`UnsafeCell<T>` and `T` have the same layout so if `T` is `Zeroable`Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
then so should `UnsafeCell<T>` be. This allows using the derive macro
for `Zeroable` on types that contain an `UnsafeCell<T>`.
Since `Opaque<T>` contains a `MaybeUninit<T>`, all bytes zero is a valid
bit pattern for that type.
Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
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