How do you access state in a custom Extractor?

[nick-kang]

I know how to get state using Extension, but am having trouble getting the state using State.

////////////// AppState
#[derive(Clone, FromRef)]
pub struct AppState {
    pub db: DbState,
}

///////////// Extractor
#[async_trait]
impl<S> FromRequestParts<S> for Authentication
where
    S: Send + Sync,
{
    type Rejection = AppError;

    async fn from_request_parts(
        parts: &mut Parts,
        state: &S,
    ) -> std::result::Result<Self, Self::Rejection> {

        // this works
        // let Extension(app_state) = parts
        //   .extract::<Extension<AppState>>()
        //  .await
        //  .map_err(|e| AppError::InternalServerError(e.into()))?;

        // this throws an error
        let state = parts
            .extract_with_state::<AppState, _>(state)
            .await
            .map_err(|e| AppError::InternalServerError(e.into()))?;
    }
}

//////////////////// Router
Router::new()
        .api_route("/api/healthcheck", get(healthcheck::controller))
        .with_state(app_state)

I get the following compiler error, which says that FromRequestParts is not implemented for AppState. However, I'm not sure how I'm suppose create that implementation.

error[E0277]: the trait bound `app_state::AppState: axum::extract::FromRequestParts<_>` is not satisfied
  --> src/extractors/authentication.rs:62:14
   |
62 |             .extract_with_state::<AppState, _>(state)
   |              ^^^^^^^^^^^^^^^^^^ the trait `axum::extract::FromRequestParts<_>` is not implemented for `app_state::AppState`
   |
   = help: the following other types implement trait `axum::extract::FromRequestParts<S>`:
             <() as axum::extract::FromRequestParts<S>>
             <(T1, T2) as axum::extract::FromRequestParts<S>>
             <(T1, T2, T3) as axum::extract::FromRequestParts<S>>
             <(T1, T2, T3, T4) as axum::extract::FromRequestParts<S>>
             <(T1, T2, T3, T4, T5) as axum::extract::FromRequestParts<S>>
             <(T1, T2, T3, T4, T5, T6) as axum::extract::FromRequestParts<S>>
             <(T1, T2, T3, T4, T5, T6, T7) as axum::extract::FromRequestParts<S>>
             <(T1, T2, T3, T4, T5, T6, T7, T8) as axum::extract::FromRequestParts<S>>
           and 27 others

#[async_trait]
impl<S> FromRequestParts<S> for AppState
where
    S: Send + Sync,
{
    type Rejection = AppError;

    async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
        // How do I extract the AppState here?
       // when I console log the `state` parameter, it looks like it's the AppState, but I'm not sure how to convert the type to AppState
    }
}

[nick-kang]

After posting this, I think I was able to solve this, but would love to see if there is a better way.

///////////// Extractor
#[async_trait]
impl<S> FromRequestParts<S> for Authentication
where
    AppState: FromRef<S>, // <---- added this line
    S: Send + Sync,
{
    type Rejection = AppError;

    async fn from_request_parts(
        parts: &mut Parts,
        state: &S,
    ) -> std::result::Result<Self, Self::Rejection> {

        let state = parts
            .extract_with_state::<AppState, _>(state)
            .await
            .map_err(|e| AppError::InternalServerError(e.into()))?;
    }
}

#[async_trait]
impl<S> FromRequestParts<S> for AppState
where
    Self: FromRef<S>, // <---- added this line
    S: Send + Sync + Debug,
{
    type Rejection = AppError;

    async fn from_request_parts(_parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
        Ok(Self::from_ref(state)) // <---- added this line
    }
}

[davidpdrsn]

That is the way. See https://docs.rs/axum/0.6.4/axum/extract/struct.State.html#for-library-authors

[nick-kang]

Thanks, David! Axum is an amazing library

[arcstur]

Can't you use AppState::from_ref(state) inside the first code block directly? Without having to implement FromRequestParts for it.

[samuela]

Can't you use AppState::from_ref(state) inside the first code block directly? Without having to implement FromRequestParts for it.

I attempted this but ran into #2721