Rust Error E0191: Specifying Associated Types for Trait Objects

Understanding Rust Error E0191

Rust Error E0191 occurs when an associated type isn't specified for a trait object. In Rust, traits can have associated types, which allows them to define placeholder types that implementors of the trait must define themselves.

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

In this example, the trait `Trait` has an associated type called `Bar`. The error arises where the type alias `Foo` is set to `Trait`, but no associated type `Bar` is specified.

Resolving Rust Error E0191

To fix this error, you must specify the associated type for the trait object. You can do this by using the syntax `Trait<Bar=Type>` while defining the type alias.

Corrected code example:

trait Trait {
    type Bar;
}

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

In the corrected example, the associated type `Bar` is set to `i32`. Now, the error is resolved, and the code compiles successfully.

Working with Generic Associated Types

To achieve further flexibility, you may use generic associated types. This allows you to use a single trait to cover more use cases while also enforcing type constraints.

Example with generic associated types:

trait Trait {
    type Bar: Default;
}

struct Struct<T: Default>;

impl<T: Default> Trait for Struct<T> {
    type Bar = T;
}

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

type Foo2 = Trait<Bar=String>; // ok!

In this example, the associated type `Bar` is now enforces a constraint by requiring the `Default` trait. The `Struct` is now generic with a type parameter `T`, and the implementation of the `Trait` trait for `Struct<T>` sets the `Bar` associated type to `T`. This allows the trait object to be used for various types like `i32`, `String`, etc., while maintaining type safety.

Recommended Reading

• Advanced Traits