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# Error code E0378

The `DispatchFromDyn` trait was implemented on something which is not a pointer or a newtype wrapper around a pointer.

Erroneous code example:

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

    struct WrapperExtraField<T> {
        ptr: T,
        extra_stuff: i32,
    }

    impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
    where
        T: DispatchFromDyn<U>,
    {}
    }

The `DispatchFromDyn` trait currently can only be implemented for builtin pointer types and structs that are newtype wrappers around them — that is, the struct must have only one field (except for `PhantomData`), and that field must itself implement `DispatchFromDyn`.

    #![allow(unused)]
    #![feature(dispatch_from_dyn, unsize)]
    fn main() {
    use std::{
        marker::Unsize,
        ops::DispatchFromDyn,
    };

    struct Ptr<T: ?Sized>(*const T);

    impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T>
    where
        T: Unsize<U>,
    {}
    }

Another example:

    #![allow(unused)]
    #![feature(dispatch_from_dyn)]
    fn main() {
    use std::{
        ops::DispatchFromDyn,
        marker::PhantomData,
    };

    struct Wrapper<T> {
        ptr: T,
        _phantom: PhantomData<()>,
    }

    impl<T, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T>
    where
        T: DispatchFromDyn<U>,
    {}
    }