Convert basic futures combinators to futures-core 0.3

This commit includes the introduction of a `FutureResult` trait along
the lines of the RFC (with some slight revisions).

It tackles all the "basic" combinators, but leaves off `shared`,
`select`, and `join` for the moment.

Cargo.toml

@@ -10,5 +10,5 @@ members = [
 #  "futures-macro-await",
 #  "futures-sink",
 #  "futures-stable",
-#  "futures-util",
+  "futures-util",
 ]

futures-util/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "futures-util"
-version = "0.2.0"
+version = "0.3.0-alpha"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 license = "MIT/Apache-2.0"
 repository = "https://github.com/rust-lang-nursery/futures-rs"
@@ -11,18 +11,20 @@ Common utilities and extension traits for the futures-rs library.
 """
 
 [features]
-std = ["futures-core/std", "futures-io/std", "futures-sink/std", "either/use_std"]
-default = ["std", "futures-core/either", "futures-sink/either"]
+# std = ["futures-core/std", "futures-io/std", "futures-sink/std", "either/use_std"]
+std = ["futures-core/std", "either/use_std"]
+# default = ["std", "futures-core/either", "futures-sink/either"]
+default = ["std", "futures-core/either"]
 bench = []
 
 [dependencies]
-futures-core = { path = "../futures-core", version = "0.2.0", default-features = false }
-futures-channel = { path = "../futures-channel", version = "0.2.0", default-features = false }
-futures-io = { path = "../futures-io", version = "0.2.0", default-features = false }
-futures-sink = { path = "../futures-sink", version = "0.2.0", default-features = false}
+futures-core = { path = "../futures-core", version = "0.3.0-alpha", default-features = false }
+futures-channel = { path = "../futures-channel", version = "0.3.0-alpha", default-features = false }
+# futures-io = { path = "../futures-io", version = "0.2.0", default-features = false }
+# futures-sink = { path = "../futures-sink", version = "0.2.0", default-features = false}
 either = { version = "1.4", default-features = false }
 
 [dev-dependencies]
-futures = { path = "../futures", version = "0.2.0" }
-futures-executor = { path = "../futures-executor", version = "0.2.0" }
-futures-channel = { path = "../futures-channel", version = "0.2.0" }
+# futures = { path = "../futures", version = "0.2.0" }
+# futures-executor = { path = "../futures-executor", version = "0.2.0" }
+# futures-channel = { path = "../futures-channel", version = "0.2.0" }

futures-util/src/future/catch_unwind.rs

@@ -1,8 +1,9 @@
+use std::mem::Pin;
 use std::prelude::v1::*;
 use std::any::Any;
 use std::panic::{catch_unwind, UnwindSafe, AssertUnwindSafe};
 
-use futures_core::{Future, Poll, Async};
+use futures_core::{Future, Poll};
 use futures_core::task;
 
 /// Future for the `catch_unwind` combinator.
@@ -11,35 +12,25 @@ use futures_core::task;
 #[derive(Debug)]
 #[must_use = "futures do nothing unless polled"]
 pub struct CatchUnwind<F> where F: Future {
-    future: Option<F>,
+    future: F,
 }
 
 pub fn new<F>(future: F) -> CatchUnwind<F>
     where F: Future + UnwindSafe,
 {
-    CatchUnwind {
-        future: Some(future),
-    }
+    CatchUnwind { future }
 }
 
 impl<F> Future for CatchUnwind<F>
     where F: Future + UnwindSafe,
 {
-    type Item = Result<F::Item, F::Error>;
-    type Error = Box<Any + Send>;
+    type Output = Result<F::Output, Box<Any + Send>>;
 
-    fn poll(&mut self, cx: &mut task::Context) -> Poll<Self::Item, Self::Error> {
-        let mut future = self.future.take().expect("cannot poll twice");
-        let (res, future) = catch_unwind(AssertUnwindSafe(|| {
-            (future.poll(cx), future)
-        }))?;
-        match res {
-            Ok(Async::Pending) => {
-                self.future = Some(future);
-                Ok(Async::Pending)
-            }
-            Ok(Async::Ready(t)) => Ok(Async::Ready(Ok(t))),
-            Err(e) => Ok(Async::Ready(Err(e))),
+    fn poll(mut self: Pin<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
+        let fut = unsafe { pinned_field!(self, future) };
+        match catch_unwind(AssertUnwindSafe(|| fut.poll(cx))) {
+            Ok(res) => res.map(Ok),
+            Err(e) => Poll::Ready(Err(e))
         }
     }
 }

futures-util/src/future/chain.rs

@@ -1,49 +1,53 @@
-use core::mem;
+use core::mem::Pin;
 
-use futures_core::{Future, Poll, Async};
+use futures_core::{Future, Poll};
 use futures_core::task;
 
 #[must_use = "futures do nothing unless polled"]
 #[derive(Debug)]
-pub enum Chain<A, B, C> where A: Future {
-    First(A, C),
-    Second(B),
-    Done,
+pub enum Chain<Fut1, Fut2, Data> {
+    First(Fut1, Option<Data>),
+    Second(Fut2),
 }
 
-impl<A, B, C> Chain<A, B, C>
-    where A: Future,
-          B: Future,
+impl<Fut1, Fut2, Data> Chain<Fut1, Fut2, Data>
+    where Fut1: Future,
+          Fut2: Future,
 {
-    pub fn new(a: A, c: C) -> Chain<A, B, C> {
-        Chain::First(a, c)
+    pub fn new(fut1: Fut1, data: Data) -> Chain<Fut1, Fut2, Data> {
+        Chain::First(fut1, Some(data))
     }
 
-    pub fn poll<F>(&mut self, cx: &mut task::Context, f: F) -> Poll<B::Item, B::Error>
-        where F: FnOnce(Result<A::Item, A::Error>, C)
-                        -> Result<Result<B::Item, B>, B::Error>,
+    pub fn poll<F>(mut self: Pin<Self>, cx: &mut task::Context, f: F) -> Poll<Fut2::Output>
+        where F: FnOnce(Fut1::Output, Data) -> Fut2,
     {
-        let a_result = match *self {
-            Chain::First(ref mut a, _) => {
-                match a.poll(cx) {
-                    Ok(Async::Pending) => return Ok(Async::Pending),
-                    Ok(Async::Ready(t)) => Ok(t),
-                    Err(e) => Err(e),
+        let mut f = Some(f);
+
+        loop {
+            // safe to `get_mut` here because we don't move out
+            let fut2 = match *unsafe { Pin::get_mut(&mut self) } {
+                Chain::First(ref mut fut1, ref mut data) => {
+                    // safe to create a new `Pin` because `fut1` will never move
+                    // before it's dropped.
+                    match unsafe { Pin::new_unchecked(fut1) }.poll(cx) {
+                        Poll::Pending => return Poll::Pending,
+                        Poll::Ready(t) => {
+                            (f.take().unwrap())(t, data.take().unwrap())
+                        }
+                    }
                 }
-            }
-            Chain::Second(ref mut b) => return b.poll(cx),
-            Chain::Done => panic!("cannot poll a chained future twice"),
-        };
-        let data = match mem::replace(self, Chain::Done) {
-            Chain::First(_, c) => c,
-            _ => panic!(),
-        };
-        match f(a_result, data)? {
-            Ok(e) => Ok(Async::Ready(e)),
-            Err(mut b) => {
-                let ret = b.poll(cx);
-                *self = Chain::Second(b);
-                ret
+                Chain::Second(ref mut fut2) => {
+                    // safe to create a new `Pin` because `fut2` will never move
+                    // before it's dropped; once we're in `Chain::Second` we stay
+                    // there forever.
+                    return unsafe { Pin::new_unchecked(fut2) }.poll(cx)
+                }
+            };
+
+            // safe because we're using the `&mut` to do an assignment, not for moving out
+            unsafe {
+                // note: it's safe to move the `fut2` here because we haven't yet polled it
+                *Pin::get_mut(&mut self) = Chain::Second(fut2);
             }
         }
     }

futures-util/src/future/empty.rs

@@ -1,32 +1,32 @@
 //! Definition of the Empty combinator, a future that's never ready.
 
+use core::mem::Pin;
 use core::marker;
 
-use futures_core::{Future, Poll, Async};
+use futures_core::{Future, Poll};
 use futures_core::task;
 
 /// A future which is never resolved.
 ///
 /// This future can be created with the `empty` function.
 #[derive(Debug)]
 #[must_use = "futures do nothing unless polled"]
-pub struct Empty<T, E> {
-    _data: marker::PhantomData<(T, E)>,
+pub struct Empty<T> {
+    _data: marker::PhantomData<T>,
 }
 
 /// Creates a future which never resolves, representing a computation that never
 /// finishes.
 ///
 /// The returned future will forever return `Async::Pending`.
-pub fn empty<T, E>() -> Empty<T, E> {
+pub fn empty<T>() -> Empty<T> {
     Empty { _data: marker::PhantomData }
 }
 
-impl<T, E> Future for Empty<T, E> {
-    type Item = T;
-    type Error = E;
+impl<T> Future for Empty<T> {
+    type Output = T;
 
-    fn poll(&mut self, _: &mut task::Context) -> Poll<T, E> {
-        Ok(Async::Pending)
+    fn poll(self: Pin<Self>, _: &mut task::Context) -> Poll<T> {
+        Poll::Pending
     }
 }

futures-util/src/future/flatten.rs

@@ -1,6 +1,7 @@
 use core::fmt;
+use core::mem::Pin;
 
-use futures_core::{Future, IntoFuture, Poll};
+use futures_core::{Future, Poll};
 use futures_core::task;
 
 use super::chain::Chain;
@@ -11,14 +12,13 @@ use super::chain::Chain;
 ///
 /// This is created by the `Future::flatten` method.
 #[must_use = "futures do nothing unless polled"]
-pub struct Flatten<A> where A: Future, A::Item: IntoFuture {
-    state: Chain<A, <A::Item as IntoFuture>::Future, ()>,
+pub struct Flatten<A> where A: Future, A::Output: Future {
+    state: Chain<A, A::Output, ()>,
 }
 
 impl<A> fmt::Debug for Flatten<A>
     where A: Future + fmt::Debug,
-          A::Item: IntoFuture,
-          <<A as Future>::Item as IntoFuture>::Future: fmt::Debug,
+          A::Output: Future + fmt::Debug,
 {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         fmt.debug_struct("Flatten")
@@ -29,7 +29,7 @@ impl<A> fmt::Debug for Flatten<A>
 
 pub fn new<A>(future: A) -> Flatten<A>
     where A: Future,
-          A::Item: IntoFuture,
+          A::Output: Future,
 {
     Flatten {
         state: Chain::new(future, ()),
@@ -38,16 +38,11 @@ pub fn new<A>(future: A) -> Flatten<A>
 
 impl<A> Future for Flatten<A>
     where A: Future,
-          A::Item: IntoFuture,
-          <<A as Future>::Item as IntoFuture>::Error: From<<A as Future>::Error>
+          A::Output: Future,
 {
-    type Item = <<A as Future>::Item as IntoFuture>::Item;
-    type Error = <<A as Future>::Item as IntoFuture>::Error;
+    type Output = <A::Output as Future>::Output;
 
-    fn poll(&mut self, cx: &mut task::Context) -> Poll<Self::Item, Self::Error> {
-        self.state.poll(cx, |a, ()| {
-            let future = a?.into_future();
-            Ok(Err(future))
-        })
+    fn poll(mut self: Pin<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
+        unsafe { pinned_field!(self, state) }.poll(cx, |a, ()| a)
     }
 }