rust/src/librustc_error_codes/error_codes/E0437.md

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:

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:

trait Foo {}

impl Foo for i32 {}