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).

1 files changed, 45 insertions, 0 deletions

diff --git a/tokio/src/io/util/shutdown.rs b/tokio/src/io/util/shutdown.rs
new file mode 100644
index 00000000..ee7f4592
--- /dev/null
+++ b/tokio/src/io/util/shutdown.rs
@@ -0,0 +1,45 @@
+use crate::io::AsyncWrite;
+
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// A future used to shutdown an I/O object.
+///
+/// Created by the [`AsyncWriteExt::shutdown`] function.
+#[derive(Debug)]
+pub struct Shutdown<'a, A: ?Sized> {
+    a: &'a mut A,
+}
+
+/// Creates a future which will shutdown an I/O object.
+pub(super) fn shutdown<A>(a: &mut A) -> Shutdown<'_, A>
+where
+    A: AsyncWrite + Unpin + ?Sized,
+{
+    Shutdown { a }
+}
+
+impl<A> Future for Shutdown<'_, A>
+where
+    A: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let me = &mut *self;
+        Pin::new(&mut *me.a).poll_shutdown(cx)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Shutdown<'_, PhantomPinned>>();
+    }
+}