Limit `futures` dependency to `Stream` via feature flag (#1774)

In an effort to reach API stability, the `tokio` crate is shedding its
_public_ dependencies on crates that are either a) do not provide a
stable (1.0+) release with longevity guarantees or b) match the `tokio`
release cadence. Of course, implementing `std` traits fits the
requirements.

The on exception, for now, is the `Stream` trait found in `futures_core`.
It is expected that this trait will not change much and be moved into `std.
Since Tokio is not yet going reaching 1.0, I feel that it is acceptable to maintain
a dependency on this trait given how foundational it is.

Since the `Stream` implementation is optional, types that are logically
streams provide `async fn next_*` functions to obtain the next value.
Avoiding the `next()` name prevents fn conflicts with `StreamExt::next()`.

Additionally, some misc cleanup is also done:

- `tokio::io::io` -> `tokio::io::util`.
- `delay` -> `delay_until`.
- `Timeout::new` -> `timeout(...)`.
- `signal::ctrl_c()` returns a future instead of a stream.
- `{tcp,unix}::Incoming` is removed (due to lack of `Stream` trait).
- `time::Throttle` is removed (due to lack of `Stream` trait).
-  Fix: `mpsc::UnboundedSender::send(&self)` (no more conflict with `Sink` fns).

4 files changed, 23 insertions, 18 deletions

diff --git a/tokio/src/net/unix/datagram.rs b/tokio/src/net/unix/datagram.rs
index f9c47dec..55eebd6c 100644
--- a/tokio/src/net/unix/datagram.rs
+++ b/tokio/src/net/unix/datagram.rs
@@ -1,7 +1,6 @@
+use crate::future::poll_fn;
 use crate::net::util::PollEvented;
 
-use futures_core::ready;
-use futures_util::future::poll_fn;
 use std::convert::TryFrom;
 use std::fmt;
 use std::io;
diff --git a/tokio/src/net/unix/incoming.rs b/tokio/src/net/unix/incoming.rs
index a66f21da..dbe964a8 100644
--- a/tokio/src/net/unix/incoming.rs
+++ b/tokio/src/net/unix/incoming.rs
@@ -1,7 +1,5 @@
-use super::{UnixListener, UnixStream};
+use crate::net::unix::{UnixListener, UnixStream};
 
-use futures_core::ready;
-use futures_core::stream::Stream;
 use std::io;
 use std::pin::Pin;
 use std::task::{Context, Poll};
@@ -9,21 +7,31 @@ use std::task::{Context, Poll};
 /// Stream of listeners
 #[derive(Debug)]
 #[must_use = "streams do nothing unless polled"]
-pub struct Incoming {
-    inner: UnixListener,
+pub struct Incoming<'a> {
+    inner: &'a mut UnixListener,
 }
 
-impl Incoming {
-    pub(crate) fn new(listener: UnixListener) -> Incoming {
+impl Incoming<'_> {
+    pub(crate) fn new(listener: &mut UnixListener) -> Incoming<'_> {
         Incoming { inner: listener }
     }
+
+    #[doc(hidden)] // TODO: dox
+    pub fn poll_accept(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<UnixStream>> {
+        let (socket, _) = ready!(self.inner.poll_accept(cx))?;
+        Poll::Ready(Ok(socket))
+    }
 }
 
-impl Stream for Incoming {
+#[cfg(feature = "stream")]
+impl futures_core::Stream for Incoming<'_> {
     type Item = io::Result<UnixStream>;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        let (socket, _) = ready!(Pin::new(&mut self.inner).poll_accept(cx))?;
+        let (socket, _) = ready!(self.inner.poll_accept(cx))?;
         Poll::Ready(Some(Ok(socket)))
     }
 }
diff --git a/tokio/src/net/unix/listener.rs b/tokio/src/net/unix/listener.rs
index 3cf8eff3..c7de68b1 100644
--- a/tokio/src/net/unix/listener.rs
+++ b/tokio/src/net/unix/listener.rs
@@ -1,8 +1,7 @@
-use crate::net::unix::UnixStream;
+use crate::future::poll_fn;
+use crate::net::unix::{Incoming, UnixStream};
 use crate::net::util::PollEvented;
 
-use futures_core::ready;
-use futures_util::future::poll_fn;
 use mio::Ready;
 use mio_uds;
 use std::convert::TryFrom;
@@ -90,8 +89,8 @@ impl UnixListener {
     ///
     /// This method returns an implementation of the `Stream` trait which
     /// resolves to the sockets the are accepted on this listener.
-    pub fn incoming(self) -> super::Incoming {
-        super::Incoming::new(self)
+    pub fn incoming(&mut self) -> Incoming<'_> {
+        Incoming::new(self)
     }
 }
 
diff --git a/tokio/src/net/unix/stream.rs b/tokio/src/net/unix/stream.rs
index 2ea8b6c7..774d20f9 100644
--- a/tokio/src/net/unix/stream.rs
+++ b/tokio/src/net/unix/stream.rs
@@ -1,11 +1,10 @@
+use crate::future::poll_fn;
 use crate::io::{AsyncRead, AsyncWrite};
 use crate::net::unix::split::{split, ReadHalf, WriteHalf};
 use crate::net::unix::ucred::{self, UCred};
 use crate::net::util::PollEvented;
 
 use bytes::{Buf, BufMut};
-use futures_core::ready;
-use futures_util::future::poll_fn;
 use iovec::IoVec;
 use std::convert::TryFrom;
 use std::fmt;