Zero-cost, compile-time app state

[musjj]

Description of the feature

axum has an extractor called State. This is similar to poem's Data, but the difference is that axum's State is zero-cost and verified on compile-time. So it's impossible to compile handlers that requests a certain state, without providing that state during registration.

poem currently relies on http's Extensions for storing state, which is a hashmap. This means that your app can error on run-time when you forget to inject the data. This also incurs a performance cost because you need to perform a hashmap lookup on every request (probably negligible, but still).

Code example (if possible)

We can have a new State extractor as an alternative to Data:

use poem::{get, handler, listener::TcpListener, Route, Server, State};

struct AppState {}

#[handler]
fn use_state(state: State<AppState>) -> String {
    todo!()
}

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    if std::env::var_os("RUST_LOG").is_none() {
        std::env::set_var("RUST_LOG", "poem=debug");
    }
    tracing_subscriber::fmt::init();

    let state = AppState {};

    let app = Route::new()
        .at("/use-state", get(use_state))
        .with_state(state); // comment out this line to trigger a compile-time error

    Server::new(TcpListener::bind("0.0.0.0:3000"))
        .name("app-state")
        .run(app)
        .await
}