We need some way to identify that this piece of metadata (i.e. what the `DefId` you're looking up) is a tuple struct constuctor (because this is the only thing for which we should look at the parent rather than the actual node, for things like attributes) and neither `'f'` nor `encode_parent_item` would work because they are not unique. E.g. enum variants have their `encode_parent_item` called, and normal functions have family `'f'`.