#
actix_service/
pipeline.rs

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    // TODO: see if pipeline is necessary
    #![allow(dead_code)]

    use core::{
        marker::PhantomData,
        task::{Context, Poll},
    };

    use crate::{
        and_then::{AndThenService, AndThenServiceFactory},
        map::{Map, MapServiceFactory},
        map_err::{MapErr, MapErrServiceFactory},
        map_init_err::MapInitErr,
        then::{ThenService, ThenServiceFactory},
        IntoService, IntoServiceFactory, Service, ServiceFactory,
    };

    /// Construct new pipeline with one service in pipeline chain.
    pub(crate) fn pipeline<I, S, Req>(service: I) -> Pipeline<S, Req>
    where
        I: IntoService<S, Req>,
        S: Service<Req>,
    {
        Pipeline {
            service: service.into_service(),
            _phantom: PhantomData,
        }
    }

    /// Construct new pipeline factory with one service factory.
    pub(crate) fn pipeline_factory<I, SF, Req>(factory: I) -> PipelineFactory<SF, Req>
    where
        I: IntoServiceFactory<SF, Req>,
        SF: ServiceFactory<Req>,
    {
        PipelineFactory {
            factory: factory.into_factory(),
            _phantom: PhantomData,
        }
    }

    /// Pipeline service - pipeline allows to compose multiple service into one service.
    pub(crate) struct Pipeline<S, Req> {
        service: S,
        _phantom: PhantomData<fn(Req)>,
    }

    impl<S, Req> Pipeline<S, Req>
    where
        S: Service<Req>,
    {
        /// Call another service after call to this one has resolved successfully.
        ///
        /// This function can be used to chain two services together and ensure that
        /// the second service isn't called until call to the first service have
        /// finished. Result of the call to the first service is used as an
        /// input parameter for the second service's call.
        ///
        /// Note that this function consumes the receiving service and returns a
        /// wrapped version of it.
        pub fn and_then<I, S1>(
            self,
            service: I,
        ) -> Pipeline<impl Service<Req, Response = S1::Response, Error = S::Error> + Clone, Req>
        where
            Self: Sized,
            I: IntoService<S1, S::Response>,
            S1: Service<S::Response, Error = S::Error>,
        {
            Pipeline {
                service: AndThenService::new(self.service, service.into_service()),
                _phantom: PhantomData,
            }
        }

        /// Chain on a computation for when a call to the service finished,
        /// passing the result of the call to the next service `U`.
        ///
        /// Note that this function consumes the receiving pipeline and returns a
        /// wrapped version of it.
        pub fn then<F, S1>(
            self,
            service: F,
        ) -> Pipeline<impl Service<Req, Response = S1::Response, Error = S::Error> + Clone, Req>
        where
            Self: Sized,
            F: IntoService<S1, Result<S::Response, S::Error>>,
            S1: Service<Result<S::Response, S::Error>, Error = S::Error>,
        {
            Pipeline {
                service: ThenService::new(self.service, service.into_service()),
                _phantom: PhantomData,
            }
        }

        /// Map this service's output to a different type, returning a new service
        /// of the resulting type.
        ///
        /// This function is similar to the `Option::map` or `Iterator::map` where
        /// it will change the type of the underlying service.
        ///
        /// Note that this function consumes the receiving service and returns a
        /// wrapped version of it, similar to the existing `map` methods in the
        /// standard library.
        pub fn map<F, R>(self, f: F) -> Pipeline<Map<S, F, Req, R>, Req>
        where
            Self: Sized,
            F: FnMut(S::Response) -> R,
        {
            Pipeline {
                service: Map::new(self.service, f),
                _phantom: PhantomData,
            }
        }

        /// Map this service's error to a different error, returning a new service.
        ///
        /// This function is similar to the `Result::map_err` where it will change
        /// the error type of the underlying service. This is useful for example to
        /// ensure that services have the same error type.
        ///
        /// Note that this function consumes the receiving service and returns a
        /// wrapped version of it.
        pub fn map_err<F, E>(self, f: F) -> Pipeline<MapErr<S, Req, F, E>, Req>
        where
            Self: Sized,
            F: Fn(S::Error) -> E,
        {
            Pipeline {
                service: MapErr::new(self.service, f),
                _phantom: PhantomData,
            }
        }
    }

    impl<T, Req> Clone for Pipeline<T, Req>
    where
        T: Clone,
    {
        fn clone(&self) -> Self {
            Pipeline {
                service: self.service.clone(),
                _phantom: PhantomData,
            }
        }
    }

    impl<S: Service<Req>, Req> Service<Req> for Pipeline<S, Req> {
        type Response = S::Response;
        type Error = S::Error;
        type Future = S::Future;

        #[inline]
        fn poll_ready(&self, ctx: &mut Context<'_>) -> Poll<Result<(), S::Error>> {
            self.service.poll_ready(ctx)
        }

        #[inline]
        fn call(&self, req: Req) -> Self::Future {
            self.service.call(req)
        }
    }

    /// Pipeline factory
    pub(crate) struct PipelineFactory<SF, Req> {
        factory: SF,
        _phantom: PhantomData<fn(Req)>,
    }

    impl<SF, Req> PipelineFactory<SF, Req>
    where
        SF: ServiceFactory<Req>,
    {
        /// Call another service after call to this one has resolved successfully.
        pub fn and_then<I, SF1>(
            self,
            factory: I,
        ) -> PipelineFactory<
            impl ServiceFactory<
                    Req,
                    Response = SF1::Response,
                    Error = SF::Error,
                    Config = SF::Config,
                    InitError = SF::InitError,
                    Service = impl Service<Req, Response = SF1::Response, Error = SF::Error> + Clone,
                > + Clone,
            Req,
        >
        where
            Self: Sized,
            SF::Config: Clone,
            I: IntoServiceFactory<SF1, SF::Response>,
            SF1: ServiceFactory<
                SF::Response,
                Config = SF::Config,
                Error = SF::Error,
                InitError = SF::InitError,
            >,
        {
            PipelineFactory {
                factory: AndThenServiceFactory::new(self.factory, factory.into_factory()),
                _phantom: PhantomData,
            }
        }

        /// Create `NewService` to chain on a computation for when a call to the
        /// service finished, passing the result of the call to the next
        /// service `U`.
        ///
        /// Note that this function consumes the receiving pipeline and returns a
        /// wrapped version of it.
        pub fn then<I, SF1>(
            self,
            factory: I,
        ) -> PipelineFactory<
            impl ServiceFactory<
                    Req,
                    Response = SF1::Response,
                    Error = SF::Error,
                    Config = SF::Config,
                    InitError = SF::InitError,
                    Service = impl Service<Req, Response = SF1::Response, Error = SF::Error> + Clone,
                > + Clone,
            Req,
        >
        where
            Self: Sized,
            SF::Config: Clone,
            I: IntoServiceFactory<SF1, Result<SF::Response, SF::Error>>,
            SF1: ServiceFactory<
                Result<SF::Response, SF::Error>,
                Config = SF::Config,
                Error = SF::Error,
                InitError = SF::InitError,
            >,
        {
            PipelineFactory {
                factory: ThenServiceFactory::new(self.factory, factory.into_factory()),
                _phantom: PhantomData,
            }
        }

        /// Map this service's output to a different type, returning a new service
        /// of the resulting type.
        pub fn map<F, R>(self, f: F) -> PipelineFactory<MapServiceFactory<SF, F, Req, R>, Req>
        where
            Self: Sized,
            F: FnMut(SF::Response) -> R + Clone,
        {
            PipelineFactory {
                factory: MapServiceFactory::new(self.factory, f),
                _phantom: PhantomData,
            }
        }

        /// Map this service's error to a different error, returning a new service.
        pub fn map_err<F, E>(self, f: F) -> PipelineFactory<MapErrServiceFactory<SF, Req, F, E>, Req>
        where
            Self: Sized,
            F: Fn(SF::Error) -> E + Clone,
        {
            PipelineFactory {
                factory: MapErrServiceFactory::new(self.factory, f),
                _phantom: PhantomData,
            }
        }

        /// Map this factory's init error to a different error, returning a new service.
        pub fn map_init_err<F, E>(self, f: F) -> PipelineFactory<MapInitErr<SF, F, Req, E>, Req>
        where
            Self: Sized,
            F: Fn(SF::InitError) -> E + Clone,
        {
            PipelineFactory {
                factory: MapInitErr::new(self.factory, f),
                _phantom: PhantomData,
            }
        }
    }

    impl<T, Req> Clone for PipelineFactory<T, Req>
    where
        T: Clone,
    {
        fn clone(&self) -> Self {
            PipelineFactory {
                factory: self.factory.clone(),
                _phantom: PhantomData,
            }
        }
    }

    impl<SF, Req> ServiceFactory<Req> for PipelineFactory<SF, Req>
    where
        SF: ServiceFactory<Req>,
    {
        type Config = SF::Config;
        type Response = SF::Response;
        type Error = SF::Error;
        type Service = SF::Service;
        type InitError = SF::InitError;
        type Future = SF::Future;

        #[inline]
        fn new_service(&self, cfg: SF::Config) -> Self::Future {
            self.factory.new_service(cfg)
        }
    }