`CoerceUnsized` or `DispatchFromDyn` was implemented on a struct which contains
more than one field that is being unsized.

Erroneous code example:

```compile_fail,E0375
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized, U: ?Sized> {
    a: i32,
    b: T,
    c: U,
}

// error: Struct `Foo` has more than one unsized field.
impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
```

`CoerceUnsized` is used to coerce structs that have a field that can be unsized,
like a custom `MyBox<T>` being unsized to `MyBox<dyn Trait>`. `DispatchFromDyn`
is used to dispatch from `MyBox<dyn Trait>` to `MyBox<Self>` in a dyn-compatible
trait.

If the struct has multiple fields that must be unsized, then the compiler has no
way to generate a valid implementation of `CoerceUnsized` or `DispatchFromDyn`.

Note that `CoerceUnsized` and `DispatchFromDyn` is mainly used by smart pointers
like `Box`, `Rc` and `Arc` to be able to mark that they can coerce unsized types
that they are pointing at.