Rollup merge of #103110 - RalfJung:manual-send, r=thomcc

remove redundant Send impl for references

Also explain why the other instance is not redundant, move it next to the trait they are implementing, and out of the redundant module. This seems to go back all the way to 35ca50b, not sure why the module was added.

The instance for `&mut` is the default instance we get anyway, and we don't have anything similar for `Sync`, so IMO we should be consistent and not have the redundant instance here, either.

Author: matthiaskrgr

library/core/src/marker.rs

@@ -44,6 +44,12 @@ impl<T: ?Sized> !Send for *const T {}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> !Send for *mut T {}
 
+// Most instances arise automatically, but this instance is needed to link up `T: Sync` with
+// `&T: Send` (and it also removes the unsound default instance `T Send` -> `&T: Send` that would
+// otherwise exist).
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<T: Sync + ?Sized> Send for &T {}
+
 /// Types with a constant size known at compile time.
 ///
 /// All type parameters have an implicit bound of `Sized`. The special syntax
@@ -674,13 +680,6 @@ impl<T: ?Sized> StructuralPartialEq for PhantomData<T> {}
 #[unstable(feature = "structural_match", issue = "31434")]
 impl<T: ?Sized> StructuralEq for PhantomData<T> {}
 
-mod impls {
-    #[stable(feature = "rust1", since = "1.0.0")]
-    unsafe impl<T: Sync + ?Sized> Send for &T {}
-    #[stable(feature = "rust1", since = "1.0.0")]
-    unsafe impl<T: Send + ?Sized> Send for &mut T {}
-}
-
 /// Compiler-internal trait used to indicate the type of enum discriminants.
 ///
 /// This trait is automatically implemented for every type and does not add any

src/test/ui/async-await/async-fn-nonsend.stderr

@@ -27,7 +27,7 @@ error: future cannot be sent between threads safely
 LL |     assert_send(non_sync_with_method_call());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
    |
-   = help: the trait `Send` is not implemented for `dyn std::fmt::Write`
+   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `dyn std::fmt::Write`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:46:14
    |