This error indicates that the `self` parameter in a method has an invalid
"receiver type".
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.
```
# 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:
```
# struct Foo;
# trait Trait {
# fn foo(&self);
# }
impl Trait for Foo {
fn foo(self: &Foo) {}
}
```
E0307 will be emitted by the compiler when using an invalid receiver type,
like in the following example:
```compile_fail,E0307
# struct Foo;
# struct Bar;
# trait Trait {
# fn foo(&self);
# }
impl Trait for Foo {
fn foo(self: &Bar) {}
}
```
The nightly feature [Arbintrary self types][AST] extends the accepted
set of receiver types to also include any type that can dereference to
`Self`:
```
#![feature(arbitrary_self_types)]
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
}
```
[AST]: https://doc.rust-lang.org/unstable-book/language-features/arbitrary-self-types.html