Attempted to cast to/from a pointer with an unknown kind.
Erroneous code examples:
```compile_fail,E0641
let b = 0 as *const _; // error
```
Must give information for type of pointer that is being cast from/to if the
type cannot be inferred.
```
// Creating a pointer from reference: type can be inferred
let a = &(String::from("Hello world!")) as *const _; // Ok
let b = 0 as *const i32; // Ok
let c: *const i32 = 0 as *const _; // Ok
```