#
actix_service/
transform_err.rs

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    use core::{
        future::Future,
        marker::PhantomData,
        pin::Pin,
        task::{Context, Poll},
    };

    use pin_project_lite::pin_project;

    use super::Transform;

    /// Transform for the [`TransformExt::map_init_err`] combinator, changing the type of a new
    /// [`Transform`]'s initialization error.
    pub struct TransformMapInitErr<T, S, Req, F, E> {
        transform: T,
        mapper: F,
        _phantom: PhantomData<fn(Req) -> (S, E)>,
    }

    impl<T, S, F, E, Req> TransformMapInitErr<T, S, Req, F, E> {
        pub(crate) fn new(t: T, f: F) -> Self
        where
            T: Transform<S, Req>,
            F: Fn(T::InitError) -> E,
        {
            Self {
                transform: t,
                mapper: f,
                _phantom: PhantomData,
            }
        }
    }

    impl<T, S, Req, F, E> Clone for TransformMapInitErr<T, S, Req, F, E>
    where
        T: Clone,
        F: Clone,
    {
        fn clone(&self) -> Self {
            Self {
                transform: self.transform.clone(),
                mapper: self.mapper.clone(),
                _phantom: PhantomData,
            }
        }
    }

    impl<T, S, F, E, Req> Transform<S, Req> for TransformMapInitErr<T, S, Req, F, E>
    where
        T: Transform<S, Req>,
        F: Fn(T::InitError) -> E + Clone,
    {
        type Response = T::Response;
        type Error = T::Error;
        type Transform = T::Transform;

        type InitError = E;
        type Future = TransformMapInitErrFuture<T, S, F, E, Req>;

        fn new_transform(&self, service: S) -> Self::Future {
            TransformMapInitErrFuture {
                fut: self.transform.new_transform(service),
                f: self.mapper.clone(),
            }
        }
    }

    pin_project! {
        pub struct TransformMapInitErrFuture<T, S, F, E, Req>
        where
        T: Transform<S, Req>,
        F: Fn(T::InitError) -> E,
        {
            #[pin]
            fut: T::Future,
            f: F,
        }
    }

    impl<T, S, F, E, Req> Future for TransformMapInitErrFuture<T, S, F, E, Req>
    where
        T: Transform<S, Req>,
        F: Fn(T::InitError) -> E + Clone,
    {
        type Output = Result<T::Transform, E>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let this = self.project();
            if let Poll::Ready(res) = this.fut.poll(cx) {
                Poll::Ready(res.map_err(this.f))
            } else {
                Poll::Pending
            }
        }
    }