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 } }