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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).
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