error[E0606]: cannot cast `usize` to a pointer that is wide
  --> $DIR/cast-macro-lhs.rs:8:23
   |
LL |     let x = foo!() as *const [u8];
   |             ------    ^^^^^^^^^^^ creating a `*const [u8]` requires both an address and a length
   |             |
   |             consider casting this expression to `*const ()`, then using `core::ptr::from_raw_parts`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0606`.