Understanding and Resolving Rust Error E0411

Introduction to Rust Error E0411

Rust Error E0411 occurs when the `Self` keyword is used outside of an impl, trait, or type definition. The `Self` keyword represents the current type and is only allowed inside an impl, trait, or type definition to access the associated items of a type.

Erroneous code example

#![allow(unused)]
fn main() {
    <Self>::foo; // error: use of `Self` outside of an impl, trait, or type
                 // definition
}

To resolve Rust Error E0411, ensure you only use the `Self` keyword inside an impl, trait, or type definition. Let's explore some correct usage of the `Self` keyword.

Correct usage of the `Self` keyword

#![allow(unused)]
fn main() {
trait Foo {
    type Bar;
}

trait Baz : Foo {
    fn bar() -> Self::Bar; // like this
}
}

However, if two types have a common associated type, the compiler may raise an error due to ambiguous associated types. Consider the following erroneous example:

Erroneous code example with ambiguous associated types

#![allow(unused)]
fn main() {
trait Foo {
    type Bar;
}

trait Foo2 {
    type Bar;
}

trait Baz : Foo + Foo2 {
    fn bar() -> Self::Bar;
    // error: ambiguous associated type `Bar` in bounds of `Self`
}
}

To resolve this error, you need to specify which trait's associated type you want to use. In the following example, we disambiguate the associated type by specifying that we want to use the `Bar` type from the `Foo` trait.

Correct usage of the `Self` keyword with multiple associated types

#![allow(unused)]
fn main() {
trait Foo {
    type Bar;
}

trait Foo2 {
    type Bar;
}

trait Baz : Foo + Foo2 {
    fn bar() -> <Self as Foo>::Bar; // ok!
}
}