Not sure if it helps, but I've sorta found a pattern I like recently for coupling types, by using a marker trait with a marker type. I use it in postcard-rpc:

```rust
/// A marker trait denoting a single endpoint
///
/// Typically used with the [endpoint] macro.
pub trait Endpoint {
    /// The type of the Request (client to server)
    type Request: Schema;
    /// The type of the Response (server to client)
    type Response: Schema;
    /// The path associated with this Endpoint
    const PATH: &'static str;
    /// The unique [Key] identifying the Request
    const REQ_KEY: Key;
    /// The unique [Key] identifying the Response
    const RESP_KEY: Key;
}
```