Add BoundLocalPool and BoundLocalSpawner

Adds derivatives of LocalPool and BoundLocalSpawner with a generic
lifetime parameter that replaces the previously used 'static, and
aliases both types with a 'static parameter to the originals.

Add a BoundLocalSpawn trait in a similar fashion. At the same time
simplify the implementation of Spawn, LocalSpawn and BoundLocalSpawn to
just work on any type implementing Deref to a type implementing a spawn
trait.

futures-executor/src/lib.rs

@@ -54,7 +54,10 @@ extern crate std;
 #[cfg(feature = "std")]
 mod local_pool;
 #[cfg(feature = "std")]
-pub use crate::local_pool::{block_on, block_on_stream, BlockingStream, LocalPool, LocalSpawner};
+pub use crate::local_pool::{
+    block_on, block_on_stream, BlockingStream, BoundLocalPool, BoundLocalSpawner, LocalPool,
+    LocalSpawner,
+};
 
 #[cfg(feature = "thread-pool")]
 #[cfg_attr(docsrs, doc(cfg(feature = "thread-pool")))]

futures-executor/src/local_pool.rs

@@ -2,7 +2,7 @@ use crate::enter;
 use futures_core::future::Future;
 use futures_core::stream::Stream;
 use futures_core::task::{Context, Poll};
-use futures_task::{waker_ref, ArcWake};
+use futures_task::{waker_ref, ArcWake, BoundLocalSpawn};
 use futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError};
 use futures_util::pin_mut;
 use futures_util::stream::FuturesUnordered;
@@ -17,6 +17,36 @@ use std::sync::{
 use std::thread::{self, Thread};
 use std::vec::Vec;
 
+/// A single-threaded task pool with bound lifetime for polling futures to
+/// completion.
+///
+/// This executor allows you to multiplex any number of tasks onto a single
+/// thread. It's appropriate to poll strictly I/O-bound futures that do very
+/// little work in between I/O actions. The lifetime of the executor is bound by
+/// a generic parameter. Futures associated with the executor need only outlive
+/// this lifetime. That uncompleted futures are dropped when the lifetime of the
+/// executor expires.
+///
+/// To get a handle to the pool that implements [`Spawn`](futures_task::Spawn),
+/// use the [`spawner()`](BoundLocalPool::spawner) method. Because the executor
+/// is single-threaded, it supports a special form of task spawning for
+/// non-`Send` futures, via
+/// [`spawn_local_obj`](futures_task::LocalSpawn::spawn_local_obj).
+/// Additionally, tasks with a limited lifetime can be spawned via
+/// [`spawn_bound_local_obj`](futures_task::BoundLocalSpawn::spawn_bound_local_obj).
+#[derive(Debug)]
+pub struct BoundLocalPool<'a> {
+    pool: FuturesUnordered<LocalFutureObj<'a, ()>>,
+    incoming: Rc<Incoming<'a>>,
+}
+
+/// A handle to a [`BoundLocalPool`] that implements
+/// [`BoundLocalSpawn`](futures_task::BoundLocalSpawn).
+#[derive(Clone, Debug)]
+pub struct BoundLocalSpawner<'a> {
+    incoming: Weak<Incoming<'a>>,
+}
+
 /// A single-threaded task pool for polling futures to completion.
 ///
 /// This executor allows you to multiplex any number of tasks onto a single
@@ -28,19 +58,13 @@ use std::vec::Vec;
 /// [`spawner()`](LocalPool::spawner) method. Because the executor is
 /// single-threaded, it supports a special form of task spawning for non-`Send`
 /// futures, via [`spawn_local_obj`](futures_task::LocalSpawn::spawn_local_obj).
-#[derive(Debug)]
-pub struct LocalPool {
-    pool: FuturesUnordered<LocalFutureObj<'static, ()>>,
-    incoming: Rc<Incoming>,
-}
+pub type LocalPool = BoundLocalPool<'static>;
 
-/// A handle to a [`LocalPool`] that implements [`Spawn`](futures_task::Spawn).
-#[derive(Clone, Debug)]
-pub struct LocalSpawner {
-    incoming: Weak<Incoming>,
-}
+/// A handle to a [`LocalPool`] that implements [`Spawn`](futures_task::Spawn)
+/// and [`LocalSpawn`](futures_task::LocalSpawn).
+pub type LocalSpawner = BoundLocalSpawner<'static>;
 
-type Incoming = RefCell<Vec<LocalFutureObj<'static, ()>>>;
+type Incoming<'a> = RefCell<Vec<LocalFutureObj<'a, ()>>>;
 
 pub(crate) struct ThreadNotify {
     /// The (single) executor thread.
@@ -107,15 +131,15 @@ fn woken() -> bool {
     CURRENT_THREAD_NOTIFY.with(|thread_notify| thread_notify.unparked.load(Ordering::Acquire))
 }
 
-impl LocalPool {
+impl<'a> BoundLocalPool<'a> {
     /// Create a new, empty pool of tasks.
     pub fn new() -> Self {
         Self { pool: FuturesUnordered::new(), incoming: Default::default() }
     }
 
     /// Get a clonable handle to the pool as a [`Spawn`].
-    pub fn spawner(&self) -> LocalSpawner {
-        LocalSpawner { incoming: Rc::downgrade(&self.incoming) }
+    pub fn spawner(&self) -> BoundLocalSpawner<'a> {
+        BoundLocalSpawner { incoming: Rc::downgrade(&self.incoming) }
     }
 
     /// Run all tasks in the pool to completion.
@@ -362,7 +386,7 @@ impl<S: Stream + Unpin> Iterator for BlockingStream<S> {
     }
 }
 
-impl Spawn for LocalSpawner {
+impl Spawn for BoundLocalSpawner<'_> {
     fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
         if let Some(incoming) = self.incoming.upgrade() {
             incoming.borrow_mut().push(future.into());
@@ -381,7 +405,7 @@ impl Spawn for LocalSpawner {
     }
 }
 
-impl LocalSpawn for LocalSpawner {
+impl LocalSpawn for BoundLocalSpawner<'_> {
     fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
         if let Some(incoming) = self.incoming.upgrade() {
             incoming.borrow_mut().push(future);
@@ -399,3 +423,14 @@ impl LocalSpawn for LocalSpawner {
         }
     }
 }
+
+impl<'a> BoundLocalSpawn<'a> for BoundLocalSpawner<'a> {
+    fn spawn_bound_local_obj(&self, future: LocalFutureObj<'a, ()>) -> Result<(), SpawnError> {
+        if let Some(incoming) = self.incoming.upgrade() {
+            incoming.borrow_mut().push(future);
+            Ok(())
+        } else {
+            Err(SpawnError::shutdown())
+        }
+    }
+}

futures-task/src/lib.rs

@@ -16,7 +16,7 @@ extern crate alloc;
 extern crate std;
 
 mod spawn;
-pub use crate::spawn::{LocalSpawn, Spawn, SpawnError};
+pub use crate::spawn::{BoundLocalSpawn, LocalSpawn, Spawn, SpawnError};
 
 #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 #[cfg(feature = "alloc")]

futures-task/src/spawn.rs

@@ -51,6 +51,22 @@ pub trait LocalSpawn {
     }
 }
 
+/// The `BoundLocalSpawn` is similar to [`LocalSpawn`], but allows spawning
+/// futures that don't implement `Send` and have a lifetime that only needs to
+/// exceed that of the associated executor.
+pub trait BoundLocalSpawn<'a> {
+    /// Spawns a future that will be run to completion or until the executor is
+    /// dropped.
+    ///
+    /// # Errors
+    ///
+    /// The executor may be unable to spawn tasks. Spawn errors should
+    /// represent relatively rare scenarios, such as the executor
+    /// having been shut down so that it is no longer able to accept
+    /// tasks.
+    fn spawn_bound_local_obj(&self, future: LocalFutureObj<'a, ()>) -> Result<(), SpawnError>;
+}
+
 /// An error that occurred during spawning.
 pub struct SpawnError {
     _priv: (),
@@ -83,17 +99,10 @@ impl SpawnError {
     }
 }
 
-impl<Sp: ?Sized + Spawn> Spawn for &Sp {
-    fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
-        Sp::spawn_obj(self, future)
-    }
-
-    fn status(&self) -> Result<(), SpawnError> {
-        Sp::status(self)
-    }
-}
-
-impl<Sp: ?Sized + Spawn> Spawn for &mut Sp {
+impl<T, Sp: ?Sized + Spawn> Spawn for T
+where
+    T: std::ops::Deref<Target = Sp>,
+{
     fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
         Sp::spawn_obj(self, future)
     }
@@ -103,17 +112,10 @@ impl<Sp: ?Sized + Spawn> Spawn for &mut Sp {
     }
 }
 
-impl<Sp: ?Sized + LocalSpawn> LocalSpawn for &Sp {
-    fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
-        Sp::spawn_local_obj(self, future)
-    }
-
-    fn status_local(&self) -> Result<(), SpawnError> {
-        Sp::status_local(self)
-    }
-}
-
-impl<Sp: ?Sized + LocalSpawn> LocalSpawn for &mut Sp {
+impl<T, Sp: ?Sized + LocalSpawn> LocalSpawn for T
+where
+    T: std::ops::Deref<Target = Sp>,
+{
     fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
         Sp::spawn_local_obj(self, future)
     }
@@ -123,70 +125,11 @@ impl<Sp: ?Sized + LocalSpawn> LocalSpawn for &mut Sp {
     }
 }
 
-#[cfg(feature = "alloc")]
-mod if_alloc {
-    use super::*;
-    use alloc::{boxed::Box, rc::Rc};
-
-    impl<Sp: ?Sized + Spawn> Spawn for Box<Sp> {
-        fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
-            (**self).spawn_obj(future)
-        }
-
-        fn status(&self) -> Result<(), SpawnError> {
-            (**self).status()
-        }
-    }
-
-    impl<Sp: ?Sized + LocalSpawn> LocalSpawn for Box<Sp> {
-        fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
-            (**self).spawn_local_obj(future)
-        }
-
-        fn status_local(&self) -> Result<(), SpawnError> {
-            (**self).status_local()
-        }
-    }
-
-    impl<Sp: ?Sized + Spawn> Spawn for Rc<Sp> {
-        fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
-            (**self).spawn_obj(future)
-        }
-
-        fn status(&self) -> Result<(), SpawnError> {
-            (**self).status()
-        }
-    }
-
-    impl<Sp: ?Sized + LocalSpawn> LocalSpawn for Rc<Sp> {
-        fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
-            (**self).spawn_local_obj(future)
-        }
-
-        fn status_local(&self) -> Result<(), SpawnError> {
-            (**self).status_local()
-        }
-    }
-
-    #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
-    impl<Sp: ?Sized + Spawn> Spawn for alloc::sync::Arc<Sp> {
-        fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
-            (**self).spawn_obj(future)
-        }
-
-        fn status(&self) -> Result<(), SpawnError> {
-            (**self).status()
-        }
-    }
-
-    #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
-    impl<Sp: ?Sized + LocalSpawn> LocalSpawn for alloc::sync::Arc<Sp> {
-        fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
-            (**self).spawn_local_obj(future)
-        }
-
-        fn status_local(&self) -> Result<(), SpawnError> {
-            (**self).status_local()
-        }
+impl<'a, T, Sp: ?Sized + BoundLocalSpawn<'a>> BoundLocalSpawn<'a> for T
+where
+    T: std::ops::Deref<Target = Sp>,
+{
+    fn spawn_bound_local_obj(&self, future: LocalFutureObj<'a, ()>) -> Result<(), SpawnError> {
+        Sp::spawn_bound_local_obj(self, future)
     }
 }

futures-util/src/stream/futures_unordered/mod.rs

@@ -17,7 +17,7 @@ use core::sync::atomic::{AtomicBool, AtomicPtr};
 use futures_core::future::Future;
 use futures_core::stream::{FusedStream, Stream};
 use futures_core::task::{Context, Poll};
-use futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError};
+use futures_task::{BoundLocalSpawn, FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError};
 
 mod abort;
 
@@ -78,6 +78,13 @@ impl LocalSpawn for FuturesUnordered<LocalFutureObj<'_, ()>> {
     }
 }
 
+impl<'a> BoundLocalSpawn<'a> for FuturesUnordered<LocalFutureObj<'a, ()>> {
+    fn spawn_bound_local_obj(&self, future_obj: LocalFutureObj<'a, ()>) -> Result<(), SpawnError> {
+        self.push(future_obj);
+        Ok(())
+    }
+}
+
 // FuturesUnordered is implemented using two linked lists. One which links all
 // futures managed by a `FuturesUnordered` and one that tracks futures that have
 // been scheduled for polling. The first linked list allows for thread safe

futures-util/src/task/mod.rs

@@ -13,7 +13,9 @@
 #[doc(no_inline)]
 pub use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 
-pub use futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError, UnsafeFutureObj};
+pub use futures_task::{
+    BoundLocalSpawn, FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError, UnsafeFutureObj,
+};
 
 pub use futures_task::noop_waker;
 pub use futures_task::noop_waker_ref;
@@ -37,4 +39,4 @@ pub use futures_task::{waker_ref, WakerRef};
 pub use futures_core::task::__internal::AtomicWaker;
 
 mod spawn;
-pub use self::spawn::{LocalSpawnExt, SpawnExt};
+pub use self::spawn::{BoundLocalSpawnExt, LocalSpawnExt, SpawnExt};