Best practice of returning different implementations of Responder on different code paths.

[Jason5Lee]

Mostly for error handling. I need to respond a JSON serialized from different serde types in different code paths, which makes using something like rocket::response::content::Json<R> hard. I have the following thoughts.

1. Define a responder and implement the API as the respond_to method. Then I can just return different Response in the different code paths. The downside is that it's a bit cumbersome to make use of rocket::response::content::* . In order to get the response from a Responder implementation under that namespace, I need to call the respond_to method on them and pass the request which needs to be passed all the way through the function called, while the method implementation may not use the request at all.
2. Use 1, but not using rocket::response::content::* . Instead, just manually set up content type, body, status code, etc. I think it can be helpful to have some methods to create the response like how the respond_to methods of the types under rocket::response::content::* do if the methods don't use the request parameter. I cannot find these methods in the standard library or in a third-party library. If someone else has done this, please let me know, otherwise, I might consider making one.
3. Implement Responder for the dynamic-dispatch closure. Responder cannot be turned into a trait object because the respond_to method uses self as the parameter. But Rust has a unique rule which allows Box<dyn FnOnce(&'_ Request<'_>) -> Result<'_>> . If the library can implement Responder for this closure type, or a third-party library provides a wrapper of the closure that implement Responder , then I can use it to unify each Responder . If someone else has done this, please let me know, otherwise, I might consider making one.
4. Define the response struct very large with lots of Option fields, including every possible field for each code path. Then I can use one Json<R> to represent all of them. I personally dislike this approach but if there are some good reasons to do this please tell me.

[Jason5Lee]

Some points I miss

5. use Either . I think this is by far the best solution.
6. Define an enum for the different types and implement it for Responder . This is also another of the best solution.