Compare only the data part of fat pointers

Two fat pointers may point to the same data, but with different vtables
(the compiler do not guarantee that vtables are unique).

Such pointers should be considered equal by std::ptr::eq(), so cast them
to thin pointers to compare only their data part.

See <rust-lang#48795>.

Author: rom1v

src/libcore/ptr.rs

@@ -2105,7 +2105,8 @@ impl<T: ?Sized> Eq for *mut T {}
 #[stable(feature = "ptr_eq", since = "1.17.0")]
 #[inline]
 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
-    a == b
+    // cast to thin pointers to ignore the vtable part
+    a as *const () == b as *const ()
 }
 
 // Impls for function pointers