src/librustc_error_codes/error_codes/E0378.md

   1 The `DispatchFromDyn` trait currently can only be implemented for
   2 builtin pointer types and structs that are newtype wrappers around them
   3 — that is, the struct must have only one field (except for`PhantomData`),
   4 and that field must itself implement `DispatchFromDyn`.
   5
   6 Examples:
   7
   8 ```
   9 #![feature(dispatch_from_dyn, unsize)]
  10 use std::{
  11     marker::Unsize,
  12     ops::DispatchFromDyn,
  13 };
  14
  15 struct Ptr<T: ?Sized>(*const T);
  16
  17 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T>
  18 where
  19     T: Unsize<U>,
  20 {}
  21 ```
  22
  23 ```
  24 #![feature(dispatch_from_dyn)]
  25 use std::{
  26     ops::DispatchFromDyn,
  27     marker::PhantomData,
  28 };
  29
  30 struct Wrapper<T> {
  31     ptr: T,
  32     _phantom: PhantomData<()>,
  33 }
  34
  35 impl<T, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T>
  36 where
  37     T: DispatchFromDyn<U>,
  38 {}
  39 ```
  40
  41 Example of illegal `DispatchFromDyn` implementation
  42 (illegal because of extra field)
  43
  44 ```compile-fail,E0378
  45 #![feature(dispatch_from_dyn)]
  46 use std::ops::DispatchFromDyn;
  47
  48 struct WrapperExtraField<T> {
  49     ptr: T,
  50     extra_stuff: i32,
  51 }
  52
  53 impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
  54 where
  55     T: DispatchFromDyn<U>,
  56 {}
  57 ```