rust/src/librustc_error_codes/error_codes/E0191.md

An associated type wasn't specified for a trait object.

Erroneous code example:

trait Trait {
    type Bar;
}

type Foo = Trait; // error: the value of the associated type `Bar` (from
                  //        the trait `Trait`) must be specified

Trait objects need to have all associated types specified. Please verify that all associated types of the trait were specified and the correct trait was used. Example:

trait Trait {
    type Bar;
}

type Foo = Trait<Bar=i32>; // ok!