# Error code E0307

The `self` parameter in a method has an invalid "receiver type".

Erroneous code example:

    #![allow(unused)]
    fn main() {
    struct Foo;
    struct Bar;

    trait Trait {
        fn foo(&self);
    }

    impl Trait for Foo {
        fn foo(self: &Bar) {}
    }
    }

Methods take a special first parameter, of which there are three variants: `self`, `&self`, and `&mut self`. These are syntactic sugar for `self: Self`, `self: &Self`, and `self: &mut Self` respectively.

    #![allow(unused)]
    fn main() {
    struct Foo;
    trait Trait {
        fn foo(&self);
    //         ^^^^^ `self` here is a reference to the receiver object
    }

    impl Trait for Foo {
        fn foo(&self) {}
    //         ^^^^^ the receiver type is `&Foo`
    }
    }

The type `Self` acts as an alias to the type of the current trait implementer, or "receiver type". Besides the already mentioned `Self`, `&Self` and `&mut Self` valid receiver types, the following are also valid: `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, and `self: Pin<P>` (where P is one of the previous types except `Self`). Note that `Self` can also be the underlying implementing type, like `Foo` in the following example:

    #![allow(unused)]
    fn main() {
    struct Foo;
    trait Trait {
        fn foo(&self);
    }
    impl Trait for Foo {
        fn foo(self: &Foo) {}
    }
    }

This error will be emitted by the compiler when using an invalid receiver type, like in the following example:

    #![allow(unused)]
    fn main() {
    struct Foo;
    struct Bar;
    trait Trait {
        fn foo(&self);
    }
    impl Trait for Foo {
        fn foo(self: &Bar) {}
    }
    }

The nightly feature Arbitrary self types extends the accepted set of receiver types to also include any type that can dereference to `Self`:

    #![allow(unused)]
    #![feature(arbitrary_self_types)]

    fn main() {
    struct Foo;
    struct Bar;

    // Because you can dereference `Bar` into `Foo`...
    impl std::ops::Deref for Bar {
        type Target = Foo;

        fn deref(&self) -> &Foo {
            &Foo
        }
    }

    impl Foo {
        fn foo(self: Bar) {}
    //         ^^^^^^^^^ ...it can be used as the receiver type
    }
    }