feat(runnable): reintroduce into_raw/from_raw functions

These APIs were removed prior to async-task 4.0 (in commit deb709f); they can however be useful to e.g. pass Runnable as a context argument to GCD's dispatch_async_f, which takes an opaque *mut () as an argument. Without exposing from_raw/into_raw one has to box the Runnable in order to get something that can be passed across that boundary, which seems wasteful.

src/raw.rs

@@ -1,7 +1,6 @@
 use alloc::alloc::Layout as StdLayout;
 use core::cell::UnsafeCell;
 use core::future::Future;
-use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
 use core::pin::Pin;
 use core::ptr::NonNull;
@@ -346,10 +345,7 @@ where
                             // Schedule the task. There is no need to call `Self::schedule(ptr)`
                             // because the schedule function cannot be destroyed while the waker is
                             // still alive.
-                            let task = Runnable {
-                                ptr: NonNull::new_unchecked(ptr as *mut ()),
-                                _marker: PhantomData,
-                            };
+                            let task = Runnable::from_raw(NonNull::new_unchecked(ptr as *mut ()));
                             (*raw.schedule).schedule(task, ScheduleInfo::new(false));
                         }
 
@@ -438,10 +434,7 @@ where
             _waker = Waker::from_raw(Self::clone_waker(ptr));
         }
 
-        let task = Runnable {
-            ptr: NonNull::new_unchecked(ptr as *mut ()),
-            _marker: PhantomData,
-        };
+        let task = Runnable::from_raw(NonNull::new_unchecked(ptr as *mut ()));
         (*raw.schedule).schedule(task, info);
     }
 

src/runnable.rs

@@ -524,10 +524,7 @@ impl<M> Builder<M> {
             RawTask::<Fut, Fut::Output, S, M>::allocate(future, schedule, self)
         };
 
-        let runnable = Runnable {
-            ptr,
-            _marker: PhantomData,
-        };
+        let runnable = Runnable::from_raw(ptr);
         let task = Task {
             ptr,
             _marker: PhantomData,
@@ -820,6 +817,77 @@ impl<M> Runnable<M> {
     fn header(&self) -> &Header<M> {
         unsafe { &*(self.ptr.as_ptr() as *const Header<M>) }
     }
+
+    /// Converts this task into a raw pointer.
+    ///
+    /// To avoid a memory leak the pointer must be converted back to a Runnable using [`Runnable<M>::from_raw`][from_raw].
+    ///
+    /// `into_raw` does not change the state of the [`Task`], but there is no guarantee that it will be in the same state after calling [`Runnable<M>::from_raw`][from_raw],
+    /// as the corresponding [`Task`] might have been dropped or cancelled.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use async_task::{Runnable, spawn};
+
+    /// let (runnable, task) = spawn(async {}, |_| {});
+    /// let runnable_pointer = runnable.into_raw();
+    ///
+    /// unsafe {
+    ///     // Convert back to an `Runnable` to prevent leak.
+    ///     let runnable = Runnable::<()>::from_raw(runnable_pointer);
+    ///     runnable.run();
+    ///     // Further calls to `Runnable::from_raw(runnable_pointer)` would be memory-unsafe.
+    /// }
+    /// // The memory was freed when `x` went out of scope above, so `runnable_pointer` is now dangling!
+    /// ```
+    /// [from_raw]: #method.from_raw
+    pub fn into_raw(self) -> NonNull<()> {
+        let ptr = self.ptr;
+        mem::forget(self);
+        ptr
+    }
+
+    /// Converts a raw pointer into a Runnable.
+    ///
+    /// # Safety
+    ///
+    /// This method should only be used with raw pointers returned from [`Runnable<M>::into_raw`][into_raw].
+    /// It is not safe to use the provided pointer once it is passed to `from_raw`.
+    /// Crucially, it is unsafe to call `from_raw` multiple times with the same pointer - even if the resulting [`Runnable`] is not used -
+    /// as internally `async-task` uses reference counting.
+    ///
+    /// It is however safe to call [`Runnable<M>::into_raw`][into_raw] on a [`Runnable`] created with `from_raw` or
+    /// after the [`Task`] associated with a given Runnable has been dropped or cancelled.
+    ///
+    /// The state of the [`Runnable`] created with `from_raw` is not specified.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use async_task::{Runnable, spawn};
+
+    /// let (runnable, task) = spawn(async {}, |_| {});
+    /// let runnable_pointer = runnable.into_raw();
+    ///
+    /// drop(task);
+    /// unsafe {
+    ///     // Convert back to an `Runnable` to prevent leak.
+    ///     let runnable = Runnable::<()>::from_raw(runnable_pointer);
+    ///     let did_poll = runnable.run();
+    ///     assert!(!did_poll);
+    ///     // Further calls to `Runnable::from_raw(runnable_pointer)` would be memory-unsafe.
+    /// }
+    /// // The memory was freed when `x` went out of scope above, so `runnable_pointer` is now dangling!
+    /// ```
+
+    /// [into_raw]: #method.into_raw
+    pub unsafe fn from_raw(ptr: NonNull<()>) -> Self {
+        Self {
+            ptr,
+            _marker: Default::default(),
+        }
+    }
 }
 
 impl<M> Drop for Runnable<M> {

tests/basic.rs

@@ -1,6 +1,8 @@
 use std::future::Future;
 use std::pin::Pin;
-use std::sync::atomic::{AtomicUsize, Ordering};
+use std::ptr::NonNull;
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::Arc;
 use std::task::{Context, Poll};
 
 use async_task::Runnable;
@@ -297,3 +299,27 @@ fn waker() {
     waker.wake();
     r.recv().unwrap();
 }
+
+#[test]
+fn raw() {
+    // Dispatch schedules a function for execution at a later point. For tests, we execute it straight away.
+    fn dispatch(trampoline: extern "C" fn(NonNull<()>), context: NonNull<()>) {
+        trampoline(context)
+    }
+    extern "C" fn trampoline(runnable: NonNull<()>) {
+        let task = unsafe { Runnable::<()>::from_raw(runnable) };
+        task.run();
+    }
+
+    let task_got_executed = Arc::new(AtomicBool::new(false));
+    let (runnable, _handle) = async_task::spawn(
+        {
+            let task_got_executed = task_got_executed.clone();
+            async move { task_got_executed.store(true, Ordering::SeqCst) }
+        },
+        |runnable: Runnable<()>| dispatch(trampoline, runnable.into_raw()),
+    );
+    runnable.schedule();
+
+    assert!(task_got_executed.load(Ordering::SeqCst));
+}