`IntoResponse` and `ApiOperationResponse` derive macro for enum support

[m4tx]

ApiOperationResponse is currently implemented for many common types, such as Json, Result, etc. However, it might be useful to use an enum, indicating that one of many responses might be returned. Currently though, one has to implement both IntoResponse and ApiOperationResponse manually for such enum. An example how the enum could look like can be seen below:

enum MyResponse {
    A(Json<VariantAResponse>),
    B(Json<VariantBResponse>),
}

This sounds like a perfect candidate for creating derive macros which would just take all the types from the variants and create IntoResponse and ApiOperationResponse that would call respective methods depending on the variant matched on self.

[kumarUjjawal]

@m4tx Shouldn't this be closed after the merge of #388

[xerion12]

Great work on this feature and the implementation in #388. I’ve worked extensively with Rust procedural macros like syn, quote and frameworks like axum and utoipa, and I’d be happy to help extend or refine the derive support.

I’m particularly interested in improving support for generic variants, custom attributes, and better macro diagnostics. If there’s room for follow-up contributions, I can open proposal or draft PR to explore these enhancements.