Rust error E0801: invalid generic `self` parameter type: `R`

The compiler team's explanation

The self parameter in a method has an invalid generic "receiver type".

Erroneous code example:

struct Foo;

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

or alternatively,

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:

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>
}

From rustc --explain E0801 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.