Surprising behaviour of handle_error

[sidred]

This is just an observation, not sure if there is any actionable here

Adding handle_error to error-handling-and-dependency-injection example will cause a compile error

        .layer(AddExtensionLayer::new(user_repo)) 
        .handle_error(|err: BoxError| 
            Ok::<_, Infallible>(( 
                StatusCode::INTERNAL_SERVER_ERROR, 
                format!("Unhandled error: {}", err),
            ))
        )

However after adding tower middleware layer, this compiles correctly

        .layer(AddExtensionLayer::new(user_repo)) 
        .layer(TimeoutLayer::new(Duration::from_secs(10)))
        .handle_error(|err: BoxError| 
            Ok::<_, Infallible>(( 
                StatusCode::INTERNAL_SERVER_ERROR, 
                format!("Unhandled error: {}", err),
            ))
        )

Is it expected that handle_error is only used in cases where check_infallible fails to compile? Intuitively I has assumed that handle error should always be added so that we handle all errors and don't swallow any errors.

The compilation error for the first case is also entirely unhelpful

  --> examples/error-handling-and-dependency-injection/src/main.rs:60:5

60 | /     axum::Server::bind(&addr)
61 | |         .serve(app.into_make_service())
62 | |         .await
   | |______________^ the trait `hyper::common::exec::ConnStreamExec<......long string.......>` is not implemented for `hyper::common::exec::Exec`


60 | /     axum::Server::bind(&addr)
61 | |         .serve(app.into_make_service())
62 | |         .await
   | |______________^ the trait `hyper::common::exec::NewSvcExec<........long string ........>` is not implemented for `hyper::common::exec::Exec`



error[E0631]: type mismatch in closure arguments
  --> examples/error-handling-and-dependency-injection/src/main.rs:60:5
   |
49 |           .handle_error(|err: BoxError|
   |  _______________________-
50 | |             Ok::<_, Infallible>((
51 | |                 StatusCode::INTERNAL_SERVER_ERROR,
52 | |                 format!("Unhandled error: {}", err),
53 | |             ))
   | |______________- found signature of `fn(Box<(dyn StdError + Send + Sync + 'static)>) -> _`
...
60 | /     axum::Server::bind(&addr)
61 | |         .serve(app.into_make_service())
62 | |         .await
   | |______________^ expected signature of `fn(Infallible) -> _`

[davidpdrsn]

Until you add .layer(TimeoutLayer::new(Duration::from_secs(10))) the applications error type is in Infallible meaning there is no error to handle. TimeoutLayer changes the error type to BoxError, which becomes the argument to handle_error.