Error code E0666

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

Erroneous code example:

trait MyGenericTrait<T> {}
trait MyInnerTrait {}

fn foo(
    bar: impl MyGenericTrait<impl MyInnerTrait>, // error!
) {}

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>, // ok!
) {}