Always mention Box::pin when dealing with !Unpin

Author: estebank

Diff for: library/core/src/marker.rs

@@ -765,7 +765,7 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 /// [`pin` module]: crate::pin
 #[stable(feature = "pin", since = "1.33.0")]
 #[rustc_on_unimplemented(
-    on(_Self = "std::future::Future", note = "consider using `Box::pin`",),
+    note = "consider using `Box::pin`",
     message = "`{Self}` cannot be unpinned"
 )]
 #[lang = "unpin"]

Diff for: src/test/ui/async-await/pin-needed-to-poll-2.stderr

@@ -4,6 +4,7 @@ error[E0277]: `PhantomPinned` cannot be unpinned
 LL |         Pin::new(&mut self.sleep).poll(cx)
    |         ^^^^^^^^ within `Sleep`, the trait `Unpin` is not implemented for `PhantomPinned`
    |
+   = note: consider using `Box::pin`
 note: required because it appears within the type `Sleep`
   --> $DIR/pin-needed-to-poll-2.rs:8:8
    |

Diff for: src/test/ui/generator/static-not-unpin.stderr

@@ -6,6 +6,8 @@ LL | fn assert_unpin<T: Unpin>(_: T) {
 ...
 LL |     assert_unpin(generator);
    |                  ^^^^^^^^^ the trait `Unpin` is not implemented for `[static generator@$DIR/static-not-unpin.rs:11:25: 13:6]`
+   |
+   = note: consider using `Box::pin`
 
 error: aborting due to previous error