E0411 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0411

The `Self` keyword was used outside an impl, trait, or type definition.

Erroneous code example:

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

The `Self` keyword represents the current type, which explains why it can only be used inside an impl, trait, or type definition. It gives access to the associated items of a type:

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

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

However, be careful when two types have a common associated type:

    #![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`
    }
    }

This problem can be solved by specifying from which trait we want to use the `Bar` type:

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

    trait Foo2 {
        type Bar;
    }

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