The ZSTs are unit types. They have one value, which we usually write as just the type name, so e.g.

    struct Goose;

    let x = Goose; // The variable x has type Goose, but also value Goose, the only value of that type

The choice to underscore that Rust's unit types have size zero is to contrast with languages like C or C++ where these types, which don't need representing, must nevertheless take up a whole byte of storage and it's just wasted.

But what we're talking about here are empty types. In Rust we'd write this:

    enum Donkey {}

    // We can't make any variables with the Donkey type, because there are no values of this type and a variable needs a value