Implement Future for Option<F>

library/core/src/option.rs

@@ -551,9 +551,12 @@ use crate::marker::Destruct;
 use crate::panicking::{panic, panic_str};
 use crate::pin::Pin;
 use crate::{
-    cmp, convert, hint, mem,
+    cmp, convert,
+    future::Future,
+    hint, mem,
     ops::{self, ControlFlow, Deref, DerefMut},
     slice,
+    task::Poll,
 };
 
 /// The `Option` type. See [the module level documentation](self) for more.
@@ -2293,6 +2296,19 @@ impl SpecOptionPartialEq for cmp::Ordering {
     }
 }
 
+#[stable(feature = "option_future", since = "CURRENT_RUSTC_VERSION")]
+impl<F: Future> Future for Option<F> {
+    type Output = Option<F::Output>;
+
+    #[inline]
+    fn poll(self: Pin<&mut Self>, cx: &mut crate::task::Context<'_>) -> Poll<Self::Output> {
+        match self.as_pin_mut() {
+            Some(f) => f.poll(cx).map(Some),
+            None => Poll::Ready(None),
+        }
+    }
+}
+
 /////////////////////////////////////////////////////////////////////////////
 // The Option Iterators
 /////////////////////////////////////////////////////////////////////////////

tests/ui/async-await/drop-track-bad-field-in-fru.rs

@@ -5,6 +5,5 @@ fn main() {}
 
 async fn foo() {
     None { value: (), ..Default::default() }.await;
-    //~^ ERROR `Option<_>` is not a future
-    //~| ERROR variant `Option<_>::None` has no field named `value`
+    //~^ ERROR variant `Option<_>::None` has no field named `value`
 }

tests/ui/async-await/drop-track-bad-field-in-fru.stderr

@@ -4,20 +4,6 @@ error[E0559]: variant `Option<_>::None` has no field named `value`
 LL |     None { value: (), ..Default::default() }.await;
    |            ^^^^^ `Option<_>::None` does not have this field
 
-error[E0277]: `Option<_>` is not a future
-  --> $DIR/drop-track-bad-field-in-fru.rs:7:45
-   |
-LL |     None { value: (), ..Default::default() }.await;
-   |                                             ^^^^^^
-   |                                             |
-   |                                             `Option<_>` is not a future
-   |                                             help: remove the `.await`
-   |
-   = help: the trait `Future` is not implemented for `Option<_>`
-   = note: Option<_> must be a future or must implement `IntoFuture` to be awaited
-   = note: required for `Option<_>` to implement `IntoFuture`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0277, E0559.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0559`.