Hi,

Assuming we have 2 simple wrappers on some type (new type pattern):

Full example here: demo

struct W1<T>(T);
struct W2<T>(T);

I assume converting from W1<_> to W2<_> is zero cost?

pub fn w1_to_w2<T>(a: W1<T>) -> W2<T> {
    W2(a.0)
}

But my problem is converting Arc<W1<T>> to Arc<W2<T>> because I also want it to be safe and zero-cost, meaning - no extra allocation, no extra copy/clone.

From the documentation of Arc I found the solution (in unsafe RUST) with using of from_raw / into_raw:

pub fn arc_w1_to_arc_w2<T>(a: Arc<W1<T>>) -> Arc<W2<T>> 
{
    let a_ptr = Arc::into_raw(a);
    unsafe {
        let a = Arc::from_raw(a_ptr as *const W2<T>);
        return a;
    }
}

Q1: Is this code above safe?

Do I need to mark these wrappers W1/W2 with some #[repr(C)] to make sure size/allignment is identical for the same T?

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