> Еще можно глянуть пример PhantomData как частный вариант ZST.
> https://doc.rust-lang.org/stable/core/marker/struct.PhantomD...

Я бы дал ссылку на
https://doc.rust-lang.org/nomicon/phantom-data.html

> PhantomData consumes no space, but simulates a field of the given type for the purpose of static analysis. This was deemed to be less error-prone than explicitly telling the type-system the kind of variance that you want, while also providing other useful things such as the information needed by drop check.

там и пример для ZST более лаконичен и поняте не-рустовцам
https://doc.rust-lang.org/nomicon/exotic-sizes.html
> On their own, Zero Sized Types (ZSTs) are, for obvious reasons, pretty useless.

...
> One of the most extreme examples of this is Sets and Maps. Given a Map<Key, Value>, it is common to implement a Set<Key> as just a thin wrapper around Map<Key, UselessJunk>. In many languages, this would necessitate allocating space for UselessJunk and doing work to store and load UselessJunk only to discard it. Proving this unnecessary would be a difficult analysis for the compiler.
> However in Rust, we can just say that Set<Key> = Map<Key, ()>. Now Rust statically knows that every load and store is useless, and no allocation has any size. The result is that the monomorphized code is basically a custom implementation of a HashSet with none of the overhead that HashMap would have to support values.