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`).