Remove StreamObj, caution against using FutureObj

Author: Nemo157

futures-core/src/future/future_obj.rs

@@ -9,9 +9,9 @@ use core::{
 /// A custom trait object for polling futures, roughly akin to
 /// `Box<dyn Future<Output = T> + 'a>`.
 ///
-/// This custom trait object was introduced for two reasons:
-/// - Currently it is not possible to take `dyn Trait` by value and
-///   `Box<dyn Trait>` is not available in no_std contexts.
+/// This custom trait object was introduced as currently it is not possible to
+/// take `dyn Trait` by value and `Box<dyn Trait>` is not available in no_std
+/// contexts.
 pub struct LocalFutureObj<'a, T> {
     ptr: *mut (),
     poll_fn: unsafe fn(*mut (), &mut Context<'_>) -> Poll<T>,
@@ -79,14 +79,13 @@ impl<T> Drop for LocalFutureObj<'_, T> {
 /// A custom trait object for polling futures, roughly akin to
 /// `Box<dyn Future<Output = T> + Send + 'a>`.
 ///
-/// This custom trait object was introduced for two reasons:
-/// - Currently it is not possible to take `dyn Trait` by value and
-///   `Box<dyn Trait>` is not available in no_std contexts.
-/// - The `Future` trait is currently not object safe: The `Future::poll`
-///   method makes uses the arbitrary self types feature and traits in which
-///   this feature is used are currently not object safe due to current compiler
-///   limitations. (See tracking issue for arbitrary self types for more
-///   information #44874)
+/// This custom trait object was introduced as currently it is not possible to
+/// take `dyn Trait` by value and `Box<dyn Trait>` is not available in no_std
+/// contexts.
+///
+/// You should generally not need to use this type outside of `no_std` or when
+/// implementing `Spawn`, consider using [`BoxFuture`](crate::future::BoxFuture)
+/// instead.
 pub struct FutureObj<'a, T>(LocalFutureObj<'a, T>);
 
 impl<T> Unpin for FutureObj<'_, T> {}

futures-core/src/future/mod.rs

@@ -8,6 +8,11 @@ pub use core::future::Future;
 mod future_obj;
 pub use self::future_obj::{FutureObj, LocalFutureObj, UnsafeFutureObj};
 
+#[cfg(feature = "alloc")]
+/// An owned dynamically typed [`Future`] for use in cases where you can't
+/// statically type your result or need to add some indirection.
+pub type BoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + Send + 'a>>;
+
 /// A `Future` or `TryFuture` which tracks whether or not the underlying future
 /// should no longer be polled.
 ///

futures-core/src/stream/mod.rs

@@ -4,8 +4,10 @@ use core::ops;
 use core::pin::Pin;
 use core::task::{Context, Poll};
 
-mod stream_obj;
-pub use self::stream_obj::{StreamObj,LocalStreamObj,UnsafeStreamObj};
+#[cfg(feature = "alloc")]
+/// An owned dynamically typed [`Stream`] for use in cases where you can't
+/// statically type your result or need to add some indirection.
+pub type BoxStream<'a, T> = Pin<alloc::boxed::Box<dyn Stream<Item = T> + Send + 'a>>;
 
 /// A stream of values produced asynchronously.
 ///

futures-core/src/stream/stream_obj.rs

@@ -9,6 +9,8 @@ use futures_core::stream::Stream;
 use futures_core::task::{Context, Poll};
 #[cfg(feature = "alloc")]
 use alloc::boxed::Box;
+#[cfg(feature = "alloc")]
+use futures_core::future::BoxFuture;
 
 // re-export for `select!`
 #[doc(hidden)]
@@ -491,8 +493,8 @@ pub trait FutureExt: Future {
 
     /// Wrap the future in a Box, pinning it.
     #[cfg(feature = "alloc")]
-    fn boxed(self) -> Pin<Box<Self>>
-        where Self: Sized
+    fn boxed(self) -> BoxFuture<'static, Self::Output>
+        where Self: Sized + Send + 'static
     {
         Box::pin(self)
     }

futures-util/src/future/mod.rs

@@ -189,6 +189,9 @@ pub mod future {
         FutureObj, LocalFutureObj, UnsafeFutureObj,
     };
 
+    #[cfg(feature = "alloc")]
+    pub use futures_core::future::BoxFuture;
+
     pub use futures_util::future::{
         empty, Empty,
         lazy, Lazy,
@@ -335,9 +338,11 @@ pub mod stream {
 
     pub use futures_core::stream::{
         Stream, TryStream, FusedStream,
-        StreamObj, LocalStreamObj, UnsafeStreamObj,
     };
 
+    #[cfg(feature = "alloc")]
+    pub use futures_core::stream::BoxStream;
+
     pub use futures_util::stream::{
         iter, Iter,
         repeat, Repeat,

futures/src/lib.rs

@@ -6,7 +6,7 @@ use futures::task::{Context, Poll};
 
 #[test]
 fn dropping_does_not_segfault() {
-    FutureObj::new(async { String::new() }.boxed());
+    FutureObj::new(Box::new(async { String::new() }));
 }
 
 #[test]
@@ -29,7 +29,7 @@ fn dropping_drops_the_future() {
         }
     }
 
-    FutureObj::new(Inc(&mut times_dropped).boxed());
+    FutureObj::new(Box::new(Inc(&mut times_dropped)));
 
     assert_eq!(times_dropped, 1);
 }