`std::marker::PhantomUnsized` marker type

Yes, but it doesn't expose new syntax (the change to core::marker uses an extern type, but part of the intent is to keep that syntax perma-unstable).

Having PhantomUnsized also documents the existence and behavior of this a bit better than stuffing it under the type keyword in the Rust documentation. And imo it makes sense for it to be a marker type.