E0801 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0801

The `self` parameter in a method has an invalid generic “receiver type”.

Erroneous code example:

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

    impl Foo {
        fn foo<R: std::ops::Deref<Target=Self>>(self: R) {}
    }
    }

or alternatively,

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

    impl Foo {
        fn foo(self: impl std::ops::Deref<Target=Self>) {}
    }
    }

Methods take a special first parameter, termed `self`. It’s normal to use `self`, `&self` or `&mut self`, which are syntactic sugar for `self: Self`, `self: &Self`, and `self: &mut Self` respectively. But it’s also possible to use more sophisticated types of `self` parameter, for instance `std::rc::Rc<Self>`. The set of allowable `Self` types is extensible using the nightly feature Arbitrary self types. This will extend the valid set of `Self` types to anything which implements `std::ops::Deref<Target=Self>`, for example `Rc<Self>`, `Box<Self>`, or your own smart pointers that do the same.

However, even with that feature, the `self` type must be concrete. Generic `self` types are not permitted. Specifically, a `self` type will be rejected if it is a type parameter defined on the method.

These are OK:

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

    impl Foo {
        fn foo(self) {}
        fn foo2(self: std::rc::Rc<Self>) {} // or some other similar
            // smart pointer if you enable arbitrary self types and
            // the pointer implements Deref<Target=Self>
    }
    }