Rationale:
- In Rust, one can omit a semicolon after a function's final expression to make
its value the function's return value. It's common for people to include a
semicolon after the last expression by mistake - common enough that the Rust
compiler suggests removing the semicolon when there's a type mismatch between
the function's signature and body. By implementing Responder for (), Actix makes
this common mistake a silent error in handler functions.
- Functions returning an empty body should return HTTP status 204 ("No Content"),
so the current Responder impl for (), which returns status 200 ("OK"), is not
really what one wants anyway.
- It's not much of a burden to ask handlers to explicitly return
`HttpResponse::Ok()` if that is what they want; all the examples in the
documentation do this already.
