Make the intention of the miri test more clear

Author: adwinwhite

tests/fail/uninit/padding-enum.rs

@@ -18,9 +18,10 @@ fn main() {
 
         // Turns out the discriminant is (currently) stored
         // in the 1st pointer, so the second half is padding.
-        let c = (&p as *const mem::MaybeUninit<E>).byte_add(mem::size_of::<&'static ()>()) as *const u8;
+        let c = &p as *const _ as *const u8;
+        let padding_offset = mem::size_of::<&'static ()>();
         // Read a padding byte.
-        let _val = *c.add(0);
+        let _val = *c.add(padding_offset);
         //~^ERROR: uninitialized
     }
 }

tests/fail/uninit/padding-enum.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> tests/fail/uninit/padding-enum.rs:LL:CC
    |
-LL |         let _val = *c.add(0);
-   |                    ^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+LL |         let _val = *c.add(padding_offset);
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information