Implement Deref for extractors

[jakobhellermann]

If the extract::* types like Json would implement std::ops::Deref you could write

#[derive(Deserialize, Serialize)]
struct Request {
    foo: u8,
}

async fn handler(req: Json<Request>) {
    println!("{}", req.foo);
}

instead of

async fn handler(Json(req): Json<Request>) {
    println!("{}", req.foo);
}

or

async fn handler(req: Json<Request>) {
    let req = req.0;
    println!("{}", req.foo);
}

which IMO is easier to read.

The disadvantage is that it introduces some "Magic" that might be difficult to understand for newer rust developers.

Prior art

The bevy game engine can write ECS systems similar to how axum's request handlers work (example) and there you can write e.g.

fn system(resource: Res<Foo>) {
  println!("{}", resource.foo);
}

which works really well and leads to very readably systems.