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    //! Connectors used by the `Client`.
    //!
    //! This module contains:
    //!
    //! - A default [`HttpConnector`][] that does DNS resolution and establishes
    //!   connections over TCP.
    //! - Types to build custom connectors.
    //!
    //! # Connectors
    //!
    //! A "connector" is a [`Service`][] that takes a [`Uri`][] destination, and
    //! its `Response` is some type implementing [`AsyncRead`][], [`AsyncWrite`][],
    //! and [`Connection`][].
    //!
    //! ## Custom Connectors
    //!
    //! A simple connector that ignores the `Uri` destination and always returns
    //! a TCP connection to the same address could be written like this:
    //!
    //! ```rust,ignore
    //! let connector = tower::service_fn(|_dst| async {
    //!     tokio::net::TcpStream::connect("127.0.0.1:1337")
    //! })
    //! ```
    //!
    //! Or, fully written out:
    //!
    //! ```
    //! # #[cfg(feature = "runtime")]
    //! # mod rt {
    //! use std::{future::Future, net::SocketAddr, pin::Pin, task::{self, Poll}};
    //! use hyper::{service::Service, Uri};
    //! use tokio::net::TcpStream;
    //!
    //! #[derive(Clone)]
    //! struct LocalConnector;
    //!
    //! impl Service<Uri> for LocalConnector {
    //!     type Response = TcpStream;
    //!     type Error = std::io::Error;
    //!     // We can't "name" an `async` generated future.
    //!     type Future = Pin<Box<
    //!         dyn Future<Output = Result<Self::Response, Self::Error>> + Send
    //!     >>;
    //!
    //!     fn poll_ready(&mut self, _: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>> {
    //!         // This connector is always ready, but others might not be.
    //!         Poll::Ready(Ok(()))
    //!     }
    //!
    //!     fn call(&mut self, _: Uri) -> Self::Future {
    //!         Box::pin(TcpStream::connect(SocketAddr::from(([127, 0, 0, 1], 1337))))
    //!     }
    //! }
    //! # }
    //! ```
    //!
    //! It's worth noting that for `TcpStream`s, the [`HttpConnector`][] is a
    //! better starting place to extend from.
    //!
    //! Using either of the above connector examples, it can be used with the
    //! `Client` like this:
    //!
    //! ```
    //! # #[cfg(feature = "runtime")]
    //! # fn rt () {
    //! # let connector = hyper::client::HttpConnector::new();
    //! // let connector = ...
    //!
    //! let client = hyper::Client::builder()
    //!     .build::<_, hyper::Body>(connector);
    //! # }
    //! ```
    //!
    //!
    //! [`HttpConnector`]: HttpConnector
    //! [`Service`]: crate::service::Service
    //! [`Uri`]: ::http::Uri
    //! [`AsyncRead`]: tokio::io::AsyncRead
    //! [`AsyncWrite`]: tokio::io::AsyncWrite
    //! [`Connection`]: Connection
    use std::fmt;

    use ::http::Extensions;

    cfg_feature! {
        #![feature = "tcp"]

        pub use self::http::{HttpConnector, HttpInfo};

        pub mod dns;
        mod http;
    }

    cfg_feature! {
        #![any(feature = "http1", feature = "http2")]

        pub use self::sealed::Connect;
    }

    /// Describes a type returned by a connector.
    pub trait Connection {
        /// Return metadata describing the connection.
        fn connected(&self) -> Connected;
    }

    /// Extra information about the connected transport.
    ///
    /// This can be used to inform recipients about things like if ALPN
    /// was used, or if connected to an HTTP proxy.
    #[derive(Debug)]
    pub struct Connected {
        pub(super) alpn: Alpn,
        pub(super) is_proxied: bool,
        pub(super) extra: Option<Extra>,
    }

    pub(super) struct Extra(Box<dyn ExtraInner>);

    #[derive(Clone, Copy, Debug, PartialEq)]
    pub(super) enum Alpn {
        H2,
        None,
    }

    impl Connected {
        /// Create new `Connected` type with empty metadata.
        pub fn new() -> Connected {
            Connected {
                alpn: Alpn::None,
                is_proxied: false,
                extra: None,
            }
        }

        /// Set whether the connected transport is to an HTTP proxy.
        ///
        /// This setting will affect if HTTP/1 requests written on the transport
        /// will have the request-target in absolute-form or origin-form:
        ///
        /// - When `proxy(false)`:
        ///
        /// ```http
        /// GET /guide HTTP/1.1
        /// ```
        ///
        /// - When `proxy(true)`:
        ///
        /// ```http
        /// GET http://hyper.rs/guide HTTP/1.1
        /// ```
        ///
        /// Default is `false`.
        pub fn proxy(mut self, is_proxied: bool) -> Connected {
            self.is_proxied = is_proxied;
            self
        }

        /// Determines if the connected transport is to an HTTP proxy.
        pub fn is_proxied(&self) -> bool {
            self.is_proxied
        }

        /// Set extra connection information to be set in the extensions of every `Response`.
        pub fn extra<T: Clone + Send + Sync + 'static>(mut self, extra: T) -> Connected {
            if let Some(prev) = self.extra {
                self.extra = Some(Extra(Box::new(ExtraChain(prev.0, extra))));
            } else {
                self.extra = Some(Extra(Box::new(ExtraEnvelope(extra))));
            }
            self
        }

        /// Copies the extra connection information into an `Extensions` map.
        pub fn get_extras(&self, extensions: &mut Extensions) {
            if let Some(extra) = &self.extra {
                extra.set(extensions);
            }
        }

        /// Set that the connected transport negotiated HTTP/2 as its next protocol.
        pub fn negotiated_h2(mut self) -> Connected {
            self.alpn = Alpn::H2;
            self
        }

        /// Determines if the connected transport negotiated HTTP/2 as its next protocol.
        pub fn is_negotiated_h2(&self) -> bool {
            self.alpn == Alpn::H2
        }

        // Don't public expose that `Connected` is `Clone`, unsure if we want to
        // keep that contract...
        #[cfg(feature = "http2")]
        pub(super) fn clone(&self) -> Connected {
            Connected {
                alpn: self.alpn.clone(),
                is_proxied: self.is_proxied,
                extra: self.extra.clone(),
            }
        }
    }

    // ===== impl Extra =====

    impl Extra {
        pub(super) fn set(&self, res: &mut Extensions) {
            self.0.set(res);
        }
    }

    impl Clone for Extra {
        fn clone(&self) -> Extra {
            Extra(self.0.clone_box())
        }
    }

    impl fmt::Debug for Extra {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Extra").finish()
        }
    }

    trait ExtraInner: Send + Sync {
        fn clone_box(&self) -> Box<dyn ExtraInner>;
        fn set(&self, res: &mut Extensions);
    }

    // This indirection allows the `Connected` to have a type-erased "extra" value,
    // while that type still knows its inner extra type. This allows the correct
    // TypeId to be used when inserting into `res.extensions_mut()`.
    #[derive(Clone)]
    struct ExtraEnvelope<T>(T);

    impl<T> ExtraInner for ExtraEnvelope<T>
    where
        T: Clone + Send + Sync + 'static,
    {
        fn clone_box(&self) -> Box<dyn ExtraInner> {
            Box::new(self.clone())
        }

        fn set(&self, res: &mut Extensions) {
            res.insert(self.0.clone());
        }
    }

    struct ExtraChain<T>(Box<dyn ExtraInner>, T);

    impl<T: Clone> Clone for ExtraChain<T> {
        fn clone(&self) -> Self {
            ExtraChain(self.0.clone_box(), self.1.clone())
        }
    }

    impl<T> ExtraInner for ExtraChain<T>
    where
        T: Clone + Send + Sync + 'static,
    {
        fn clone_box(&self) -> Box<dyn ExtraInner> {
            Box::new(self.clone())
        }

        fn set(&self, res: &mut Extensions) {
            self.0.set(res);
            res.insert(self.1.clone());
        }
    }

    #[cfg(any(feature = "http1", feature = "http2"))]
    pub(super) mod sealed {
        use std::error::Error as StdError;

        use ::http::Uri;
        use tokio::io::{AsyncRead, AsyncWrite};

        use super::Connection;
        use crate::common::{Future, Unpin};

        /// Connect to a destination, returning an IO transport.
        ///
        /// A connector receives a [`Uri`](::http::Uri) and returns a `Future` of the
        /// ready connection.
        ///
        /// # Trait Alias
        ///
        /// This is really just an *alias* for the `tower::Service` trait, with
        /// additional bounds set for convenience *inside* hyper. You don't actually
        /// implement this trait, but `tower::Service<Uri>` instead.
        // The `Sized` bound is to prevent creating `dyn Connect`, since they cannot
        // fit the `Connect` bounds because of the blanket impl for `Service`.
        pub trait Connect: Sealed + Sized {
            #[doc(hidden)]
            type _Svc: ConnectSvc;
            #[doc(hidden)]
            fn connect(self, internal_only: Internal, dst: Uri) -> <Self::_Svc as ConnectSvc>::Future;
        }

        pub trait ConnectSvc {
            type Connection: AsyncRead + AsyncWrite + Connection + Unpin + Send + 'static;
            type Error: Into<Box<dyn StdError + Send + Sync>>;
            type Future: Future<Output = Result<Self::Connection, Self::Error>> + Unpin + Send + 'static;

            fn connect(self, internal_only: Internal, dst: Uri) -> Self::Future;
        }

        impl<S, T> Connect for S
        where
            S: tower_service::Service<Uri, Response = T> + Send + 'static,
            S::Error: Into<Box<dyn StdError + Send + Sync>>,
            S::Future: Unpin + Send,
            T: AsyncRead + AsyncWrite + Connection + Unpin + Send + 'static,
        {
            type _Svc = S;

            fn connect(self, _: Internal, dst: Uri) -> crate::service::Oneshot<S, Uri> {
                crate::service::oneshot(self, dst)
            }
        }

        impl<S, T> ConnectSvc for S
        where
            S: tower_service::Service<Uri, Response = T> + Send + 'static,
            S::Error: Into<Box<dyn StdError + Send + Sync>>,
            S::Future: Unpin + Send,
            T: AsyncRead + AsyncWrite + Connection + Unpin + Send + 'static,
        {
            type Connection = T;
            type Error = S::Error;
            type Future = crate::service::Oneshot<S, Uri>;

            fn connect(self, _: Internal, dst: Uri) -> Self::Future {
                crate::service::oneshot(self, dst)
            }
        }

        impl<S, T> Sealed for S
        where
            S: tower_service::Service<Uri, Response = T> + Send,
            S::Error: Into<Box<dyn StdError + Send + Sync>>,
            S::Future: Unpin + Send,
            T: AsyncRead + AsyncWrite + Connection + Unpin + Send + 'static,
        {
        }

        pub trait Sealed {}
        #[allow(missing_debug_implementations)]
        pub struct Internal;
    }

    #[cfg(test)]
    mod tests {
        use super::Connected;

        #[derive(Clone, Debug, PartialEq)]
        struct Ex1(usize);

        #[derive(Clone, Debug, PartialEq)]
        struct Ex2(&'static str);

        #[derive(Clone, Debug, PartialEq)]
        struct Ex3(&'static str);

        #[test]
        fn test_connected_extra() {
            let c1 = Connected::new().extra(Ex1(41));

            let mut ex = ::http::Extensions::new();

            assert_eq!(ex.get::<Ex1>(), None);

            c1.extra.as_ref().expect("c1 extra").set(&mut ex);

            assert_eq!(ex.get::<Ex1>(), Some(&Ex1(41)));
        }

        #[test]
        fn test_connected_extra_chain() {
            // If a user composes connectors and at each stage, there's "extra"
            // info to attach, it shouldn't override the previous extras.

            let c1 = Connected::new()
                .extra(Ex1(45))
                .extra(Ex2("zoom"))
                .extra(Ex3("pew pew"));

            let mut ex1 = ::http::Extensions::new();

            assert_eq!(ex1.get::<Ex1>(), None);
            assert_eq!(ex1.get::<Ex2>(), None);
            assert_eq!(ex1.get::<Ex3>(), None);

            c1.extra.as_ref().expect("c1 extra").set(&mut ex1);

            assert_eq!(ex1.get::<Ex1>(), Some(&Ex1(45)));
            assert_eq!(ex1.get::<Ex2>(), Some(&Ex2("zoom")));
            assert_eq!(ex1.get::<Ex3>(), Some(&Ex3("pew pew")));

            // Just like extensions, inserting the same type overrides previous type.
            let c2 = Connected::new()
                .extra(Ex1(33))
                .extra(Ex2("hiccup"))
                .extra(Ex1(99));

            let mut ex2 = ::http::Extensions::new();

            c2.extra.as_ref().expect("c2 extra").set(&mut ex2);

            assert_eq!(ex2.get::<Ex1>(), Some(&Ex1(99)));
            assert_eq!(ex2.get::<Ex2>(), Some(&Ex2("hiccup")));
        }
    }