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    use std::error::Error as StdError;
    use std::fmt;
    use std::mem;
    use std::time::Duration;

    use futures_channel::oneshot;
    use futures_util::future::{self, Either, FutureExt as _, TryFutureExt as _};
    use http::header::{HeaderValue, HOST};
    use http::uri::Scheme;
    use http::{Method, Request, Response, Uri, Version};

    use super::conn;
    use super::connect::{self, sealed::Connect, Alpn, Connected, Connection};
    use super::pool::{self, Key as PoolKey, Pool, Poolable, Pooled, Reservation};
    #[cfg(feature = "tcp")]
    use super::HttpConnector;
    use crate::body::{Body, HttpBody};
    use crate::common::{exec::BoxSendFuture, lazy as hyper_lazy, task, Future, Lazy, Pin, Poll};
    use crate::rt::Executor;

    /// A Client to make outgoing HTTP requests.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "http1", feature = "http2"))))]
    pub struct Client<C, B = Body> {
        config: Config,
        conn_builder: conn::Builder,
        connector: C,
        pool: Pool<PoolClient<B>>,
    }

    #[derive(Clone, Copy, Debug)]
    struct Config {
        retry_canceled_requests: bool,
        set_host: bool,
        ver: Ver,
    }

    /// A `Future` that will resolve to an HTTP Response.
    ///
    /// This is returned by `Client::request` (and `Client::get`).
    #[must_use = "futures do nothing unless polled"]
    pub struct ResponseFuture {
        inner: Pin<Box<dyn Future<Output = crate::Result<Response<Body>>> + Send>>,
    }

    // ===== impl Client =====

    #[cfg(feature = "tcp")]
    impl Client<HttpConnector, Body> {
        /// Create a new Client with the default [config](Builder).
        ///
        /// # Note
        ///
        /// The default connector does **not** handle TLS. Speaking to `https`
        /// destinations will require [configuring a connector that implements
        /// TLS](https://hyper.rs/guides/client/configuration).
        #[inline]
        pub fn new() -> Client<HttpConnector, Body> {
            Builder::default().build_http()
        }
    }

    #[cfg(feature = "tcp")]
    impl Default for Client<HttpConnector, Body> {
        fn default() -> Client<HttpConnector, Body> {
            Client::new()
        }
    }

    impl Client<(), Body> {
        /// Create a builder to configure a new `Client`.
        ///
        /// # Example
        ///
        /// ```
        /// # #[cfg(feature  = "runtime")]
        /// # fn run () {
        /// use std::time::Duration;
        /// use hyper::Client;
        ///
        /// let client = Client::builder()
        ///     .pool_idle_timeout(Duration::from_secs(30))
        ///     .http2_only(true)
        ///     .build_http();
        /// # let infer: Client<_, hyper::Body> = client;
        /// # drop(infer);
        /// # }
        /// # fn main() {}
        /// ```
        #[inline]
        pub fn builder() -> Builder {
            Builder::default()
        }
    }

    impl<C, B> Client<C, B>
    where
        C: Connect + Clone + Send + Sync + 'static,
        B: HttpBody + Send + 'static,
        B::Data: Send,
        B::Error: Into<Box<dyn StdError + Send + Sync>>,
    {
        /// Send a `GET` request to the supplied `Uri`.
        ///
        /// # Note
        ///
        /// This requires that the `HttpBody` type have a `Default` implementation.
        /// It *should* return an "empty" version of itself, such that
        /// `HttpBody::is_end_stream` is `true`.
        ///
        /// # Example
        ///
        /// ```
        /// # #[cfg(feature  = "runtime")]
        /// # fn run () {
        /// use hyper::{Client, Uri};
        ///
        /// let client = Client::new();
        ///
        /// let future = client.get(Uri::from_static("http://httpbin.org/ip"));
        /// # }
        /// # fn main() {}
        /// ```
        pub fn get(&self, uri: Uri) -> ResponseFuture
        where
            B: Default,
        {
            let body = B::default();
            if !body.is_end_stream() {
                warn!("default HttpBody used for get() does not return true for is_end_stream");
            }

            let mut req = Request::new(body);
            *req.uri_mut() = uri;
            self.request(req)
        }

        /// Send a constructed `Request` using this `Client`.
        ///
        /// # Example
        ///
        /// ```
        /// # #[cfg(feature  = "runtime")]
        /// # fn run () {
        /// use hyper::{Body, Client, Request};
        ///
        /// let client = Client::new();
        ///
        /// let req = Request::builder()
        ///     .method("POST")
        ///     .uri("http://httpbin.org/post")
        ///     .body(Body::from("Hallo!"))
        ///     .expect("request builder");
        ///
        /// let future = client.request(req);
        /// # }
        /// # fn main() {}
        /// ```
        pub fn request(&self, mut req: Request<B>) -> ResponseFuture {
            let is_http_connect = req.method() == Method::CONNECT;
            match req.version() {
                Version::HTTP_11 => (),
                Version::HTTP_10 => {
                    if is_http_connect {
                        warn!("CONNECT is not allowed for HTTP/1.0");
                        return ResponseFuture::new(Box::new(future::err(
                            crate::Error::new_user_unsupported_request_method(),
                        )));
                    }
                }
                other_h2 @ Version::HTTP_2 => {
                    if self.config.ver != Ver::Http2 {
                        return ResponseFuture::error_version(other_h2);
                    }
                }
                // completely unsupported HTTP version (like HTTP/0.9)!
                other => return ResponseFuture::error_version(other),
            };

            let pool_key = match extract_domain(req.uri_mut(), is_http_connect) {
                Ok(s) => s,
                Err(err) => {
                    return ResponseFuture::new(Box::new(future::err(err)));
                }
            };

            ResponseFuture::new(Box::new(self.retryably_send_request(req, pool_key)))
        }

        fn retryably_send_request(
            &self,
            req: Request<B>,
            pool_key: PoolKey,
        ) -> impl Future<Output = crate::Result<Response<Body>>> {
            let client = self.clone();
            let uri = req.uri().clone();

            let mut send_fut = client.send_request(req, pool_key.clone());
            future::poll_fn(move |cx| loop {
                match ready!(Pin::new(&mut send_fut).poll(cx)) {
                    Ok(resp) => return Poll::Ready(Ok(resp)),
                    Err(ClientError::Normal(err)) => return Poll::Ready(Err(err)),
                    Err(ClientError::Canceled {
                        connection_reused,
                        mut req,
                        reason,
                    }) => {
                        if !client.config.retry_canceled_requests || !connection_reused {
                            // if client disabled, don't retry
                            // a fresh connection means we definitely can't retry
                            return Poll::Ready(Err(reason));
                        }

                        trace!(
                            "unstarted request canceled, trying again (reason={:?})",
                            reason
                        );
                        *req.uri_mut() = uri.clone();
                        send_fut = client.send_request(req, pool_key.clone());
                    }
                }
            })
        }

        fn send_request(
            &self,
            mut req: Request<B>,
            pool_key: PoolKey,
        ) -> impl Future<Output = Result<Response<Body>, ClientError<B>>> + Unpin {
            let conn = self.connection_for(pool_key);

            let set_host = self.config.set_host;
            let executor = self.conn_builder.exec.clone();
            conn.and_then(move |mut pooled| {
                if pooled.is_http1() {
                    if set_host {
                        let uri = req.uri().clone();
                        req.headers_mut().entry(HOST).or_insert_with(|| {
                            let hostname = uri.host().expect("authority implies host");
                            if let Some(port) = uri.port() {
                                let s = format!("{}:{}", hostname, port);
                                HeaderValue::from_str(&s)
                            } else {
                                HeaderValue::from_str(hostname)
                            }
                            .expect("uri host is valid header value")
                        });
                    }

                    // CONNECT always sends authority-form, so check it first...
                    if req.method() == Method::CONNECT {
                        authority_form(req.uri_mut());
                    } else if pooled.conn_info.is_proxied {
                        absolute_form(req.uri_mut());
                    } else {
                        origin_form(req.uri_mut());
                    };
                } else if req.method() == Method::CONNECT {
                    debug!("client does not support CONNECT requests over HTTP2");
                    return Either::Left(future::err(ClientError::Normal(
                        crate::Error::new_user_unsupported_request_method(),
                    )));
                }

                let fut = pooled
                    .send_request_retryable(req)
                    .map_err(ClientError::map_with_reused(pooled.is_reused()));

                // If the Connector included 'extra' info, add to Response...
                let extra_info = pooled.conn_info.extra.clone();
                let fut = fut.map_ok(move |mut res| {
                    if let Some(extra) = extra_info {
                        extra.set(res.extensions_mut());
                    }
                    res
                });

                // As of futures@0.1.21, there is a race condition in the mpsc
                // channel, such that sending when the receiver is closing can
                // result in the message being stuck inside the queue. It won't
                // ever notify until the Sender side is dropped.
                //
                // To counteract this, we must check if our senders 'want' channel
                // has been closed after having tried to send. If so, error out...
                if pooled.is_closed() {
                    return Either::Right(Either::Left(fut));
                }

                Either::Right(Either::Right(fut.map_ok(move |mut res| {
                    // If pooled is HTTP/2, we can toss this reference immediately.
                    //
                    // when pooled is dropped, it will try to insert back into the
                    // pool. To delay that, spawn a future that completes once the
                    // sender is ready again.
                    //
                    // This *should* only be once the related `Connection` has polled
                    // for a new request to start.
                    //
                    // It won't be ready if there is a body to stream.
                    if pooled.is_http2() || !pooled.is_pool_enabled() || pooled.is_ready() {
                        drop(pooled);
                    } else if !res.body().is_end_stream() {
                        let (delayed_tx, delayed_rx) = oneshot::channel();
                        res.body_mut().delayed_eof(delayed_rx);
                        let on_idle = future::poll_fn(move |cx| pooled.poll_ready(cx)).map(move |_| {
                            // At this point, `pooled` is dropped, and had a chance
                            // to insert into the pool (if conn was idle)
                            drop(delayed_tx);
                        });

                        executor.execute(on_idle);
                    } else {
                        // There's no body to delay, but the connection isn't
                        // ready yet. Only re-insert when it's ready
                        let on_idle = future::poll_fn(move |cx| pooled.poll_ready(cx)).map(|_| ());

                        executor.execute(on_idle);
                    }
                    res
                })))
            })
        }

        fn connection_for(
            &self,
            pool_key: PoolKey,
        ) -> impl Future<Output = Result<Pooled<PoolClient<B>>, ClientError<B>>> {
            // This actually races 2 different futures to try to get a ready
            // connection the fastest, and to reduce connection churn.
            //
            // - If the pool has an idle connection waiting, that's used
            //   immediately.
            // - Otherwise, the Connector is asked to start connecting to
            //   the destination Uri.
            // - Meanwhile, the pool Checkout is watching to see if any other
            //   request finishes and tries to insert an idle connection.
            // - If a new connection is started, but the Checkout wins after
            //   (an idle connection became available first), the started
            //   connection future is spawned into the runtime to complete,
            //   and then be inserted into the pool as an idle connection.
            let checkout = self.pool.checkout(pool_key.clone());
            let connect = self.connect_to(pool_key);

            let executor = self.conn_builder.exec.clone();
            // The order of the `select` is depended on below...
            future::select(checkout, connect).then(move |either| match either {
                // Checkout won, connect future may have been started or not.
                //
                // If it has, let it finish and insert back into the pool,
                // so as to not waste the socket...
                Either::Left((Ok(checked_out), connecting)) => {
                    // This depends on the `select` above having the correct
                    // order, such that if the checkout future were ready
                    // immediately, the connect future will never have been
                    // started.
                    //
                    // If it *wasn't* ready yet, then the connect future will
                    // have been started...
                    if connecting.started() {
                        let bg = connecting
                            .map_err(|err| {
                                trace!("background connect error: {}", err);
                            })
                            .map(|_pooled| {
                                // dropping here should just place it in
                                // the Pool for us...
                            });
                        // An execute error here isn't important, we're just trying
                        // to prevent a waste of a socket...
                        executor.execute(bg);
                    }
                    Either::Left(future::ok(checked_out))
                }
                // Connect won, checkout can just be dropped.
                Either::Right((Ok(connected), _checkout)) => Either::Left(future::ok(connected)),
                // Either checkout or connect could get canceled:
                //
                // 1. Connect is canceled if this is HTTP/2 and there is
                //    an outstanding HTTP/2 connecting task.
                // 2. Checkout is canceled if the pool cannot deliver an
                //    idle connection reliably.
                //
                // In both cases, we should just wait for the other future.
                Either::Left((Err(err), connecting)) => Either::Right(Either::Left({
                    if err.is_canceled() {
                        Either::Left(connecting.map_err(ClientError::Normal))
                    } else {
                        Either::Right(future::err(ClientError::Normal(err)))
                    }
                })),
                Either::Right((Err(err), checkout)) => Either::Right(Either::Right({
                    if err.is_canceled() {
                        Either::Left(checkout.map_err(ClientError::Normal))
                    } else {
                        Either::Right(future::err(ClientError::Normal(err)))
                    }
                })),
            })
        }

        fn connect_to(
            &self,
            pool_key: PoolKey,
        ) -> impl Lazy<Output = crate::Result<Pooled<PoolClient<B>>>> + Unpin {
            let executor = self.conn_builder.exec.clone();
            let pool = self.pool.clone();
            #[cfg(not(feature = "http2"))]
            let conn_builder = self.conn_builder.clone();
            #[cfg(feature = "http2")]
            let mut conn_builder = self.conn_builder.clone();
            let ver = self.config.ver;
            let is_ver_h2 = ver == Ver::Http2;
            let connector = self.connector.clone();
            let dst = domain_as_uri(pool_key.clone());
            hyper_lazy(move || {
                // Try to take a "connecting lock".
                //
                // If the pool_key is for HTTP/2, and there is already a
                // connection being established, then this can't take a
                // second lock. The "connect_to" future is Canceled.
                let connecting = match pool.connecting(&pool_key, ver) {
                    Some(lock) => lock,
                    None => {
                        let canceled =
                            crate::Error::new_canceled().with("HTTP/2 connection in progress");
                        return Either::Right(future::err(canceled));
                    }
                };
                Either::Left(
                    connector
                        .connect(connect::sealed::Internal, dst)
                        .map_err(crate::Error::new_connect)
                        .and_then(move |io| {
                            let connected = io.connected();
                            // If ALPN is h2 and we aren't http2_only already,
                            // then we need to convert our pool checkout into
                            // a single HTTP2 one.
                            let connecting = if connected.alpn == Alpn::H2 && !is_ver_h2 {
                                match connecting.alpn_h2(&pool) {
                                    Some(lock) => {
                                        trace!("ALPN negotiated h2, updating pool");
                                        lock
                                    }
                                    None => {
                                        // Another connection has already upgraded,
                                        // the pool checkout should finish up for us.
                                        let canceled = crate::Error::new_canceled()
                                            .with("ALPN upgraded to HTTP/2");
                                        return Either::Right(future::err(canceled));
                                    }
                                }
                            } else {
                                connecting
                            };

                            #[cfg_attr(not(feature = "http2"), allow(unused))]
                            let is_h2 = is_ver_h2 || connected.alpn == Alpn::H2;
                            #[cfg(feature = "http2")]
                            {
                                conn_builder.http2_only(is_h2);
                            }

                            Either::Left(Box::pin(
                                conn_builder
                                    .handshake(io)
                                    .and_then(move |(tx, conn)| {
                                        trace!(
                                            "handshake complete, spawning background dispatcher task"
                                        );
                                        executor.execute(
                                            conn.map_err(|e| debug!("client connection error: {}", e))
                                                .map(|_| ()),
                                        );

                                        // Wait for 'conn' to ready up before we
                                        // declare this tx as usable
                                        tx.when_ready()
                                    })
                                    .map_ok(move |tx| {
                                        let tx = {
                                            #[cfg(feature = "http2")]
                                            {
                                                if is_h2 {
                                                    PoolTx::Http2(tx.into_http2())
                                                } else {
                                                    PoolTx::Http1(tx)
                                                }
                                            }
                                            #[cfg(not(feature = "http2"))]
                                            PoolTx::Http1(tx)
                                        };
                                        pool.pooled(
                                            connecting,
                                            PoolClient {
                                                conn_info: connected,
                                                tx,
                                            },
                                        )
                                    }),
                            ))
                        }),
                )
            })
        }
    }

    impl<C, B> tower_service::Service<Request<B>> for Client<C, B>
    where
        C: Connect + Clone + Send + Sync + 'static,
        B: HttpBody + Send + 'static,
        B::Data: Send,
        B::Error: Into<Box<dyn StdError + Send + Sync>>,
    {
        type Response = Response<Body>;
        type Error = crate::Error;
        type Future = ResponseFuture;

        fn poll_ready(&mut self, _: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>> {
            Poll::Ready(Ok(()))
        }

        fn call(&mut self, req: Request<B>) -> Self::Future {
            self.request(req)
        }
    }

    impl<C, B> tower_service::Service<Request<B>> for &'_ Client<C, B>
    where
        C: Connect + Clone + Send + Sync + 'static,
        B: HttpBody + Send + 'static,
        B::Data: Send,
        B::Error: Into<Box<dyn StdError + Send + Sync>>,
    {
        type Response = Response<Body>;
        type Error = crate::Error;
        type Future = ResponseFuture;

        fn poll_ready(&mut self, _: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>> {
            Poll::Ready(Ok(()))
        }

        fn call(&mut self, req: Request<B>) -> Self::Future {
            self.request(req)
        }
    }

    impl<C: Clone, B> Clone for Client<C, B> {
        fn clone(&self) -> Client<C, B> {
            Client {
                config: self.config.clone(),
                conn_builder: self.conn_builder.clone(),
                connector: self.connector.clone(),
                pool: self.pool.clone(),
            }
        }
    }

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

    // ===== impl ResponseFuture =====

    impl ResponseFuture {
        fn new(fut: Box<dyn Future<Output = crate::Result<Response<Body>>> + Send>) -> Self {
            Self { inner: fut.into() }
        }

        fn error_version(ver: Version) -> Self {
            warn!("Request has unsupported version \"{:?}\"", ver);
            ResponseFuture::new(Box::new(future::err(
                crate::Error::new_user_unsupported_version(),
            )))
        }
    }

    impl fmt::Debug for ResponseFuture {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Future<Response>")
        }
    }

    impl Future for ResponseFuture {
        type Output = crate::Result<Response<Body>>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            Pin::new(&mut self.inner).poll(cx)
        }
    }

    // ===== impl PoolClient =====

    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    struct PoolClient<B> {
        conn_info: Connected,
        tx: PoolTx<B>,
    }

    enum PoolTx<B> {
        Http1(conn::SendRequest<B>),
        #[cfg(feature = "http2")]
        Http2(conn::Http2SendRequest<B>),
    }

    impl<B> PoolClient<B> {
        fn poll_ready(&mut self, cx: &mut task::Context<'_>) -> Poll<crate::Result<()>> {
            match self.tx {
                PoolTx::Http1(ref mut tx) => tx.poll_ready(cx),
                #[cfg(feature = "http2")]
                PoolTx::Http2(_) => Poll::Ready(Ok(())),
            }
        }

        fn is_http1(&self) -> bool {
            !self.is_http2()
        }

        fn is_http2(&self) -> bool {
            match self.tx {
                PoolTx::Http1(_) => false,
                #[cfg(feature = "http2")]
                PoolTx::Http2(_) => true,
            }
        }

        fn is_ready(&self) -> bool {
            match self.tx {
                PoolTx::Http1(ref tx) => tx.is_ready(),
                #[cfg(feature = "http2")]
                PoolTx::Http2(ref tx) => tx.is_ready(),
            }
        }

        fn is_closed(&self) -> bool {
            match self.tx {
                PoolTx::Http1(ref tx) => tx.is_closed(),
                #[cfg(feature = "http2")]
                PoolTx::Http2(ref tx) => tx.is_closed(),
            }
        }
    }

    impl<B: HttpBody + 'static> PoolClient<B> {
        fn send_request_retryable(
            &mut self,
            req: Request<B>,
        ) -> impl Future<Output = Result<Response<Body>, (crate::Error, Option<Request<B>>)>>
        where
            B: Send,
        {
            match self.tx {
                #[cfg(not(feature = "http2"))]
                PoolTx::Http1(ref mut tx) => tx.send_request_retryable(req),
                #[cfg(feature = "http2")]
                PoolTx::Http1(ref mut tx) => Either::Left(tx.send_request_retryable(req)),
                #[cfg(feature = "http2")]
                PoolTx::Http2(ref mut tx) => Either::Right(tx.send_request_retryable(req)),
            }
        }
    }

    impl<B> Poolable for PoolClient<B>
    where
        B: Send + 'static,
    {
        fn is_open(&self) -> bool {
            match self.tx {
                PoolTx::Http1(ref tx) => tx.is_ready(),
                #[cfg(feature = "http2")]
                PoolTx::Http2(ref tx) => tx.is_ready(),
            }
        }

        fn reserve(self) -> Reservation<Self> {
            match self.tx {
                PoolTx::Http1(tx) => Reservation::Unique(PoolClient {
                    conn_info: self.conn_info,
                    tx: PoolTx::Http1(tx),
                }),
                #[cfg(feature = "http2")]
                PoolTx::Http2(tx) => {
                    let b = PoolClient {
                        conn_info: self.conn_info.clone(),
                        tx: PoolTx::Http2(tx.clone()),
                    };
                    let a = PoolClient {
                        conn_info: self.conn_info,
                        tx: PoolTx::Http2(tx),
                    };
                    Reservation::Shared(a, b)
                }
            }
        }

        fn can_share(&self) -> bool {
            self.is_http2()
        }
    }

    // ===== impl ClientError =====

    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    enum ClientError<B> {
        Normal(crate::Error),
        Canceled {
            connection_reused: bool,
            req: Request<B>,
            reason: crate::Error,
        },
    }

    impl<B> ClientError<B> {
        fn map_with_reused(conn_reused: bool) -> impl Fn((crate::Error, Option<Request<B>>)) -> Self {
            move |(err, orig_req)| {
                if let Some(req) = orig_req {
                    ClientError::Canceled {
                        connection_reused: conn_reused,
                        reason: err,
                        req,
                    }
                } else {
                    ClientError::Normal(err)
                }
            }
        }
    }

    /// A marker to identify what version a pooled connection is.
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    pub(super) enum Ver {
        Auto,
        Http2,
    }

    fn origin_form(uri: &mut Uri) {
        let path = match uri.path_and_query() {
            Some(path) if path.as_str() != "/" => {
                let mut parts = ::http::uri::Parts::default();
                parts.path_and_query = Some(path.clone());
                Uri::from_parts(parts).expect("path is valid uri")
            }
            _none_or_just_slash => {
                debug_assert!(Uri::default() == "/");
                Uri::default()
            }
        };
        *uri = path
    }

    fn absolute_form(uri: &mut Uri) {
        debug_assert!(uri.scheme().is_some(), "absolute_form needs a scheme");
        debug_assert!(
            uri.authority().is_some(),
            "absolute_form needs an authority"
        );
        // If the URI is to HTTPS, and the connector claimed to be a proxy,
        // then it *should* have tunneled, and so we don't want to send
        // absolute-form in that case.
        if uri.scheme() == Some(&Scheme::HTTPS) {
            origin_form(uri);
        }
    }

    fn authority_form(uri: &mut Uri) {
        if let Some(path) = uri.path_and_query() {
            // `https://hyper.rs` would parse with `/` path, don't
            // annoy people about that...
            if path != "/" {
                warn!("HTTP/1.1 CONNECT request stripping path: {:?}", path);
            }
        }
        *uri = match uri.authority() {
            Some(auth) => {
                let mut parts = ::http::uri::Parts::default();
                parts.authority = Some(auth.clone());
                Uri::from_parts(parts).expect("authority is valid")
            }
            None => {
                unreachable!("authority_form with relative uri");
            }
        };
    }

    fn extract_domain(uri: &mut Uri, is_http_connect: bool) -> crate::Result<PoolKey> {
        let uri_clone = uri.clone();
        match (uri_clone.scheme(), uri_clone.authority()) {
            (Some(scheme), Some(auth)) => Ok((scheme.clone(), auth.clone())),
            (None, Some(auth)) if is_http_connect => {
                let scheme = match auth.port_u16() {
                    Some(443) => {
                        set_scheme(uri, Scheme::HTTPS);
                        Scheme::HTTPS
                    }
                    _ => {
                        set_scheme(uri, Scheme::HTTP);
                        Scheme::HTTP
                    }
                };
                Ok((scheme, auth.clone()))
            }
            _ => {
                debug!("Client requires absolute-form URIs, received: {:?}", uri);
                Err(crate::Error::new_user_absolute_uri_required())
            }
        }
    }

    fn domain_as_uri((scheme, auth): PoolKey) -> Uri {
        http::uri::Builder::new()
            .scheme(scheme)
            .authority(auth)
            .path_and_query("/")
            .build()
            .expect("domain is valid Uri")
    }

    fn set_scheme(uri: &mut Uri, scheme: Scheme) {
        debug_assert!(
            uri.scheme().is_none(),
            "set_scheme expects no existing scheme"
        );
        let old = mem::replace(uri, Uri::default());
        let mut parts: ::http::uri::Parts = old.into();
        parts.scheme = Some(scheme);
        parts.path_and_query = Some("/".parse().expect("slash is a valid path"));
        *uri = Uri::from_parts(parts).expect("scheme is valid");
    }

    /// A builder to configure a new [`Client`](Client).
    ///
    /// # Example
    ///
    /// ```
    /// # #[cfg(feature  = "runtime")]
    /// # fn run () {
    /// use std::time::Duration;
    /// use hyper::Client;
    ///
    /// let client = Client::builder()
    ///     .pool_idle_timeout(Duration::from_secs(30))
    ///     .http2_only(true)
    ///     .build_http();
    /// # let infer: Client<_, hyper::Body> = client;
    /// # drop(infer);
    /// # }
    /// # fn main() {}
    /// ```
    #[cfg_attr(docsrs, doc(cfg(any(feature = "http1", feature = "http2"))))]
    #[derive(Clone)]
    pub struct Builder {
        client_config: Config,
        conn_builder: conn::Builder,
        pool_config: pool::Config,
    }

    impl Default for Builder {
        fn default() -> Self {
            Self {
                client_config: Config {
                    retry_canceled_requests: true,
                    set_host: true,
                    ver: Ver::Auto,
                },
                conn_builder: conn::Builder::new(),
                pool_config: pool::Config {
                    idle_timeout: Some(Duration::from_secs(90)),
                    max_idle_per_host: std::usize::MAX,
                },
            }
        }
    }

    impl Builder {
        #[doc(hidden)]
        #[deprecated(
            note = "name is confusing, to disable the connection pool, call pool_max_idle_per_host(0)"
        )]
        pub fn keep_alive(&mut self, val: bool) -> &mut Self {
            if !val {
                // disable
                self.pool_max_idle_per_host(0)
            } else if self.pool_config.max_idle_per_host == 0 {
                // enable
                self.pool_max_idle_per_host(std::usize::MAX)
            } else {
                // already enabled
                self
            }
        }

        #[doc(hidden)]
        #[deprecated(note = "renamed to `pool_idle_timeout`")]
        pub fn keep_alive_timeout<D>(&mut self, val: D) -> &mut Self
        where
            D: Into<Option<Duration>>,
        {
            self.pool_idle_timeout(val)
        }

        /// Set an optional timeout for idle sockets being kept-alive.
        ///
        /// Pass `None` to disable timeout.
        ///
        /// Default is 90 seconds.
        pub fn pool_idle_timeout<D>(&mut self, val: D) -> &mut Self
        where
            D: Into<Option<Duration>>,
        {
            self.pool_config.idle_timeout = val.into();
            self
        }

        #[doc(hidden)]
        #[deprecated(note = "renamed to `pool_max_idle_per_host`")]
        pub fn max_idle_per_host(&mut self, max_idle: usize) -> &mut Self {
            self.pool_config.max_idle_per_host = max_idle;
            self
        }

        /// Sets the maximum idle connection per host allowed in the pool.
        ///
        /// Default is `usize::MAX` (no limit).
        pub fn pool_max_idle_per_host(&mut self, max_idle: usize) -> &mut Self {
            self.pool_config.max_idle_per_host = max_idle;
            self
        }

        // HTTP/1 options

        /// Sets the exact size of the read buffer to *always* use.
        ///
        /// Note that setting this option unsets the `http1_max_buf_size` option.
        ///
        /// Default is an adaptive read buffer.
        pub fn http1_read_buf_exact_size(&mut self, sz: usize) -> &mut Self {
            self.conn_builder.h1_read_buf_exact_size(Some(sz));
            self
        }

        /// Set the maximum buffer size for the connection.
        ///
        /// Default is ~400kb.
        ///
        /// Note that setting this option unsets the `http1_read_exact_buf_size` option.
        ///
        /// # Panics
        ///
        /// The minimum value allowed is 8192. This method panics if the passed `max` is less than the minimum.
        #[cfg(feature = "http1")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http1")))]
        pub fn http1_max_buf_size(&mut self, max: usize) -> &mut Self {
            self.conn_builder.h1_max_buf_size(max);
            self
        }

        /// Set whether HTTP/1 connections will write header names as title case at
        /// the socket level.
        ///
        /// Note that this setting does not affect HTTP/2.
        ///
        /// Default is false.
        pub fn http1_title_case_headers(&mut self, val: bool) -> &mut Self {
            self.conn_builder.h1_title_case_headers(val);
            self
        }

        /// Set whether the connection **must** use HTTP/2.
        ///
        /// The destination must either allow HTTP2 Prior Knowledge, or the
        /// `Connect` should be configured to do use ALPN to upgrade to `h2`
        /// as part of the connection process. This will not make the `Client`
        /// utilize ALPN by itself.
        ///
        /// Note that setting this to true prevents HTTP/1 from being allowed.
        ///
        /// Default is false.
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_only(&mut self, val: bool) -> &mut Self {
            self.client_config.ver = if val { Ver::Http2 } else { Ver::Auto };
            self
        }

        /// Sets the [`SETTINGS_INITIAL_WINDOW_SIZE`][spec] option for HTTP2
        /// stream-level flow control.
        ///
        /// Passing `None` will do nothing.
        ///
        /// If not set, hyper will use a default.
        ///
        /// [spec]: https://http2.github.io/http2-spec/#SETTINGS_INITIAL_WINDOW_SIZE
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_initial_stream_window_size(&mut self, sz: impl Into<Option<u32>>) -> &mut Self {
            self.conn_builder
                .http2_initial_stream_window_size(sz.into());
            self
        }

        /// Sets the max connection-level flow control for HTTP2
        ///
        /// Passing `None` will do nothing.
        ///
        /// If not set, hyper will use a default.
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_initial_connection_window_size(
            &mut self,
            sz: impl Into<Option<u32>>,
        ) -> &mut Self {
            self.conn_builder
                .http2_initial_connection_window_size(sz.into());
            self
        }

        /// Sets whether to use an adaptive flow control.
        ///
        /// Enabling this will override the limits set in
        /// `http2_initial_stream_window_size` and
        /// `http2_initial_connection_window_size`.
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_adaptive_window(&mut self, enabled: bool) -> &mut Self {
            self.conn_builder.http2_adaptive_window(enabled);
            self
        }

        /// Sets the maximum frame size to use for HTTP2.
        ///
        /// Passing `None` will do nothing.
        ///
        /// If not set, hyper will use a default.
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_max_frame_size(&mut self, sz: impl Into<Option<u32>>) -> &mut Self {
            self.conn_builder.http2_max_frame_size(sz);
            self
        }

        /// Sets an interval for HTTP2 Ping frames should be sent to keep a
        /// connection alive.
        ///
        /// Pass `None` to disable HTTP2 keep-alive.
        ///
        /// Default is currently disabled.
        ///
        /// # Cargo Feature
        ///
        /// Requires the `runtime` cargo feature to be enabled.
        #[cfg(feature = "runtime")]
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_keep_alive_interval(
            &mut self,
            interval: impl Into<Option<Duration>>,
        ) -> &mut Self {
            self.conn_builder.http2_keep_alive_interval(interval);
            self
        }

        /// Sets a timeout for receiving an acknowledgement of the keep-alive ping.
        ///
        /// If the ping is not acknowledged within the timeout, the connection will
        /// be closed. Does nothing if `http2_keep_alive_interval` is disabled.
        ///
        /// Default is 20 seconds.
        ///
        /// # Cargo Feature
        ///
        /// Requires the `runtime` cargo feature to be enabled.
        #[cfg(feature = "runtime")]
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_keep_alive_timeout(&mut self, timeout: Duration) -> &mut Self {
            self.conn_builder.http2_keep_alive_timeout(timeout);
            self
        }

        /// Sets whether HTTP2 keep-alive should apply while the connection is idle.
        ///
        /// If disabled, keep-alive pings are only sent while there are open
        /// request/responses streams. If enabled, pings are also sent when no
        /// streams are active. Does nothing if `http2_keep_alive_interval` is
        /// disabled.
        ///
        /// Default is `false`.
        ///
        /// # Cargo Feature
        ///
        /// Requires the `runtime` cargo feature to be enabled.
        #[cfg(feature = "runtime")]
        #[cfg(feature = "http2")]
        #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
        pub fn http2_keep_alive_while_idle(&mut self, enabled: bool) -> &mut Self {
            self.conn_builder.http2_keep_alive_while_idle(enabled);
            self
        }

        /// Set whether to retry requests that get disrupted before ever starting
        /// to write.
        ///
        /// This means a request that is queued, and gets given an idle, reused
        /// connection, and then encounters an error immediately as the idle
        /// connection was found to be unusable.
        ///
        /// When this is set to `false`, the related `ResponseFuture` would instead
        /// resolve to an `Error::Cancel`.
        ///
        /// Default is `true`.
        #[inline]
        pub fn retry_canceled_requests(&mut self, val: bool) -> &mut Self {
            self.client_config.retry_canceled_requests = val;
            self
        }

        /// Set whether to automatically add the `Host` header to requests.
        ///
        /// If true, and a request does not include a `Host` header, one will be
        /// added automatically, derived from the authority of the `Uri`.
        ///
        /// Default is `true`.
        #[inline]
        pub fn set_host(&mut self, val: bool) -> &mut Self {
            self.client_config.set_host = val;
            self
        }

        /// Provide an executor to execute background `Connection` tasks.
        pub fn executor<E>(&mut self, exec: E) -> &mut Self
        where
            E: Executor<BoxSendFuture> + Send + Sync + 'static,
        {
            self.conn_builder.executor(exec);
            self
        }

        /// Builder a client with this configuration and the default `HttpConnector`.
        #[cfg(feature = "tcp")]
        pub fn build_http<B>(&self) -> Client<HttpConnector, B>
        where
            B: HttpBody + Send,
            B::Data: Send,
        {
            let mut connector = HttpConnector::new();
            if self.pool_config.is_enabled() {
                connector.set_keepalive(self.pool_config.idle_timeout);
            }
            self.build(connector)
        }

        /// Combine the configuration of this builder with a connector to create a `Client`.
        pub fn build<C, B>(&self, connector: C) -> Client<C, B>
        where
            C: Connect + Clone,
            B: HttpBody + Send,
            B::Data: Send,
        {
            Client {
                config: self.client_config,
                conn_builder: self.conn_builder.clone(),
                connector,
                pool: Pool::new(self.pool_config, &self.conn_builder.exec),
            }
        }
    }

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

    #[cfg(test)]
    mod unit_tests {
        use super::*;

        #[test]
        fn set_relative_uri_with_implicit_path() {
            let mut uri = "http://hyper.rs".parse().unwrap();
            origin_form(&mut uri);
            assert_eq!(uri.to_string(), "/");
        }

        #[test]
        fn test_origin_form() {
            let mut uri = "http://hyper.rs/guides".parse().unwrap();
            origin_form(&mut uri);
            assert_eq!(uri.to_string(), "/guides");

            let mut uri = "http://hyper.rs/guides?foo=bar".parse().unwrap();
            origin_form(&mut uri);
            assert_eq!(uri.to_string(), "/guides?foo=bar");
        }

        #[test]
        fn test_absolute_form() {
            let mut uri = "http://hyper.rs/guides".parse().unwrap();
            absolute_form(&mut uri);
            assert_eq!(uri.to_string(), "http://hyper.rs/guides");

            let mut uri = "https://hyper.rs/guides".parse().unwrap();
            absolute_form(&mut uri);
            assert_eq!(uri.to_string(), "/guides");
        }

        #[test]
        fn test_authority_form() {
            let _ = pretty_env_logger::try_init();

            let mut uri = "http://hyper.rs".parse().unwrap();
            authority_form(&mut uri);
            assert_eq!(uri.to_string(), "hyper.rs");

            let mut uri = "hyper.rs".parse().unwrap();
            authority_form(&mut uri);
            assert_eq!(uri.to_string(), "hyper.rs");
        }

        #[test]
        fn test_extract_domain_connect_no_port() {
            let mut uri = "hyper.rs".parse().unwrap();
            let (scheme, host) = extract_domain(&mut uri, true).expect("extract domain");
            assert_eq!(scheme, *"http");
            assert_eq!(host, "hyper.rs");
        }
    }