Update Tokio to use std::future. (#1120)

A first pass at updating Tokio to use `std::future`.

Implementations of `Future` from the futures crate are updated to implement
`Future` from std. Implementations of `Stream` are moved to a feature flag.

This commits disables a number of crates that have not yet been updated.

tokio-executor/src/enter.rs

@@ -1,9 +1,8 @@
-use futures::{self, Future};
 use std::cell::{Cell, RefCell};
 use std::error::Error;
 use std::fmt;
+use std::future::Future;
 use std::marker::PhantomData;
-use std::prelude::v1::*;
 
 thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
 
@@ -65,8 +64,25 @@ pub fn enter() -> Result<Enter, EnterError> {
 impl Enter {
     /// Blocks the thread on the specified future, returning the value with
     /// which that future completes.
-    pub fn block_on<F: Future>(&mut self, f: F) -> Result<F::Item, F::Error> {
-        futures::executor::spawn(f).wait_future()
+    pub fn block_on<F: Future>(&mut self, mut f: F) -> F::Output {
+        use crate::park::{Park, ParkThread};
+        use std::pin::Pin;
+        use std::task::Context;
+        use std::task::Poll::Ready;
+
+        let park = ParkThread::new();
+        let waker = park.unpark().into_waker();
+        let mut cx = Context::from_waker(&waker);
+
+        // `block_on` takes ownership of `f`. Once it is pinned here, the original `f` binding can
+        // no longer be accessed, making the pinning safe.
+        let mut f = unsafe { Pin::new_unchecked(&mut f) };
+
+        loop {
+            if let Ready(v) = f.as_mut().poll(&mut cx) {
+                return v;
+            }
+        }
     }
 }
 

tokio-executor/src/executor.rs

@@ -1,5 +1,6 @@
 use crate::SpawnError;
-use futures::Future;
+use std::future::Future;
+use std::pin::Pin;
 
 /// A value that executes futures.
 ///
@@ -82,16 +83,14 @@ pub trait Executor {
     /// use futures::future::lazy;
     ///
     /// # fn docs(my_executor: &mut dyn Executor) {
-    /// my_executor.spawn(Box::new(lazy(|| {
+    /// my_executor.spawn(Box::pin(lazy(|| {
     ///     println!("running on the executor");
     ///     Ok(())
     /// }))).unwrap();
     /// # }
     /// ```
-    fn spawn(
-        &mut self,
-        future: Box<dyn Future<Item = (), Error = ()> + Send>,
-    ) -> Result<(), SpawnError>;
+    fn spawn(&mut self, future: Pin<Box<dyn Future<Output = ()> + Send>>)
+        -> Result<(), SpawnError>;
 
     /// Provides a best effort **hint** to whether or not `spawn` will succeed.
     ///
@@ -116,7 +115,7 @@ pub trait Executor {
     ///
     /// # fn docs(my_executor: &mut dyn Executor) {
     /// if my_executor.status().is_ok() {
-    ///     my_executor.spawn(Box::new(lazy(|| {
+    ///     my_executor.spawn(Box::pin(lazy(|| {
     ///         println!("running on the executor");
     ///         Ok(())
     ///     }))).unwrap();
@@ -133,7 +132,7 @@ pub trait Executor {
 impl<E: Executor + ?Sized> Executor for Box<E> {
     fn spawn(
         &mut self,
-        future: Box<dyn Future<Item = (), Error = ()> + Send>,
+        future: Pin<Box<dyn Future<Output = ()> + Send>>,
     ) -> Result<(), SpawnError> {
         (**self).spawn(future)
     }

tokio-executor/src/global.rs

@@ -1,6 +1,7 @@
 use super::{Enter, Executor, SpawnError};
-use futures::{future, Future};
 use std::cell::Cell;
+use std::future::Future;
+use std::pin::Pin;
 
 /// Executes futures on the default executor for the current execution context.
 ///
@@ -70,7 +71,7 @@ thread_local! {
 impl super::Executor for DefaultExecutor {
     fn spawn(
         &mut self,
-        future: Box<dyn Future<Item = (), Error = ()> + Send>,
+        future: Pin<Box<dyn Future<Output = ()> + Send>>,
     ) -> Result<(), SpawnError> {
         DefaultExecutor::with_current(|executor| executor.spawn(future))
             .unwrap_or_else(|| Err(SpawnError::shutdown()))
@@ -84,10 +85,10 @@ impl super::Executor for DefaultExecutor {
 
 impl<T> super::TypedExecutor<T> for DefaultExecutor
 where
-    T: Future<Item = (), Error = ()> + Send + 'static,
+    T: Future<Output = ()> + Send + 'static,
 {
     fn spawn(&mut self, future: T) -> Result<(), SpawnError> {
-        super::Executor::spawn(self, Box::new(future))
+        super::Executor::spawn(self, Box::pin(future))
     }
 
     fn status(&self) -> Result<(), SpawnError> {
@@ -95,26 +96,6 @@ where
     }
 }
 
-impl<T> future::Executor<T> for DefaultExecutor
-where
-    T: Future<Item = (), Error = ()> + Send + 'static,
-{
-    fn execute(&self, future: T) -> Result<(), future::ExecuteError<T>> {
-        if let Err(e) = super::Executor::status(self) {
-            let kind = if e.is_at_capacity() {
-                future::ExecuteErrorKind::NoCapacity
-            } else {
-                future::ExecuteErrorKind::Shutdown
-            };
-
-            return Err(future::ExecuteError::new(kind, future));
-        }
-
-        let _ = DefaultExecutor::with_current(|executor| executor.spawn(Box::new(future)));
-        Ok(())
-    }
-}
-
 // ===== global spawn fns =====
 
 /// Submits a future for execution on the default executor -- usually a
@@ -153,9 +134,9 @@ where
 /// ```
 pub fn spawn<T>(future: T)
 where
-    T: Future<Item = (), Error = ()> + Send + 'static,
+    T: Future<Output = ()> + Send + 'static,
 {
-    DefaultExecutor::current().spawn(Box::new(future)).unwrap()
+    DefaultExecutor::current().spawn(Box::pin(future)).unwrap()
 }
 
 /// Set the default executor for the duration of the closure

tokio-executor/src/park.rs

@@ -46,8 +46,10 @@
 
 use crossbeam_utils::sync::{Parker, Unparker};
 use std::marker::PhantomData;
+use std::mem;
 use std::rc::Rc;
 use std::sync::Arc;
+use std::task::{RawWaker, RawWakerVTable, Waker};
 use std::time::Duration;
 
 /// Block the current thread.
@@ -223,3 +225,44 @@ impl Unpark for UnparkThread {
         self.inner.unpark();
     }
 }
+
+static VTABLE: RawWakerVTable = RawWakerVTable::new(clone, wake, wake_by_ref, drop);
+
+impl UnparkThread {
+    pub(crate) fn into_waker(self) -> Waker {
+        unsafe {
+            let raw = unparker_to_raw_waker(self.inner);
+            Waker::from_raw(raw)
+        }
+    }
+}
+
+unsafe fn unparker_to_raw_waker(unparker: Unparker) -> RawWaker {
+    RawWaker::new(Unparker::into_raw(unparker), &VTABLE)
+}
+
+unsafe fn clone(raw: *const ()) -> RawWaker {
+    let unparker = Unparker::from_raw(raw);
+
+    // Increment the ref count
+    mem::forget(unparker.clone());
+
+    unparker_to_raw_waker(unparker)
+}
+
+unsafe fn wake(raw: *const ()) {
+    let unparker = Unparker::from_raw(raw);
+    unparker.unpark();
+}
+
+unsafe fn wake_by_ref(raw: *const ()) {
+    let unparker = Unparker::from_raw(raw);
+    unparker.unpark();
+
+    // We don't actually own a reference to the unparker
+    mem::forget(unparker);
+}
+
+unsafe fn drop(raw: *const ()) {
+    let _ = Unparker::from_raw(raw);
+}