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

Example of erroneous code:

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>) {}