error[E0605]: non-primitive cast: `NonNullary` as `isize`
  --> $DIR/cast-enum-to-primitive-error.rs:24:15
   |
LL |     let val = v as isize;
   |               ^^^^^^^^^^ an `as` expression can be used to convert enum types to numeric types only if the enum type is unit-only or field-less
   |
   = note: see https://doc.rust-lang.org/reference/items/enumerations.html#casting for more information
help: consider using the `From` trait instead
   |
LL -     let val = v as isize;
LL +     let val = isize::from(v);
   |

error: aborting due to 1 previous error

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