Is the following code sound?

#[repr(C)]
union Ptr {
    raw: *mut (),
    atomic: ManuallyDrop<AtomicPtr<()>>,
}

impl Ptr {
    /// Retrieve the pointer stored, no matter if it was stored with `raw` or `atomic`
    fn get_mut(&mut self) -> &mut *mut () {
        // SAFETY: both union members have exactly the same layout, and 
        // accessing an atomic pointer from a mutable reference does not
        // require any synchronization.
        unsafe { &mut self.raw }
    }
}

miri doesn't seem to see any issue with it, and I would say it's sound, but I'd rather ask to be sure on this topic.

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