> Rust which has language changes as well

Rust doesn't have backwards incompatible language changes.

> This looks like a breaking change on a stable API.

No. It is a method addition. It is breaking only in the sense that code that didn't explicitly invoke the previous "as_ref" method might call this new method instead. It's the moral equivalent of:

    type A struct { ... }

    type B struct {
        A
    }

    func (a ﹡A) Foo() {}

and then a later version of Go adds a method:

    func (b ﹡B) Foo() {}

Such that code that called Foo() on an instance of ﹡B might call the new method instead. Go can make those changes.