feat(lib): update to std::future::Future

BREAKING CHANGE: All usage of async traits (`Future`, `Stream`,
`AsyncRead`, `AsyncWrite`, etc) are updated to newer versions.

src/common/task.rs

@@ -1,40 +1,10 @@
-use futures::{Async, Poll, task::Task};
-
+pub(crate) use std::task::{Context, Poll};
 use super::Never;
 
-/// A type to help "yield" a future, such that it is re-scheduled immediately.
+/// A function to help "yield" a future, such that it is re-scheduled immediately.
 ///
 /// Useful for spin counts, so a future doesn't hog too much time.
-#[derive(Debug)]
-pub(crate) struct YieldNow {
-    cached_task: Option<Task>,
-}
-
-impl YieldNow {
-    pub(crate) fn new() -> YieldNow {
-        YieldNow {
-            cached_task: None,
-        }
-    }
-
-    /// Returns `Ok(Async::NotReady)` always, while also notifying the
-    /// current task so that it is rescheduled immediately.
-    ///
-    /// Since it never returns `Async::Ready` or `Err`, those types are
-    /// set to `Never`.
-    pub(crate) fn poll_yield(&mut self) -> Poll<Never, Never> {
-        // Check for a cached `Task` first...
-        if let Some(ref t) = self.cached_task {
-            if t.will_notify_current() {
-                t.notify();
-                return Ok(Async::NotReady);
-            }
-        }
-
-        // No cached task, or not current, so get a new one...
-        let t = ::futures::task::current();
-        t.notify();
-        self.cached_task = Some(t);
-        Ok(Async::NotReady)
-    }
+pub(crate) fn yield_now(cx: &mut Context) -> Poll<Never> {
+    cx.waker().wake_by_ref();
+    Poll::Pending
 }