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 } }