Macro coerce_stackptr

macro_rules! coerce_stackptr {
    ($sp:expr, $ty:ty) => { ... };
}

An implementation of std::ops::CoerceUnsized on stable rust. On nightly, you can convert a StackPtr<T> into a StackPtr<U> if T implements U, with let sp = sp as StackPtr<U>;, but this requires the unstable CoerceUnsized trait. On stable you can do let sp = coerce_stackptr!(sp, U);.