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! } }