miri: don't use int2ptr casts for invalid pointers (#553)

tests/test_bytes_vec_alloc.rs

@@ -45,7 +45,7 @@ impl Ledger {
             if entry_ptr
                 .compare_exchange(
                     ptr,
-                    usize::MAX as *mut u8,
+                    invalid_ptr(usize::MAX),
                     Ordering::SeqCst,
                     Ordering::SeqCst,
                 )
@@ -103,3 +103,12 @@ fn test_bytes_truncate_and_advance() {
     bytes.advance(1);
     drop(bytes);
 }
+
+/// Returns a dangling pointer with the given address. This is used to store
+/// integer data in pointer fields.
+#[inline]
+fn invalid_ptr<T>(addr: usize) -> *mut T {
+    let ptr = std::ptr::null_mut::<u8>().wrapping_add(addr);
+    debug_assert_eq!(ptr as usize, addr);
+    ptr.cast::<T>()
+}