Understanding Rust Error E0221: Ambiguous Associated Type

Rust Error E0221: Ambiguous Associated Type

Rust error E0221 occurs when an associated type is ambiguous due to multiple definitions, making it unclear which type is being referred to. This can happen when multiple traits define associated types with the same name. In this article, we will analyze the cause of error E0221 in depth, explain how to resolve it and provide code examples to illustrate the concepts.

Examining Rust Error E0221

Let's start by examining the erroneous code example provided in the official Rust documentation:

trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: Self::A;
    }
}

In this example, two traits, T1 and T2, define associated types. The Foo trait introduces an associated type A with the constraint that it implements trait T1. The Bar trait, which requires implementing Foo, also defines an associated type A with the constraint that it implements trait T2. When we reach the function `do_something`, we encounter an error with the type declaration `Self::A`. Rust cannot determine which associated type A to use – should it use the one from Foo, or the one from Bar?

Fixing Rust Error E0221

There are two main ways to resolve error E0221:

1. Renaming one of the associated types

The simplest solution is to rename one of the conflicting associated types, so that there is no ambiguity. The example code below demonstrates this approach:

trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type B: T2;
    fn do_something() {
        let _: Self::B;
    }
}

We've renamed the associated type A in the Bar trait to B, and the error is resolved.

2. Specifying the intended associated type

An alternative solution is to specify which associated type you want to use by using the following syntax: `<Self as TraitName>::AssociatedType`. This way, you don't need to rename any associated types. The example code below demonstrates this approach:

trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: <Self as Bar>::A;
    }
}

We've explicitly specified the associated type A from the Bar trait using `<Self as Bar>::A`, thus eliminating the ambiguity and resolving the error.