cmp/export_unsafe.go
func retrieveUnexportedField(v reflect.Value, f reflect.StructField) reflect.Value {
	return reflect.NewAt(f.Type, unsafe.Pointer(v.UnsafeAddr()+f.Offset)).Elem()
}
func retrieveUnexportedField(v reflect.Value, f reflect.StructField) reflect.Value {
	return reflect.NewAt(f.Type, unsafe.Pointer(v.UnsafeAddr()+f.Offset)).Elem()
}
// retrieveUnexportedField uses unsafe to forcibly retrieve any field from
// a struct such that the value has read-write permissions.
//
// The parent struct, v, must be addressable, while f must be a StructField
// describing the field to retrieve.