| The trait `CoerceUnsized` may only be implemented for a coercion between |
| structures with the same definition. |
| |
| Example of erroneous code: |
| |
| ```compile_fail,E0377 |
| #![feature(coerce_unsized)] |
| use std::ops::CoerceUnsized; |
| |
| pub struct Foo<T: ?Sized> { |
| field_with_unsized_type: T, |
| } |
| |
| pub struct Bar<T: ?Sized> { |
| field_with_unsized_type: T, |
| } |
| |
| // error: the trait `CoerceUnsized` may only be implemented for a coercion |
| // between structures with the same definition |
| impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {} |
| ``` |
| |
| When attempting to implement `CoerceUnsized`, the `impl` signature must look |
| like: `impl CoerceUnsized<Type<U>> for Type<T> where T: CoerceUnsized<U>`; |
| the *implementer* and *`CoerceUnsized` type parameter* must be the same |
| type. In this example, `Bar` and `Foo` (even though structurally identical) |
| are *not* the same type and are rejected. Learn more about the `CoerceUnsized` |
| trait and DST coercion in |
| [the `CoerceUnsized` docs](../std/ops/trait.CoerceUnsized.html). |
