Error code E0224

A trait object was declared with no traits.

Erroneous code example:

#![allow(unused)]
fn main() {
type Foo = dyn 'static +;
}

Rust does not currently support this.

To solve, ensure that the trait object has at least one trait:

#![allow(unused)]
fn main() {
type Foo = dyn 'static + Copy;
}