`impl Trait` types cannot appear nested in the
generic arguments of other `impl Trait` types.

Example of erroneous code:

```compile_fail,E0666
trait MyGenericTrait<T> {}
trait MyInnerTrait {}

fn foo(bar: impl MyGenericTrait<impl MyInnerTrait>) {}
```

Type parameters for `impl Trait` types must be
explicitly defined as named generic parameters:

```
trait MyGenericTrait<T> {}
trait MyInnerTrait {}

fn foo<T: MyInnerTrait>(bar: impl MyGenericTrait<T>) {}
```