Visibility of Src?

In the below example, is downstream::as_bytes sound?

mod upstream {
    /// Implement this to promise that your type's layout consists
    /// solely of initialized bytes with no library invariants.
    pub unsafe trait POD {}
}

mod downstream {
    use super::*;

    pub fn as_bytes<'t, T>(t: &'t T) -> &'t [T]
    where
        T: upstream::POD,
    {
        use core::{slice, mem::size_of};
        
        unsafe {
            slice::from_raw_parts(t as *const T as *const u8, size_of::<T>())
        }
    }
}

The answer to this question impacts the implementation details of BikeshedIntrinsicFrom.

If yes, then the three aforementioned visibility conditions are sufficient.

If no, there is a fourth condition necessary for safety:

In order to be safe, a well-defined transmutation must also not allow you to:

1. construct instances of a hidden Dst type
2. mutate hidden fields of the Src type
3. construct hidden fields of the Dst type
4. read hidden fields of the Src type