go - Is there any way to access private fields of a struct from another package?

Question

I have a struct in one package that has private fields:

package foo

type Foo struct {
    x int
    y *Foo
}

And another package (for example, a white-box testing package) needs access to them:

package bar

import "../foo"

func change_foo(f *Foo) {
    f.y = nil
}

Is there a way to declare bar to be a sort of "friend" package or any other way to be able to access foo.Foo's private members from bar, but still keep them private for all other packages (perhaps something in unsafe)?

Answer 1

Waitting for answers