refactor(lib): drop futures-util except in ffi (#3890)

Make hyper usable for h1/h2 and client/server without this heavyweight
dependency. It's about 17k lines of code and takes up to 1.7 seconds to
compile on my machine, but hyper is only using a tiny fraction of it.
Larger applications probably still pull in futures-util by other means,
but it's no longer as unavoidable as in the early days of the ecosystem.

To remove futures-util without raising MSRV, I took these steps:

* When futures-util just re-exports something from its dependencies,
  use it directly from the source.
* Inline trivial helpers like `poll_unpin` that "only" communicate
  intent a little better but don't save any significant amount of code.
* Refactor the h2 client code to avoid `StreamFuture` for the "Client
  has been dropped" detection -- just poll the mpsc channel directly.
* Implement a couple of small helpers from scratch when they're
  straightforward and fit on one screen each. The majority of this is
  polyfills for standard library APIs that would require a higher MSRV.
* Use `AtomicWaker` from the `atomic-waker` crate, a separately
  published copy of the futures-util type of the same name. While the
  two crates are owned by different organizations (smol-rs vs.
  rust-lang), it's mostly the same people maintaining both copies.

The uses of future-util in hyper's tests/benches/examples and in the
`ffi` module seem much harder to remove entirely, so I did not touch
those modules at all.

Cargo.toml

@@ -27,14 +27,17 @@ tokio = { version = "1", features = ["sync"] }
 
 # Optional
 
+atomic-waker = { version = "1.1.2", optional = true }
 futures-channel = { version = "0.3", optional = true }
-futures-util = { version = "0.3", default-features = false, optional = true }
+futures-core = { version = "0.3.31", optional = true }
+futures-util = { version = "0.3", default-features = false, features = ["alloc"], optional = true }
 h2 = { version = "0.4.2", optional = true }
 http-body-util = { version = "0.1", optional = true }
 httparse = { version = "1.9", optional = true }
 httpdate = { version = "1.0", optional = true }
 itoa = { version = "1", optional = true }
 pin-project-lite = { version = "0.2.4", optional = true }
+pin-utils = { version = "0.1", optional = true } # TODO: replace with std::pin::pin! once MSRV >= 1.68
 smallvec = { version = "1.12", features = ["const_generics", "const_new"], optional = true }
 tracing = { version = "0.1", default-features = false, features = ["std"], optional = true }
 want = { version = "0.3", optional = true }
@@ -77,15 +80,15 @@ full = [
 ]
 
 # HTTP versions
-http1 = ["dep:futures-channel", "dep:futures-util", "dep:httparse", "dep:itoa"]
-http2 = ["dep:futures-channel", "dep:futures-util", "dep:h2"]
+http1 = ["dep:atomic-waker", "dep:futures-channel", "dep:futures-core", "dep:httparse", "dep:itoa", "dep:pin-utils"]
+http2 = ["dep:futures-channel", "dep:futures-core", "dep:h2"]
 
 # Client/Server
 client = ["dep:want", "dep:pin-project-lite", "dep:smallvec"]
 server = ["dep:httpdate", "dep:pin-project-lite", "dep:smallvec"]
 
 # C-API support (currently unstable (no semver))
-ffi = ["dep:http-body-util", "futures-util?/alloc"]
+ffi = ["dep:http-body-util", "futures-util"]
 capi = []
 
 # Utilize tracing (currently unstable)

src/body/incoming.rs

@@ -11,9 +11,9 @@ use futures_channel::{mpsc, oneshot};
     any(feature = "http1", feature = "http2"),
     any(feature = "client", feature = "server")
 ))]
-use futures_util::ready;
+use futures_core::ready;
 #[cfg(all(feature = "http1", any(feature = "client", feature = "server")))]
-use futures_util::{stream::FusedStream, Stream}; // for mpsc::Receiver
+use futures_core::{stream::FusedStream, Stream}; // for mpsc::Receiver
 #[cfg(all(feature = "http1", any(feature = "client", feature = "server")))]
 use http::HeaderMap;
 use http_body::{Body, Frame, SizeHint};

src/client/conn/http1.rs

@@ -8,7 +8,7 @@ use std::task::{Context, Poll};
 
 use crate::rt::{Read, Write};
 use bytes::Bytes;
-use futures_util::ready;
+use futures_core::ready;
 use http::{Request, Response};
 use httparse::ParserConfig;
 
@@ -92,7 +92,7 @@ where
     /// instead run `into_parts`. This is a convenience wrapper over `poll_without_shutdown`.
     pub async fn without_shutdown(self) -> crate::Result<Parts<T>> {
         let mut conn = Some(self);
-        futures_util::future::poll_fn(move |cx| -> Poll<crate::Result<Parts<T>>> {
+        crate::common::future::poll_fn(move |cx| -> Poll<crate::Result<Parts<T>>> {
             ready!(conn.as_mut().unwrap().poll_without_shutdown(cx))?;
             Poll::Ready(Ok(conn.take().unwrap().into_parts()))
         })
@@ -148,7 +148,7 @@ impl<B> SendRequest<B> {
     ///
     /// If the associated connection is closed, this returns an Error.
     pub async fn ready(&mut self) -> crate::Result<()> {
-        futures_util::future::poll_fn(|cx| self.poll_ready(cx)).await
+        crate::common::future::poll_fn(|cx| self.poll_ready(cx)).await
     }
 
     /// Checks if the connection is currently ready to send a request.

src/client/conn/http2.rs

@@ -10,7 +10,7 @@ use std::task::{Context, Poll};
 use std::time::Duration;
 
 use crate::rt::{Read, Write};
-use futures_util::ready;
+use futures_core::ready;
 use http::{Request, Response};
 
 use super::super::dispatch::{self, TrySendError};
@@ -99,7 +99,7 @@ impl<B> SendRequest<B> {
     ///
     /// If the associated connection is closed, this returns an Error.
     pub async fn ready(&mut self) -> crate::Result<()> {
-        futures_util::future::poll_fn(|cx| self.poll_ready(cx)).await
+        crate::common::future::poll_fn(|cx| self.poll_ready(cx)).await
     }
 
     /// Checks if the connection is currently ready to send a request.

src/client/dispatch.rs

@@ -199,8 +199,7 @@ impl<T, U> Receiver<T, U> {
 
     #[cfg(feature = "http1")]
     pub(crate) fn try_recv(&mut self) -> Option<(T, Callback<T, U>)> {
-        use futures_util::FutureExt;
-        match self.inner.recv().now_or_never() {
+        match crate::common::task::now_or_never(self.inner.recv()) {
             Some(Some(mut env)) => env.0.take(),
             _ => None,
         }

src/common/either.rs

@@ -0,0 +1,46 @@
+use pin_project_lite::pin_project;
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+pin_project! {
+    /// One of two possible futures that have the same output type.
+    #[project = EitherProj]
+    pub(crate) enum Either<F1, F2> {
+        Left {
+            #[pin]
+            fut: F1
+        },
+        Right {
+            #[pin]
+            fut: F2,
+        },
+    }
+}
+
+impl<F1, F2> Either<F1, F2> {
+    pub(crate) fn left(fut: F1) -> Self {
+        Either::Left { fut }
+    }
+
+    pub(crate) fn right(fut: F2) -> Self {
+        Either::Right { fut }
+    }
+}
+
+impl<F1, F2> Future for Either<F1, F2>
+where
+    F1: Future,
+    F2: Future<Output = F1::Output>,
+{
+    type Output = F1::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.project() {
+            EitherProj::Left { fut } => fut.poll(cx),
+            EitherProj::Right { fut } => fut.poll(cx),
+        }
+    }
+}

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.as_mut().f)(cx)
+    }
+}

src/common/mod.rs

@@ -2,6 +2,13 @@
 pub(crate) mod buf;
 #[cfg(all(feature = "server", any(feature = "http1", feature = "http2")))]
 pub(crate) mod date;
+#[cfg(all(feature = "client", feature = "http2"))]
+pub(crate) mod either;
+#[cfg(any(
+    all(feature = "client", any(feature = "http1", feature = "http2")),
+    all(feature = "server", feature = "http1"),
+))]
+pub(crate) mod future;
 pub(crate) mod io;
 #[cfg(all(any(feature = "client", feature = "server"), feature = "http1"))]
 pub(crate) mod task;

src/common/task.rs

@@ -1,4 +1,6 @@
 use std::task::{Context, Poll};
+#[cfg(feature = "client")]
+use std::task::{RawWaker, RawWakerVTable, Waker};
 
 /// A function to help "yield" a future, such that it is re-scheduled immediately.
 ///
@@ -7,3 +9,37 @@ pub(crate) fn yield_now(cx: &mut Context<'_>) -> Poll<std::convert::Infallible>
     cx.waker().wake_by_ref();
     Poll::Pending
 }
+
+// TODO: replace with `std::task::Waker::noop()` once MSRV >= 1.85
+#[cfg(feature = "client")]
+fn noop_waker() -> Waker {
+    const NOOP_RAW_WAKER: RawWaker = RawWaker::new(std::ptr::null(), &NOOP_VTABLE);
+    const NOOP_VTABLE: RawWakerVTable = RawWakerVTable::new(
+        // `clone` returns the same noop waker again
+        |_: *const ()| NOOP_RAW_WAKER,
+        // `wake`, `wake_by_ref`, and `drop` do nothing
+        |_: *const ()| {},
+        |_: *const ()| {},
+        |_: *const ()| {},
+    );
+
+    // SAFETY: all functions in the vtable are safe to call, and Waker's safety does not require
+    // them to actually do anything.
+    unsafe { Waker::from_raw(NOOP_RAW_WAKER) }
+}
+
+/// Poll the future once and return `Some` if it is ready, else `None`.
+///
+/// If the future wasn't ready, it future likely can't be driven to completion any more: the polling
+/// uses a no-op waker, so knowledge of what the pending future was waiting for is lost.
+#[cfg(feature = "client")]
+pub(crate) fn now_or_never<F: std::future::Future>(fut: F) -> Option<F::Output> {
+    let waker = noop_waker();
+    let mut cx = Context::from_waker(&waker);
+    // TODO: replace with std::pin::pin! and drop pin-utils once MSRV >= 1.68
+    pin_utils::pin_mut!(fut);
+    match fut.poll(&mut cx) {
+        Poll::Ready(res) => Some(res),
+        Poll::Pending => None,
+    }
+}

src/common/time.rs

@@ -40,7 +40,7 @@ impl Time {
         }
     }
 
-    #[cfg(feature = "http1")]
+    #[cfg(all(feature = "server", feature = "http1"))]
     pub(crate) fn sleep_until(&self, deadline: Instant) -> Pin<Box<dyn Sleep>> {
         match *self {
             Time::Empty => {

src/common/watch.rs

@@ -4,7 +4,7 @@
 //! - The consumer is only notified if the value is different.
 //! - The value `0` is reserved for closed.
 
-use futures_util::task::AtomicWaker;
+use atomic_waker::AtomicWaker;
 use std::sync::{
     atomic::{AtomicUsize, Ordering},
     Arc,

src/proto/h1/conn.rs

@@ -10,7 +10,7 @@ use std::time::{Duration, Instant};
 
 use crate::rt::{Read, Write};
 use bytes::{Buf, Bytes};
-use futures_util::ready;
+use futures_core::ready;
 use http::header::{HeaderValue, CONNECTION, TE};
 use http::{HeaderMap, Method, Version};
 use http_body::Frame;

src/proto/h1/decode.rs

@@ -4,7 +4,7 @@ use std::io;
 use std::task::{Context, Poll};
 
 use bytes::{BufMut, Bytes, BytesMut};
-use futures_util::ready;
+use futures_core::ready;
 use http::{HeaderMap, HeaderName, HeaderValue};
 use http_body::Frame;
 

src/proto/h1/dispatch.rs

@@ -8,7 +8,7 @@ use std::{
 
 use crate::rt::{Read, Write};
 use bytes::{Buf, Bytes};
-use futures_util::ready;
+use futures_core::ready;
 use http::Request;
 
 use super::{Http1Transaction, Wants};

src/proto/h1/io.rs

@@ -6,7 +6,7 @@ use std::task::{Context, Poll};
 
 use crate::rt::{Read, ReadBuf, Write};
 use bytes::{Buf, BufMut, Bytes, BytesMut};
-use futures_util::ready;
+use futures_core::ready;
 
 use super::{Http1Transaction, ParseContext, ParsedMessage};
 use crate::common::buf::BufList;

src/proto/h2/client.rs

@@ -11,9 +11,7 @@ use crate::rt::{Read, Write};
 use bytes::Bytes;
 use futures_channel::mpsc::{Receiver, Sender};
 use futures_channel::{mpsc, oneshot};
-use futures_util::future::{Either, FusedFuture, FutureExt as _};
-use futures_util::ready;
-use futures_util::stream::{StreamExt as _, StreamFuture};
+use futures_core::{ready, FusedFuture, FusedStream, Stream};
 use h2::client::{Builder, Connection, SendRequest};
 use h2::SendStream;
 use http::{Method, StatusCode};
@@ -23,6 +21,7 @@ use super::ping::{Ponger, Recorder};
 use super::{ping, H2Upgraded, PipeToSendStream, SendBuf};
 use crate::body::{Body, Incoming as IncomingBody};
 use crate::client::dispatch::{Callback, SendWhen, TrySendError};
+use crate::common::either::Either;
 use crate::common::io::Compat;
 use crate::common::time::Time;
 use crate::ext::Protocol;
@@ -164,11 +163,9 @@ where
     // 'Client' has been dropped. This is to get around a bug
     // in h2 where dropping all SendRequests won't notify a
     // parked Connection.
-    let (conn_drop_ref, rx) = mpsc::channel(1);
+    let (conn_drop_ref, conn_drop_rx) = mpsc::channel(1);
     let (cancel_tx, conn_eof) = oneshot::channel();
 
-    let conn_drop_rx = rx.into_future();
-
     let ping_config = new_ping_config(config);
 
     let (conn, ping) = if ping_config.is_enabled() {
@@ -176,9 +173,9 @@ where
         let (recorder, ponger) = ping::channel(pp, ping_config, timer);
 
         let conn: Conn<_, B> = Conn::new(ponger, conn);
-        (Either::Left(conn), recorder)
+        (Either::left(conn), recorder)
     } else {
-        (Either::Right(conn), ping::disabled())
+        (Either::right(conn), ping::disabled())
     };
     let conn: ConnMapErr<T, B> = ConnMapErr {
         conn,
@@ -305,7 +302,7 @@ pin_project! {
         T: Unpin,
     {
         #[pin]
-        drop_rx: StreamFuture<Receiver<Infallible>>,
+        drop_rx: Receiver<Infallible>,
         #[pin]
         cancel_tx: Option<oneshot::Sender<Infallible>>,
         #[pin]
@@ -320,7 +317,7 @@ where
 {
     fn new(
         conn: ConnMapErr<T, B>,
-        drop_rx: StreamFuture<Receiver<Infallible>>,
+        drop_rx: Receiver<Infallible>,
         cancel_tx: oneshot::Sender<Infallible>,
     ) -> Self {
         Self {
@@ -341,12 +338,12 @@ where
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let mut this = self.project();
 
-        if !this.conn.is_terminated() && this.conn.poll_unpin(cx).is_ready() {
+        if !this.conn.is_terminated() && Pin::new(&mut this.conn).poll(cx).is_ready() {
             // ok or err, the `conn` has finished.
             return Poll::Ready(());
         }
 
-        if !this.drop_rx.is_terminated() && this.drop_rx.poll_unpin(cx).is_ready() {
+        if !this.drop_rx.is_terminated() && Pin::new(&mut this.drop_rx).poll_next(cx).is_ready() {
             // mpsc has been dropped, hopefully polling
             // the connection some more should start shutdown
             // and then close.
@@ -468,7 +465,7 @@ where
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> std::task::Poll<Self::Output> {
         let mut this = self.project();
 
-        match this.pipe.poll_unpin(cx) {
+        match Pin::new(&mut this.pipe).poll(cx) {
             Poll::Ready(result) => {
                 if let Err(_e) = result {
                     debug!("client request body error: {}", _e);

src/proto/h2/mod.rs

@@ -6,7 +6,7 @@ use std::pin::Pin;
 use std::task::{Context, Poll};
 
 use bytes::{Buf, Bytes};
-use futures_util::ready;
+use futures_core::ready;
 use h2::{Reason, RecvStream, SendStream};
 use http::header::{HeaderName, CONNECTION, TE, TRANSFER_ENCODING, UPGRADE};
 use http::HeaderMap;

src/proto/h2/server.rs

@@ -5,7 +5,7 @@ use std::task::{Context, Poll};
 use std::time::Duration;
 
 use bytes::Bytes;
-use futures_util::ready;
+use futures_core::ready;
 use h2::server::{Connection, Handshake, SendResponse};
 use h2::{Reason, RecvStream};
 use http::{Method, Request};

src/server/conn/http1.rs

@@ -11,7 +11,7 @@ use std::time::Duration;
 use crate::rt::{Read, Write};
 use crate::upgrade::Upgraded;
 use bytes::Bytes;
-use futures_util::ready;
+use futures_core::ready;
 
 use crate::body::{Body, Incoming as IncomingBody};
 use crate::proto;
@@ -179,7 +179,7 @@ where
     /// This errors if the underlying connection protocol is not HTTP/1.
     pub fn without_shutdown(self) -> impl Future<Output = crate::Result<Parts<I, S>>> {
         let mut zelf = Some(self);
-        futures_util::future::poll_fn(move |cx| {
+        crate::common::future::poll_fn(move |cx| {
             ready!(zelf.as_mut().unwrap().conn.poll_without_shutdown(cx))?;
             Poll::Ready(Ok(zelf.take().unwrap().into_parts()))
         })

src/server/conn/http2.rs

@@ -9,7 +9,7 @@ use std::task::{Context, Poll};
 use std::time::Duration;
 
 use crate::rt::{Read, Write};
-use futures_util::ready;
+use futures_core::ready;
 use pin_project_lite::pin_project;
 
 use crate::body::{Body, Incoming as IncomingBody};