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>, {} }