From #31:

"If you have a struct which -- transitively -- contains no data of non-zero size, then the size of that struct will be zero as well. These zero-sized structs appear frequently as exceptions in other layout considerations (e.g., single-field structs). An example of such a struct is std::marker::PhantomData."

Is that a sufficient definition for zero-sized structs? This seems like an important guarantee that we frequently rely upon for performance and other purposes, so it is worth specifying.