Attribute Macro handler 

#[handler]

Validates a handler function at its definition site.

Checks that:

• The function is async
• All arguments (except the last) implement FromRequestParts
• The last argument implements either FromRequestParts or FromRequest
• The return type implements IntoResponse

The function is emitted unchanged. It already works with bind and the blanket Handler<Args> impls. This macro exists purely for early, readable compile errors instead of cryptic trait-resolution failures at the Server::new call site.

Example

#[handler]
async fn get_user(path: Path<UserByIdPath>, state: State<AppState>) -> Json<User> {
    // ...
}

Compile errors

#[handler]
fn not_async() -> String { "hello".to_string() }
// error: handler functions must be async

#[handler]
async fn bad_return() -> NotAResponse { NotAResponse }
// error: `NotAResponse` does not implement `IntoResponse`