Summary

Make Self a keyword.

Motivation

Right now, Self is just a regular identifier that happens to get a special meaning inside trait definitions and impls. Specifically, users are not forbidden from defining a type called Self, which can lead to weird situations:

struct Self;

struct Foo;

impl Foo {
    fn foo(&self, _: Self) {}
}

This piece of code defines types called Self and Foo, and a method foo() that because of the special meaning of Self has the signature fn(&Foo, Foo).

So in this case it is not possible to define a method on Foo that takes the actual type Self without renaming it or creating a renamed alias.

It would also be highly unidiomatic to actually name the type Self for a custom type, precisely because of this ambiguity, so preventing it outright seems like the right thing to do.

Making the identifier Self an keyword would prevent this situation because the user could not use it freely for custom definitions.

Detailed design

Make the identifier Self a keyword that is only legal to use inside a trait definition or impl to refer to the Self type.

Drawbacks

It might be unnecessary churn because people already don’t run into this in practice.

Alternatives

Keep the status quo. It isn’t a problem in practice, and just means Self is the special case of a contextual type definition in the language.

Unresolved questions