rust/src/librustc_error_codes/error_codes/E0720.md

An impl Trait type expands to a recursive type.

An impl Trait type must be expandable to a concrete type that contains no impl Trait types. For example the following example tries to create an impl Trait type T that is equal to [T, T]:

fn make_recursive_type() -> impl Sized {
    [make_recursive_type(), make_recursive_type()]
}