Function cast_for_sized 

unsafe fn cast_for_sized<'a, T, A, R, S>(
    ptr: Ptr<'a, [u8], (A, Aligned, Valid)>,
) -> Ptr<'a, T, (A, Unaligned, Valid)>
where
    T: FromBytes + KnownLayout + ?Sized + TransmuteFromPtr<T, A, Initialized, Valid, IdCast, S>,
    A: Aliasing,
    [u8]: MutationCompatible<T, A, Initialized, Initialized, R>,

Safety

T: Sized and ptr’s referent must have size size_of::<T>().