Understanding Rust Error E0220

What is Rust Error E0220?

Rust Error E0220 occurs when an associated type used in the code is not defined within the trait. This is usually caused either by a typo in the associated type name or by not declaring the associated type within the trait.

Example of Rust Error E0220

Consider the following erroneous code that produces Rust Error E0220:

trait T1 {
    type Bar;
}

type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`

In this example, the trait 'T1' defines an associated type named 'Bar'. However, the type 'Foo' is using 'T1' with an associated type named 'F'. Since 'F' is not defined in 'T1', the compiler raises Error E0220.

Fixing Rust Error E0220

To fix Rust Error E0220, ensure that the associated type is correctly defined within the trait, and that the name of the associated type is correct. Here's the corrected code:

trait T1 {
    type Bar;
}

type Foo = T1<Bar=i32>; // ok!

In this corrected example, we have replaced 'F' with 'Bar', which matches the associated type defined in the trait 'T1', and the error is resolved.

Another example of Rust Error E0220

Consider another erroneous code example that results in Rust Error E0220:

trait T2 {
    type Bar;

    // error: Baz is used but not declared
    fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
}

In this example, the trait 'T2' defines an associated type 'Bar', but the function 'return_bool()' tries to use another associated type named 'Baz' which is not defined in the trait, resulting in Error E0220.

Fixing the second example

To resolve Rust Error E0220 in this example, we need to define the associated type 'Baz' within the trait 'T2'. The corrected code is as follows:

trait T2 {
    type Bar;
    type Baz; // we declare `Baz` in our trait.

    // and now we can use it here:
    fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
}

With the associated type 'Baz' now defined in the trait 'T2', the function 'return_bool()' can use it without causing Rust Error E0220.