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 rust-lang/rust@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

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