Make BoxFuture and LocalBoxFuture a concrete type with #[must_use]

Currently BoxFuture is type alias for Pin<Box<...>>, that leads to accidental mistakes, when user does not call await/poll on result of function that returns BoxFuture. We had multiple problems with that

Some notes on PR:

* I understand that for some projects this might be breaking change - if someone have hardcoded Pin<Box<...>> as a type of variable returned by .boxed, they would have to change it BoxedFuture once this diff lands, but I believe this is reasonable trade off.

* I am not sure what to do with Debug implementation for BoxFuture - ok to just suppress warning for this type?

* I don't fully understand what UnsafeFutureObj is, I mostly copied implementation from Pin<Box<...>>, but please take a look at it carefully to make sure I did not miss something.

Author: Andrey Chursin

futures-core/src/future/mod.rs

@@ -10,13 +10,17 @@ mod future_obj;
 pub use self::future_obj::{FutureObj, LocalFutureObj, UnsafeFutureObj};
 
 #[cfg(feature = "alloc")]
+#[must_use = "futures do nothing unless polled"]
+#[allow(missing_debug_implementations)]
 /// 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>>;
+pub struct BoxFuture<'a, T>(Pin<alloc::boxed::Box<dyn Future<Output = T> + Send + 'a>>);
 
 #[cfg(feature = "alloc")]
+#[must_use = "futures do nothing unless polled"]
+#[allow(missing_debug_implementations)]
 /// `BoxFuture`, but without the `Send` requirement.
-pub type LocalBoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + 'a>>;
+pub struct LocalBoxFuture<'a, T>(Pin<alloc::boxed::Box<dyn Future<Output = T> + 'a>>);
 
 /// A `Future` or `TryFuture` which tracks whether or not the underlying future
 /// should no longer be polled.
@@ -81,6 +85,7 @@ impl<F, T, E> TryFuture for F
 
 #[cfg(feature = "alloc")]
 mod if_alloc {
+    use std::mem;
     use alloc::boxed::Box;
     use super::*;
 
@@ -96,4 +101,62 @@ mod if_alloc {
             <F as FusedFuture>::is_terminated(&**self)
         }
     }
+
+    impl<O> Future for BoxFuture<'_, O> {
+        type Output = O;
+
+        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+            let mut_pin = self.0.as_mut();
+            mut_pin.poll(cx)
+        }
+    }
+
+    impl<O> BoxFuture<'_, O> {
+        /// Converts Pin<Box<Future<Output=O>> to PinnedFuture<O>
+        pub fn new<'a>(f: Pin<Box<dyn Future<Output = O> + Send + 'a>>) -> BoxFuture<'a, O> {
+            BoxFuture(f)
+        }
+    }
+
+    impl<O> Future for LocalBoxFuture<'_, O> {
+        type Output = O;
+
+        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+            let mut_pin = self.0.as_mut();
+            mut_pin.poll(cx)
+        }
+    }
+
+    impl<O> LocalBoxFuture<'_, O> {
+        /// Converts Pin<Box<Future<Output=O>> to PinnedFuture<O>
+        pub fn new<'a>(f: Pin<Box<dyn Future<Output = O> + 'a>>) -> LocalBoxFuture<'a, O> {
+            LocalBoxFuture(f)
+        }
+    }
+
+    unsafe impl<'a, T> UnsafeFutureObj<'a, T> for BoxFuture<'a, T> where T: 'a
+    {
+        fn into_raw(mut self) -> *mut (dyn Future<Output = T> + 'a) {
+            let ptr = unsafe { self.0.as_mut().get_unchecked_mut() as *mut _ };
+            mem::forget(self);
+            ptr
+        }
+
+        unsafe fn drop(ptr: *mut (dyn Future<Output = T> + 'a)) {
+            drop(Pin::from(Box::from_raw(ptr)))
+        }
+    }
+
+    unsafe impl<'a, T> UnsafeFutureObj<'a, T> for LocalBoxFuture<'a, T> where T: 'a
+    {
+        fn into_raw(mut self) -> *mut (dyn Future<Output = T> + 'a) {
+            let ptr = unsafe { self.0.as_mut().get_unchecked_mut() as *mut _ };
+            mem::forget(self);
+            ptr
+        }
+
+        unsafe fn drop(ptr: *mut (dyn Future<Output = T> + 'a)) {
+            drop(Pin::from(Box::from_raw(ptr)))
+        }
+    }
 }

futures-util/src/future/mod.rs

@@ -495,7 +495,7 @@ pub trait FutureExt: Future {
     fn boxed<'a>(self) -> BoxFuture<'a, Self::Output>
         where Self: Sized + Send + 'a
     {
-        Box::pin(self)
+        BoxFuture::new(Box::pin(self))
     }
 
     /// Wrap the future in a Box, pinning it.
@@ -505,7 +505,7 @@ pub trait FutureExt: Future {
     fn boxed_local<'a>(self) -> LocalBoxFuture<'a, Self::Output>
         where Self: Sized + 'a
     {
-        Box::pin(self)
+        LocalBoxFuture::new(Box::pin(self))
     }
 
     /// Turns a [`Future<Output = T>`](Future) into a