I'm trying to get the client IP from the RightmostXForwardedFor header. The problem is the app has two configurations. If the app runs on reverse proxy, I want to use the RightmostXForwardedFor otherwise, I want to use the ConnectInfo (which I suppose is the direct client IP).

Routes configuration:

let secure_client_ip_source = if tracker.config.on_reverse_proxy {
    SecureClientIpSource::RightmostXForwardedFor
} else {
    SecureClientIpSource::ConnectInfo
};

Router::new()
    .route("/announce", get(handle).with_state(tracker.clone()))
    .layer(secure_client_ip_source.into_extension())

The handler:

pub async fn handle(
    State(tracker): State<Arc<Tracker>>,
    ExtractAnnounceRequest(announce_request): ExtractAnnounceRequest,
    secure_ip: SecureClientIp,
) -> Response {
// ...
}

But I want something like this:

pub async fn handle(
    State(tracker): State<Arc<Tracker>>,
    ExtractAnnounceRequest(announce_request): ExtractAnnounceRequest,
    maybe_secure_client_ip: Result<SecureClientIp, StringRejection>,
) -> Response {
// ...
}

But that gives me this error:

the trait bound `fn(axum::extract::State<Arc<Tracker>>, ExtractAnnounceRequest, Result<SecureClientIp, StringRejection>) -> impl futures::Future<Output = axum::http::Response<http_body::combinators::box_body::UnsyncBoxBody<axum::body::Bytes, axum::Error>>> {handle}: Handler<_, _, _>` is not satisfied

The reason I want to make it optional is that I want to send a custom error in the handler if the proxy does not provide the X-Forwarded-For header.

Alternatively, I could:

• Try to catch the extractor error, if that's possible, in Axum.
• Build a wrapper for your extractor.
• Just build my extractor and stop using this crate :-(.

For more context, this is the PR I'm working on.