The self
parameter in a method has an invalid “receiver type”.
Erroneous code example:
struct Foo;
struct Bar;
trait Trait {
fn foo(&self);
}
impl Trait for Foo {
fn foo(self: &Bar) {}
}
RunMethods 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.
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`
}
RunThe 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:
impl Trait for Foo {
fn foo(self: &Foo) {}
}
RunThis error will be emitted by the compiler when using an invalid receiver type, like in the following example:
impl Trait for Foo {
fn foo(self: &Bar) {}
}
RunThe nightly feature Arbitrary self types 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
}
Run