Error code E0720

An impl Trait type expands to a recursive type.

Erroneous code example:

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

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