refactor: limit dependency futures-util to tests and client-legacy (#192)

Enable non-trivial applications based on hyper + tokio without pulling
in this heavyweight dependency. Keep MSRV 1.63 by using `ready!` from
`futures_core` and polyfilling `poll_fn`.

Don't remove futures-util from the `client-legacy` code, because this
would require re-implementing several non-trivial combinators.

Author: hanna-kruppe

Cargo.toml

@@ -21,7 +21,8 @@ rustdoc-args = ["--cfg", "docsrs"]
 base64 = { version = "0.22", optional = true }
 bytes = "1.7.1"
 futures-channel = { version = "0.3", optional = true }
-futures-util = { version = "0.3.16", default-features = false }
+futures-core = { version = "0.3" }
+futures-util = { version = "0.3.16", default-features = false, optional = true }
 http = "1.0"
 http-body = "1.0.0"
 hyper = "1.6.0"
@@ -37,6 +38,7 @@ tower-service = { version = "0.3", optional = true }
 [dev-dependencies]
 hyper = { version = "1.4.0", features = ["full"] }
 bytes = "1"
+futures-util = { version = "0.3.16", default-features = false, features = ["alloc"] }
 http-body-util = "0.1.0"
 tokio = { version = "1", features = ["macros", "test-util", "signal"] }
 tokio-test = "0.4"
@@ -69,13 +71,13 @@ full = [
 ]
 
 client = ["hyper/client", "dep:tracing", "dep:futures-channel", "dep:tower-service"]
-client-legacy = ["client", "dep:socket2", "tokio/sync", "dep:libc"]
+client-legacy = ["client", "dep:socket2", "tokio/sync", "dep:libc", "dep:futures-util"]
 client-proxy = ["client", "dep:base64", "dep:ipnet", "dep:percent-encoding"]
 client-proxy-system = ["dep:system-configuration", "dep:windows-registry"]
 
 server = ["hyper/server"]
 server-auto = ["server", "http1", "http2"]
-server-graceful = ["server", "tokio/sync", "futures-util/alloc"]
+server-graceful = ["server", "tokio/sync"]
 
 service = ["dep:tower-service"]
 

src/client/legacy/client.rs

@@ -25,6 +25,7 @@ use super::connect::HttpConnector;
 use super::connect::{Alpn, Connect, Connected, Connection};
 use super::pool::{self, Ver};
 
+use crate::common::future::poll_fn;
 use crate::common::{lazy as hyper_lazy, timer, Exec, Lazy, SyncWrapper};
 
 type BoxSendFuture = Pin<Box<dyn Future<Output = ()> + Send>>;
@@ -360,7 +361,7 @@ where
         } 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 |_| {
+            let on_idle = 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);
@@ -370,7 +371,7 @@ where
         } 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(|_| ());
+            let on_idle = poll_fn(move |cx| pooled.poll_ready(cx)).map(|_| ());
 
             self.exec.execute(on_idle);
         }

src/client/legacy/connect/dns.rs

@@ -284,7 +284,7 @@ pub(super) async fn resolve<R>(resolver: &mut R, name: Name) -> Result<R::Addrs,
 where
     R: Resolve,
 {
-    futures_util::future::poll_fn(|cx| resolver.poll_ready(cx)).await?;
+    crate::common::future::poll_fn(|cx| resolver.poll_ready(cx)).await?;
     resolver.resolve(name).await
 }
 

src/client/legacy/connect/http.rs

@@ -9,6 +9,7 @@ use std::sync::Arc;
 use std::task::{self, Poll};
 use std::time::Duration;
 
+use futures_core::ready;
 use futures_util::future::Either;
 use http::uri::{Scheme, Uri};
 use pin_project_lite::pin_project;
@@ -464,7 +465,7 @@ where
     type Future = HttpConnecting<R>;
 
     fn poll_ready(&mut self, cx: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>> {
-        futures_util::ready!(self.resolver.poll_ready(cx)).map_err(ConnectError::dns)?;
+        ready!(self.resolver.poll_ready(cx)).map_err(ConnectError::dns)?;
         Poll::Ready(Ok(()))
     }
 

src/client/legacy/connect/proxy/tunnel.rs

@@ -4,6 +4,7 @@ use std::marker::{PhantomData, Unpin};
 use std::pin::Pin;
 use std::task::{self, Poll};
 
+use futures_core::ready;
 use http::{HeaderMap, HeaderValue, Uri};
 use hyper::rt::{Read, Write};
 use pin_project_lite::pin_project;
@@ -127,8 +128,7 @@ where
     type Future = Tunneling<C::Future, C::Response>;
 
     fn poll_ready(&mut self, cx: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>> {
-        futures_util::ready!(self.inner.poll_ready(cx))
-            .map_err(|e| TunnelError::ConnectFailed(e.into()))?;
+        ready!(self.inner.poll_ready(cx)).map_err(|e| TunnelError::ConnectFailed(e.into()))?;
         Poll::Ready(Ok(()))
     }
 

src/client/legacy/pool.rs

@@ -14,7 +14,7 @@ use std::task::{self, Poll};
 use std::time::{Duration, Instant};
 
 use futures_channel::oneshot;
-use futures_util::ready;
+use futures_core::ready;
 use tracing::{debug, trace};
 
 use hyper::rt::Sleep;

src/common/future.rs

@@ -0,0 +1,30 @@
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+// TODO: replace with `std::future::poll_fn` once MSRV >= 1.64
+pub(crate) fn poll_fn<T, F>(f: F) -> PollFn<F>
+where
+    F: FnMut(&mut Context<'_>) -> Poll<T>,
+{
+    PollFn { f }
+}
+
+pub(crate) struct PollFn<F> {
+    f: F,
+}
+
+impl<F> Unpin for PollFn<F> {}
+
+impl<T, F> Future for PollFn<F>
+where
+    F: FnMut(&mut Context<'_>) -> Poll<T>,
+{
+    type Output = T;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        (self.f)(cx)
+    }
+}

src/common/mod.rs

@@ -15,3 +15,5 @@ pub(crate) use exec::Exec;
 pub(crate) use lazy::{lazy, Started as Lazy};
 #[cfg(feature = "client")]
 pub(crate) use sync::SyncWrapper;
+
+pub(crate) mod future;

src/rt/io.rs

@@ -2,15 +2,16 @@ use std::marker::Unpin;
 use std::pin::Pin;
 use std::task::Poll;
 
-use futures_util::future;
-use futures_util::ready;
+use futures_core::ready;
 use hyper::rt::{Read, ReadBuf, Write};
 
+use crate::common::future::poll_fn;
+
 pub(crate) async fn read<T>(io: &mut T, buf: &mut [u8]) -> Result<usize, std::io::Error>
 where
     T: Read + Unpin,
 {
-    future::poll_fn(move |cx| {
+    poll_fn(move |cx| {
         let mut buf = ReadBuf::new(buf);
         ready!(Pin::new(&mut *io).poll_read(cx, buf.unfilled()))?;
         Poll::Ready(Ok(buf.filled().len()))
@@ -23,7 +24,7 @@ where
     T: Write + Unpin,
 {
     let mut n = 0;
-    future::poll_fn(move |cx| {
+    poll_fn(move |cx| {
         while n < buf.len() {
             n += ready!(Pin::new(&mut *io).poll_write(cx, &buf[n..])?);
         }

src/server/conn/auto/mod.rs

@@ -2,7 +2,6 @@
 
 pub mod upgrade;
 
-use futures_util::ready;
 use hyper::service::HttpService;
 use std::future::Future;
 use std::marker::PhantomPinned;
@@ -12,6 +11,7 @@ use std::task::{Context, Poll};
 use std::{error::Error as StdError, io, time::Duration};
 
 use bytes::Bytes;
+use futures_core::ready;
 use http::{Request, Response};
 use http_body::Body;
 use hyper::{