record-struct-middleware-1-variadics.rs

trait App<Input>
where
    Input: { request: http::Request }
{
    type Output: Into<http::Response>;

    fn call(self, input: Input) -> Self::Output;
}

struct AuthApp<T> {
    next: T,
}

impl<T, Input> App<Input> for AuthApp<T>
where
    Input: { request: http::Request },
    T: App<{ user: Result<User, AuthError>, ..Input }>
{
    type Output = T::Output;

    fn call(self, input: Input) -> Self::Output {
        let user = authenticate(&input.request);
        self.next({ user, ..input })
    }
}

struct User { /* … */ }
enum AuthError { /* … */ }
fn authenticate(_: &http::Request) -> Result<User, AuthError> { /* … */ }

record-struct-middleware-2-traits.rs

trait HasField<const Name: &'static str> {
    type Type;

    fn get_ref(&self) -> &Self::Type;
}

trait AddField<const Name: &'static str, Type> {
    type Output;

    fn add(self, value: Type) -> Self::Output;
}

trait App<Input>
where
    Input: HasField<"request", Type = http::Request>
{
    type Output: Into<http::Response>;

    fn call(self, input: Input) -> Self::Output;
}

struct AuthApp<T> {
    next: T,
}

impl<T, Input> App<Input> for AuthApp<T>
where
    Input: HasField<"request", Type = http::Request> + AddField<"user", Result<User, AuthError>>,
    T: App<<Input as AddField<"user", Result<User, AuthError>>>::Output>
{
    type Output = T::Output;

    fn call(self, input: Input) -> Self::Output {
        let user = authenticate(input.get_ref());
        self.next(input.add(user))
    }
}

struct User { /* … */ }
enum AuthError { /* … */ }
fn authenticate(_: &http::Request) -> Result<User, AuthError> { /* … */ }