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).

examples/chat.rs

@@ -116,18 +116,12 @@ impl Shared {
 
     /// Send a `LineCodec` encoded message to every peer, except
     /// for the sender.
-    async fn broadcast(
-        &mut self,
-        sender: SocketAddr,
-        message: &str,
-    ) -> Result<(), mpsc::error::UnboundedSendError> {
+    async fn broadcast(&mut self, sender: SocketAddr, message: &str) {
         for peer in self.peers.iter_mut() {
             if *peer.0 != sender {
-                peer.1.send(message.into()).await?;
+                let _ = peer.1.send(message.into());
             }
         }
-
-        Ok(())
     }
 }
 
@@ -218,7 +212,7 @@ async fn process(
         let mut state = state.lock().await;
         let msg = format!("{} has joined the chat", username);
         println!("{}", msg);
-        state.broadcast(addr, &msg).await?;
+        state.broadcast(addr, &msg).await;
     }
 
     // Process incoming messages until our stream is exhausted by a disconnect.
@@ -230,7 +224,7 @@ async fn process(
                 let mut state = state.lock().await;
                 let msg = format!("{}: {}", username, msg);
 
-                state.broadcast(addr, &msg).await?;
+                state.broadcast(addr, &msg).await;
             }
             // A message was received from a peer. Send it to the
             // current user.
@@ -254,7 +248,7 @@ async fn process(
 
         let msg = format!("{} has left the chat", username);
         println!("{}", msg);
-        state.broadcast(addr, &msg).await?;
+        state.broadcast(addr, &msg).await;
     }
 
     Ok(())

examples/connect.rs

@@ -20,7 +20,7 @@ use tokio::io;
 use tokio::sync::{mpsc, oneshot};
 use tokio_util::codec::{FramedRead, FramedWrite};
 
-use futures::{SinkExt, Stream, StreamExt};
+use futures::{Stream, StreamExt};
 use std::env;
 use std::error::Error;
 use std::net::SocketAddr;
@@ -69,12 +69,14 @@ async fn run() -> Result<(), Box<dyn Error>> {
 
 // Temporary work around for stdin blocking the stream
 fn stdin() -> impl Stream<Item = Result<Vec<u8>, io::Error>> + Unpin {
-    let mut stdin = FramedRead::new(io::stdin(), codec::Bytes).map(Ok);
+    let mut stdin = FramedRead::new(io::stdin(), codec::Bytes);
 
-    let (mut tx, rx) = mpsc::unbounded_channel();
+    let (tx, rx) = mpsc::unbounded_channel();
 
     tokio::spawn(async move {
-        tx.send_all(&mut stdin).await.unwrap();
+        while let Some(res) = stdin.next().await {
+            let _ = tx.send(res);
+        }
     });
 
     rx

examples/print_each_packet.rs

@@ -55,9 +55,9 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::net::TcpListener;
-use tokio::prelude::*;
 use tokio_util::codec::{BytesCodec, Decoder};
 
+use futures::StreamExt;
 use std::env;
 
 #[tokio::main]

examples/proxy.rs

@@ -22,7 +22,7 @@
 
 #![warn(rust_2018_idioms)]
 
-use futures::{future::try_join, FutureExt, StreamExt};
+use futures::{future::try_join, FutureExt};
 use std::{env, error::Error};
 use tokio::{
     io::AsyncReadExt,
@@ -37,9 +37,9 @@ async fn main() -> Result<(), Box<dyn Error>> {
     println!("Listening on: {}", listen_addr);
     println!("Proxying to: {}", server_addr);
 
-    let mut incoming = TcpListener::bind(listen_addr).await?.incoming();
+    let mut listener = TcpListener::bind(listen_addr).await?;
 
-    while let Some(Ok(inbound)) = incoming.next().await {
+    while let Ok((inbound, _)) = listener.accept().await {
         let transfer = transfer(inbound, server_addr.clone()).map(|r| {
             if let Err(e) = r {
                 println!("Failed to transfer; error={}", e);

examples/udp-codec.rs

@@ -8,9 +8,8 @@
 
 #![warn(rust_2018_idioms)]
 
-use tokio::future::FutureExt as TokioFutureExt;
-use tokio::io;
 use tokio::net::UdpSocket;
+use tokio::{io, time};
 use tokio_util::codec::BytesCodec;
 use tokio_util::udp::UdpFramed;
 
@@ -68,7 +67,7 @@ async fn ping(socket: &mut UdpFramed<BytesCodec>, b_addr: SocketAddr) -> Result<
 async fn pong(socket: &mut UdpFramed<BytesCodec>) -> Result<(), io::Error> {
     let timeout = Duration::from_millis(200);
 
-    while let Ok(Some(Ok((bytes, addr)))) = socket.next().timeout(timeout).await {
+    while let Ok(Some(Ok((bytes, addr)))) = time::timeout(timeout, socket.next()).await {
         println!("[b] recv: {}", String::from_utf8_lossy(&bytes));
 
         socket.send((Bytes::from(&b"PONG"[..]), addr)).await?;