Cannot register function as middleware due to Debug trait requirements

[jhorstmann]

Looking at the middleware documentation, there is a implementation for Fn(Request<State>, Next<'a, State>) -> BoxFuture<'a, Result>, which should allow me to register an async function as a middleware without implementing a struct and trait. Unfortunately the Server::middleware function additionally has a requirement that the middleware implements the Debug trait. So the following example fails

async fn handle_error<State: Send + Sync + 'static>(
    ctx: Request<State>,
    next: Next<State>,
) -> Result {
    ...
}

...

let mut app = tide::new();
app.middleware(handle_error);

error[E0277]: `fn(tide::request::Request<_>, tide::middleware::Next<'static, _>) -> impl std::future::Future {handle_error::<_>}` doesn't implement `std::fmt::Debug`
  --> src/main.rs:97:20
   |
97 |     app.middleware(handle_error);
   |                    ^^^^^^^^^^^^ `fn(tide::request::Request<_>, tide::middleware::Next<'static, _>) -> impl std::future::Future {handle_error::<_>}` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
   |
   = help: the trait `std::fmt::Debug` is not implemented for `fn(tide::request::Request<_>, tide::middleware::Next<'static, _>) -> impl std::future::Future {handle_error::<_>}`

This makes implementing a middleware much more complex, especially for rust beginners.

[yoshuawuyts]

Yeah that's fair — maybe we should get rid of the debug requirement, but add a default method on the middleware trait that contains the middleware name.

[jhorstmann]

It seems part of this was fixed in #545, to me as a rust beginner the signature there still looks overly complex:

fn user_loader<'a>(
    mut request: Request<UserDatabase>,
    next: Next<'a, UserDatabase>,
) -> Pin<Box<dyn Future<Output = Result> + Send + 'a>> {

I'm still wondering if it might be possible to register a simple async function that returns a Result. An explanation why this might not be possible would also be very appreciated.

[yoshuawuyts]

#556 makes this easier by exposing Before and After middleware constructors. We currently cannot implement the middleware trait for async fns, so this seems to be about the best we can do until the language catches up.

[jhorstmann]

Sounds reasonable. If you want you can close this issue (or I can close), unless you want to keep it open as a reminder.