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).

3 files changed, 31 insertions, 23 deletions

diff --git a/tokio/src/fs/blocking.rs b/tokio/src/fs/blocking.rs
index 695358a3..64398cbb 100644
--- a/tokio/src/fs/blocking.rs
+++ b/tokio/src/fs/blocking.rs
@@ -1,7 +1,6 @@
 use crate::fs::sys;
 use crate::io::{AsyncRead, AsyncWrite};
 
-use futures_core::ready;
 use std::cmp;
 use std::future::Future;
 use std::io;
diff --git a/tokio/src/fs/file.rs b/tokio/src/fs/file.rs
index 3f18831e..0ff45025 100644
--- a/tokio/src/fs/file.rs
+++ b/tokio/src/fs/file.rs
@@ -7,7 +7,6 @@ use crate::fs::blocking::Buf;
 use crate::fs::{asyncify, sys};
 use crate::io::{AsyncRead, AsyncWrite};
 
-use futures_core::ready;
 use std::fmt;
 use std::fs::{Metadata, Permissions};
 use std::future::Future;
@@ -430,7 +429,7 @@ impl File {
     }
 
     async fn complete_inflight(&mut self) {
-        use futures_util::future::poll_fn;
+        use crate::future::poll_fn;
 
         if let Err(e) = poll_fn(|cx| Pin::new(&mut *self).poll_flush(cx)).await {
             self.last_write_err = Some(e.kind());
diff --git a/tokio/src/fs/read_dir.rs b/tokio/src/fs/read_dir.rs
index 9492a2f4..219c7b35 100644
--- a/tokio/src/fs/read_dir.rs
+++ b/tokio/src/fs/read_dir.rs
@@ -1,7 +1,5 @@
 use crate::fs::{asyncify, sys};
 
-use futures_core::ready;
-use futures_core::stream::Stream;
 use std::ffi::OsString;
 use std::fs::{FileType, Metadata};
 use std::future::Future;
@@ -50,10 +48,15 @@ enum State {
     Pending(sys::Blocking<(Option<io::Result<std::fs::DirEntry>>, std::fs::ReadDir)>),
 }
 
-impl Stream for ReadDir {
-    type Item = io::Result<DirEntry>;
+impl ReadDir {
+    /// Returns the next entry in the directory stream.
+    pub async fn next_entry(&mut self) -> io::Result<Option<DirEntry>> {
+        use crate::future::poll_fn;
+        poll_fn(|cx| self.poll_next_entry(cx)).await
+    }
 
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+    #[doc(hidden)]
+    pub fn poll_next_entry(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<Option<DirEntry>>> {
         loop {
             match self.0 {
                 State::Idle(ref mut std) => {
@@ -68,7 +71,11 @@ impl Stream for ReadDir {
                     let (ret, std) = ready!(Pin::new(rx).poll(cx))?;
                     self.0 = State::Idle(Some(std));
 
-                    let ret = ret.map(|res| res.map(|std| DirEntry(Arc::new(std))));
+                    let ret = match ret {
+                        Some(Ok(std)) => Ok(Some(DirEntry(Arc::new(std)))),
+                        Some(Err(e)) => Err(e),
+                        None => Ok(None),
+                    };
 
                     return Poll::Ready(ret);
                 }
@@ -77,6 +84,19 @@ impl Stream for ReadDir {
     }
 }
 
+#[cfg(feature = "stream")]
+impl futures_core::Stream for ReadDir {
+    type Item = io::Result<DirEntry>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Poll::Ready(match ready!(self.poll_next_entry(cx)) {
+            Ok(Some(entry)) => Some(Ok(entry)),
+            Ok(None) => None,
+            Err(err) => Some(Err(err)),
+        })
+    }
+}
+
 /// Entries returned by the [`ReadDir`] stream.
 ///
 /// [`ReadDir`]: struct.ReadDir.html
@@ -100,13 +120,11 @@ impl DirEntry {
     ///
     /// ```no_run
     /// use tokio::fs;
-    /// use tokio::prelude::*;
     ///
     /// # async fn dox() -> std::io::Result<()> {
     /// let mut entries = fs::read_dir(".").await?;
     ///
-    /// while let Some(res) = entries.next().await {
-    ///     let entry = res?;
+    /// while let Some(entry) = entries.next_entry().await? {
     ///     println!("{:?}", entry.path());
     /// }
     /// # Ok(())
@@ -133,13 +151,11 @@ impl DirEntry {
     ///
     /// ```
     /// use tokio::fs;
-    /// use tokio::prelude::*;
     ///
     /// # async fn dox() -> std::io::Result<()> {
     /// let mut entries = fs::read_dir(".").await?;
     ///
-    /// while let Some(res) = entries.next().await {
-    ///     let entry = res?;
+    /// while let Some(entry) = entries.next_entry().await? {
     ///     println!("{:?}", entry.file_name());
     /// }
     /// # Ok(())
@@ -164,14 +180,11 @@ impl DirEntry {
     ///
     /// ```
     /// use tokio::fs;
-    /// use tokio::prelude::*;
     ///
     /// # async fn dox() -> std::io::Result<()> {
     /// let mut entries = fs::read_dir(".").await?;
     ///
-    /// while let Some(res) = entries.next().await {
-    ///     let entry = res?;
-    ///
+    /// while let Some(entry) = entries.next_entry().await? {
     ///     if let Ok(metadata) = entry.metadata().await {
     ///         // Now let's show our entry's permissions!
     ///         println!("{:?}: {:?}", entry.path(), metadata.permissions());
@@ -202,14 +215,11 @@ impl DirEntry {
     ///
     /// ```
     /// use tokio::fs;
-    /// use tokio::prelude::*;
     ///
     /// # async fn dox() -> std::io::Result<()> {
     /// let mut entries = fs::read_dir(".").await?;
     ///
-    /// while let Some(res) = entries.next().await {
-    ///     let entry = res?;
-    ///
+    /// while let Some(entry) = entries.next_entry().await? {
     ///     if let Ok(file_type) = entry.file_type().await {
     ///         // Now let's show our entry's file type!
     ///         println!("{:?}: {:?}", entry.path(), file_type);