Error code E0666

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

Erroneous code example:

#![allow(unused)] fn main() { 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:

#![allow(unused)] fn main() { trait MyGenericTrait<T> {} trait MyInnerTrait {} fn foo<T: MyInnerTrait>( bar: impl MyGenericTrait<T>, // ok! ) {} }