Does the PhantomData type in Rust do what you want? Or are you talking about something slightly different?

https://doc.rust-lang.org/nomicon/phantom-data.html

https://doc.rust-lang.org/std/marker/struct.PhantomData.html

So

  struct Sender<S> {
    /// Actual implementation of network I/O.
    inner: SenderImpl;  
    /// 0-sized field, doesn't exist at runtime.
    state: S;
  }

I believe could become

  struct Sender<S> {
    /// Actual implementation of network I/O.
    inner: SenderImpl;  
    /// 0-sized field, doesn't exist at runtime.
    _marker: PhantomData<S>;
  }

I've never used this PhantomData personally, so this might be wrong. Cheers!