Attempted to pass an invalid type of variable into a variadic function.

Erroneous code example:

```compile_fail,E0617
extern {
    fn printf(c: *const i8, ...);
}

unsafe {
    printf(::std::ptr::null(), 0f32);
    // error: cannot pass an `f32` to variadic function, cast to `c_double`
}
```

Certain Rust types must be cast before passing them to a variadic function,
because of arcane ABI rules dictated by the C standard. To fix the error,
cast the value to the type specified by the error message (which you may need
to import from `std::os::raw`).

In this case, `c_double` has the same size as `f64` so we can use it directly:

```no_run
# extern {
#     fn printf(c: *const i8, ...);
# }
unsafe {
    printf(::std::ptr::null(), 0f64); // ok!
}
```
