error[E0605]: non-primitive cast: `u32` as `()`
  --> $DIR/cast-to-nil.rs:2:21
   |
LL | fn main() { let u = 0u32 as (); }
   |                     ^^^^^^^^^^
   |
   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait

error: aborting due to previous error

For more information about this error, try `rustc --explain E0605`.