Error code E0437

An associated type whose name does not match any of the associated types in the trait was used when implementing the trait.

Erroneous code example:

#![allow(unused)]
fn main() {
trait Foo {}

impl Foo for i32 {
    type Bar = bool;
}
}

Trait implementations can only implement associated types that are members of the trait in question.

The solution to this problem is to remove the extraneous associated type:

#![allow(unused)]
fn main() {
trait Foo {}

impl Foo for i32 {}
}