doesn't have any fields of unsized types then there is no meaningful way to implement `CoerceUnsized` or `DispatchFromDyn`, since there is no coercion taking place.

Note that `CoerceUnsized` and `DispatchFromDyn` is mainly used by smart pointers like `Box`, `Rc` and `Arc` to be able to mark that they can coerce unsized types that they are pointing at.