Assumed audience: Software developers working with the Rust Axum library, particularly if you’re seeing an inscrutable message about your handler function not implementing one of the Axum route handler traits.
Over the weekend, I was whipping up a little project using Axum,1 and at one point I got a bit stuck trying to figure out why a particular handler was not being accepted.2 I eventually figured it out: any handler you use with Axum’s router needs to implement IntoResponse, because Axum uses that to define how to convert your handler’s return into, well, a response.
Axum implements IntoResponse for a lot of common types, including the majority of things you would naturally want to return, including for Result… with an important caveat: both the T and E cases on the Result also need to implement IntoResponse.
The problem I had came from a signature something like this:
async fn handler() -> Result<Response<Body>, Error> {
// ...
}
#[derive(Debug, thiserror::Error)
enum Error {
#[error("Something went wrong")]
OhTehNoes
}
The problem here, which produced a rather inscrutable error, was that Error did not implement IntoResponse, so Result<Response<Body>, Error> did not either. I fixed that pretty easily, with a trivial implementation:
impl IntoResponse for Error {
fn into_response(self) -> axum::response::Response {
(http::StatusCode::INTERNAL_SERVER_ERROR, self.to_string()).into_response()
}
}
In this case that incredibly simple implementation is sufficient because it’s an incredibly simple little piece of software and the only errors in question mean I did something wrong. I might get more granular later. I also might not!