error: pointers cannot be cast to integers during const eval
  --> shouldfail/nonconst_len_expr.rs:17:16
   |
17 |     arr1: [u8; u8_len as *const _ as usize],
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: at compile-time, pointers do not have an integer value
   = note: avoiding this restriction via `transmute`, `union`, or raw pointers leads to compile-time undefined behavior

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  --> shouldfail/nonconst_len_expr.rs:20:16
   |
20 |     arr2: [u8; next_rand()],
   |                ^^^^^^^^^^^

error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
  --> shouldfail/nonconst_len_expr.rs:26:16
   |
26 |     arr4: [u8; std::mem::transmute(u8_len) ]
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
   |
   = note: consult the function's documentation for information on how to avoid undefined behavior