Implement fmt::Pointer for pointers to unsized types

This allows printing pointers to unsized types with the {:p} formatting
directive. The following impls are extended to unsized types:
 - impl<'a, T: ?Sized> Pointer for &'a T
 - impl<'a, T: ?Sized> Pointer for &'a mut T
 - impl<T: ?Sized> Pointer for *const T
 - impl<T: ?Sized> Pointer for *mut T
 - impl<T: ?Sized> fmt::Pointer for Box<T>
 - impl<T: ?Sized> fmt::Pointer for Rc<T>
 - impl<T: ?Sized> fmt::Pointer for Arc<T>

src/libcoretest/fmt/mod.rs

@@ -20,3 +20,11 @@ fn test_format_flags() {
 
     assert_eq!(format!("{: >3}", 'a'), "  a");
 }
+
+#[test]
+fn test_pointer_formats_data_pointer() {
+    let b: &[u8] = b"";
+    let s: &str = "";
+    assert_eq!(format!("{:p}", s), format!("{:p}", s.as_ptr()));
+    assert_eq!(format!("{:p}", b), format!("{:p}", b.as_ptr()));
+}