Add spawn_local and clarify what the schedule function can do

examples/panic-propagation.rs

@@ -11,6 +11,8 @@ use futures::executor;
 use futures::future::FutureExt;
 use lazy_static::lazy_static;
 
+type Task = async_task::Task<()>;
+
 /// Spawns a future on the executor.
 fn spawn<F, R>(future: F) -> JoinHandle<R>
 where
@@ -19,8 +21,8 @@ where
 {
     lazy_static! {
         // A channel that holds scheduled tasks.
-        static ref QUEUE: Sender<async_task::Task<()>> = {
-            let (sender, receiver) = unbounded::<async_task::Task<()>>();
+        static ref QUEUE: Sender<Task> = {
+            let (sender, receiver) = unbounded::<Task>();
 
             // Start the executor thread.
             thread::spawn(|| {

examples/panic-result.rs

@@ -9,16 +9,19 @@ use futures::executor;
 use futures::future::FutureExt;
 use lazy_static::lazy_static;
 
+type Task = async_task::Task<()>;
+type JoinHandle<T> = async_task::JoinHandle<T, ()>;
+
 /// Spawns a future on the executor.
-fn spawn<F, R>(future: F) -> async_task::JoinHandle<thread::Result<R>, ()>
+fn spawn<F, R>(future: F) -> JoinHandle<thread::Result<R>>
 where
     F: Future<Output = R> + Send + 'static,
     R: Send + 'static,
 {
     lazy_static! {
         // A channel that holds scheduled tasks.
-        static ref QUEUE: Sender<async_task::Task<()>> = {
-            let (sender, receiver) = unbounded::<async_task::Task<()>>();
+        static ref QUEUE: Sender<Task> = {
+            let (sender, receiver) = unbounded::<Task>();
 
             // Start the executor thread.
             thread::spawn(|| {

examples/spawn-local.rs

@@ -0,0 +1,76 @@
+//! A simple single-threaded executor that can spawn non-`Send` futures.
+
+use std::cell::Cell;
+use std::future::Future;
+use std::rc::Rc;
+
+use crossbeam::channel::{unbounded, Receiver, Sender};
+
+type Task = async_task::Task<()>;
+type JoinHandle<T> = async_task::JoinHandle<T, ()>;
+
+thread_local! {
+    // A channel that holds scheduled tasks.
+    static QUEUE: (Sender<Task>, Receiver<Task>) = unbounded();
+}
+
+/// Spawns a future on the executor.
+fn spawn<F, R>(future: F) -> JoinHandle<R>
+where
+    F: Future<Output = R> + 'static,
+    R: 'static,
+{
+    // Create a task that is scheduled by sending itself into the channel.
+    let schedule = |t| QUEUE.with(|(s, _)| s.send(t).unwrap());
+    let (task, handle) = async_task::spawn_local(future, schedule, ());
+
+    // Schedule the task by sending it into the queue.
+    task.schedule();
+
+    handle
+}
+
+/// Runs a future to completion.
+fn run<F, R>(future: F) -> R
+where
+    F: Future<Output = R> + 'static,
+    R: 'static,
+{
+    // Spawn a task that sends its result through a channel.
+    let (s, r) = unbounded();
+    spawn(async move { s.send(future.await).unwrap() });
+
+    loop {
+        // If the original task has completed, return its result.
+        if let Ok(val) = r.try_recv() {
+            return val;
+        }
+
+        // Otherwise, take a task from the queue and run it.
+        QUEUE.with(|(_, r)| r.recv().unwrap().run());
+    }
+}
+
+fn main() {
+    let val = Rc::new(Cell::new(0));
+
+    // Run a future that increments a non-`Send` value.
+    run({
+        let val = val.clone();
+        async move {
+            // Spawn a future that increments the value.
+            let handle = spawn({
+                let val = val.clone();
+                async move {
+                    val.set(dbg!(val.get()) + 1);
+                }
+            });
+
+            val.set(dbg!(val.get()) + 1);
+            handle.await;
+        }
+    });
+
+    // The value should be 2 at the end of the program.
+    dbg!(val.get());
+}

examples/spawn-on-thread.rs

@@ -7,10 +7,12 @@ use std::thread;
 use crossbeam::channel;
 use futures::executor;
 
+type JoinHandle<T> = async_task::JoinHandle<T, ()>;
+
 /// Spawns a future on a new dedicated thread.
 ///
 /// The returned handle can be used to await the output of the future.
-fn spawn_on_thread<F, R>(future: F) -> async_task::JoinHandle<R, ()>
+fn spawn_on_thread<F, R>(future: F) -> JoinHandle<R>
 where
     F: Future<Output = R> + Send + 'static,
     R: Send + 'static,

examples/spawn.rs

@@ -8,16 +8,19 @@ use crossbeam::channel::{unbounded, Sender};
 use futures::executor;
 use lazy_static::lazy_static;
 
+type Task = async_task::Task<()>;
+type JoinHandle<T> = async_task::JoinHandle<T, ()>;
+
 /// Spawns a future on the executor.
-fn spawn<F, R>(future: F) -> async_task::JoinHandle<R, ()>
+fn spawn<F, R>(future: F) -> JoinHandle<R>
 where
     F: Future<Output = R> + Send + 'static,
     R: Send + 'static,
 {
     lazy_static! {
         // A channel that holds scheduled tasks.
-        static ref QUEUE: Sender<async_task::Task<()>> = {
-            let (sender, receiver) = unbounded::<async_task::Task<()>>();
+        static ref QUEUE: Sender<Task> = {
+            let (sender, receiver) = unbounded::<Task>();
 
             // Start the executor thread.
             thread::spawn(|| {

examples/task-id.rs

@@ -13,6 +13,9 @@ use lazy_static::lazy_static;
 #[derive(Clone, Copy, Debug)]
 struct TaskId(usize);
 
+type Task = async_task::Task<TaskId>;
+type JoinHandle<T> = async_task::JoinHandle<T, TaskId>;
+
 thread_local! {
     /// The ID of the current task.
     static TASK_ID: Cell<Option<TaskId>> = Cell::new(None);
@@ -26,15 +29,15 @@ fn task_id() -> Option<TaskId> {
 }
 
 /// Spawns a future on the executor.
-fn spawn<F, R>(future: F) -> async_task::JoinHandle<R, TaskId>
+fn spawn<F, R>(future: F) -> JoinHandle<R>
 where
     F: Future<Output = R> + Send + 'static,
     R: Send + 'static,
 {
     lazy_static! {
         // A channel that holds scheduled tasks.
-        static ref QUEUE: Sender<async_task::Task<TaskId>> = {
-            let (sender, receiver) = unbounded::<async_task::Task<TaskId>>();
+        static ref QUEUE: Sender<Task> = {
+            let (sender, receiver) = unbounded::<Task>();
 
             // Start the executor thread.
             thread::spawn(|| {

src/join_handle.rs

@@ -24,7 +24,7 @@ pub struct JoinHandle<R, T> {
     pub(crate) _marker: PhantomData<(R, T)>,
 }
 
-unsafe impl<R, T> Send for JoinHandle<R, T> {}
+unsafe impl<R: Send, T> Send for JoinHandle<R, T> {}
 unsafe impl<R, T> Sync for JoinHandle<R, T> {}
 
 impl<R, T> Unpin for JoinHandle<R, T> {}

src/lib.rs

@@ -23,7 +23,7 @@
 //! # let (task, handle) = async_task::spawn(future, schedule, ());
 //! ```
 //!
-//! A task is constructed using the [`spawn`] function:
+//! A task is constructed using either [`spawn`] or [`spawn_local`]:
 //!
 //! ```
 //! # let (sender, receiver) = crossbeam::channel::unbounded();
@@ -93,6 +93,7 @@
 //! union of the future and its output.
 //!
 //! [`spawn`]: fn.spawn.html
+//! [`spawn_local`]: fn.spawn_local.html
 //! [`Task`]: struct.Task.html
 //! [`JoinHandle`]: struct.JoinHandle.html
 
@@ -108,4 +109,4 @@ mod task;
 mod utils;
 
 pub use crate::join_handle::JoinHandle;
-pub use crate::task::{spawn, Task};
+pub use crate::task::{spawn, spawn_local, Task};

src/raw.rs

@@ -95,15 +95,13 @@ impl<F, R, S, T> Clone for RawTask<F, R, S, T> {
 
 impl<F, R, S, T> RawTask<F, R, S, T>
 where
-    F: Future<Output = R> + Send + 'static,
-    R: Send + 'static,
+    F: Future<Output = R> + 'static,
     S: Fn(Task<T>) + Send + Sync + 'static,
-    T: Send + 'static,
 {
     /// Allocates a task with the given `future` and `schedule` function.
     ///
     /// It is assumed that initially only the `Task` reference and the `JoinHandle` exist.
-    pub(crate) fn allocate(tag: T, future: F, schedule: S) -> NonNull<()> {
+    pub(crate) fn allocate(future: F, schedule: S, tag: T) -> NonNull<()> {
         // Compute the layout of the task for allocation. Abort if the computation fails.
         let task_layout = abort_on_panic(|| Self::task_layout());
 
@@ -592,17 +590,13 @@ where
         /// A guard that closes the task if polling its future panics.
         struct Guard<F, R, S, T>(RawTask<F, R, S, T>)
         where
-            F: Future<Output = R> + Send + 'static,
-            R: Send + 'static,
-            S: Fn(Task<T>) + Send + Sync + 'static,
-            T: Send + 'static;
+            F: Future<Output = R> + 'static,
+            S: Fn(Task<T>) + Send + Sync + 'static;
 
         impl<F, R, S, T> Drop for Guard<F, R, S, T>
         where
-            F: Future<Output = R> + Send + 'static,
-            R: Send + 'static,
+            F: Future<Output = R> + 'static,
             S: Fn(Task<T>) + Send + Sync + 'static,
-            T: Send + 'static,
         {
             fn drop(&mut self) {
                 let raw = self.0;

src/task.rs

@@ -1,8 +1,11 @@
 use std::fmt;
 use std::future::Future;
 use std::marker::PhantomData;
-use std::mem;
+use std::mem::{self, ManuallyDrop};
+use std::pin::Pin;
 use std::ptr::NonNull;
+use std::task::{Context, Poll};
+use std::thread::{self, ThreadId};
 
 use crate::header::Header;
 use crate::raw::RawTask;
@@ -16,8 +19,16 @@ use crate::JoinHandle;
 /// When run, the task polls `future`. When woken up, it gets scheduled for running by the
 /// `schedule` function. Argument `tag` is an arbitrary piece of data stored inside the task.
 ///
+/// The schedule function should not attempt to run the task nor to drop it. Instead, it should
+/// push the task into some kind of queue so that it can be processed later.
+///
+/// If you need to spawn a future that does not implement [`Send`], consider using the
+/// [`spawn_local`] function instead.
+///
 /// [`Task`]: struct.Task.html
 /// [`JoinHandle`]: struct.JoinHandle.html
+/// [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
+/// [`spawn_local`]: fn.spawn_local.html
 ///
 /// # Examples
 ///
@@ -43,7 +54,98 @@ where
     S: Fn(Task<T>) + Send + Sync + 'static,
     T: Send + Sync + 'static,
 {
-    let raw_task = RawTask::<F, R, S, T>::allocate(tag, future, schedule);
+    let raw_task = RawTask::<F, R, S, T>::allocate(future, schedule, tag);
+    let task = Task {
+        raw_task,
+        _marker: PhantomData,
+    };
+    let handle = JoinHandle {
+        raw_task,
+        _marker: PhantomData,
+    };
+    (task, handle)
+}
+
+/// Creates a new local task.
+///
+/// This constructor returns a [`Task`] reference that runs the future and a [`JoinHandle`] that
+/// awaits its result.
+///
+/// When run, the task polls `future`. When woken up, it gets scheduled for running by the
+/// `schedule` function. Argument `tag` is an arbitrary piece of data stored inside the task.
+///
+/// The schedule function should not attempt to run the task nor to drop it. Instead, it should
+/// push the task into some kind of queue so that it can be processed later.
+///
+/// Unlike [`spawn`], this function does not require the future to implement [`Send`]. If the
+/// [`Task`] reference is run or dropped on a thread it was not created on, a panic will occur.
+///
+/// [`Task`]: struct.Task.html
+/// [`JoinHandle`]: struct.JoinHandle.html
+/// [`spawn`]: fn.spawn.html
+/// [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam::channel;
+///
+/// // The future inside the task.
+/// let future = async {
+///     println!("Hello, world!");
+/// };
+///
+/// // If the task gets woken up, it will be sent into this channel.
+/// let (s, r) = channel::unbounded();
+/// let schedule = move |task| s.send(task).unwrap();
+///
+/// // Create a task with the future and the schedule function.
+/// let (task, handle) = async_task::spawn_local(future, schedule, ());
+/// ```
+pub fn spawn_local<F, R, S, T>(future: F, schedule: S, tag: T) -> (Task<T>, JoinHandle<R, T>)
+where
+    F: Future<Output = R> + 'static,
+    R: 'static,
+    S: Fn(Task<T>) + Send + Sync + 'static,
+    T: Send + Sync + 'static,
+{
+    thread_local! {
+        static ID: ThreadId = thread::current().id();
+    }
+
+    struct Checked<F> {
+        id: ThreadId,
+        inner: ManuallyDrop<F>,
+    }
+
+    impl<F> Drop for Checked<F> {
+        fn drop(&mut self) {
+            if ID.with(|id| *id) != self.id {
+                panic!("local task dropped by a thread that didn't spawn it");
+            }
+            unsafe {
+                ManuallyDrop::drop(&mut self.inner);
+            }
+        }
+    }
+
+    impl<F: Future> Future for Checked<F> {
+        type Output = F::Output;
+
+        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+            if ID.with(|id| *id) != self.id {
+                panic!("local task polled by a thread that didn't spawn it");
+            }
+            unsafe { self.map_unchecked_mut(|c| &mut *c.inner).poll(cx) }
+        }
+    }
+
+    let future = Checked {
+        id: ID.with(|id| *id),
+        inner: ManuallyDrop::new(future),
+    };
+
+    let raw_task = RawTask::<_, R, S, T>::allocate(future, schedule, tag);
     let task = Task {
         raw_task,
         _marker: PhantomData,