Re: [PATCH v2 09/12] rust: init: implement Zeroable for Opaque<T>

[Benno Lossin <benno.lossin@proton.me>]

On 20.07.23 15:34, Alice Ryhl wrote:
> Benno Lossin <benno.lossin@proton.me> writes:
>> 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@proton.me>
>> ---
>>   ///
>>   /// This is meant to be used with FFI objects that are never interpreted by Rust code.
>>   #[repr(transparent)]
>> +#[derive(Zeroable)]
>>   pub struct Opaque<T> {
>>       value: UnsafeCell<MaybeUninit<T>>,
>>       _pin: PhantomPinned,
>>   }
> 
> Does this actually work? I don't think we implement Zeroable for
> UnsafeCell.

Good catch, this does compile, but only because the current
implementation of the derive expands to (modulo correct paths):
```
impl<T> Zeroable for Opaque<T>
where
     UnsafeCell<MaybeUninit<T>>: Zeroable,
     PhantomPinned: Zeroable,
{}
```
This implementation is of course useless, since `UnsafeCell` is never
`Zeroable` at the moment. We could of course implement that and then this
should work, but the question is if this is actually the desired output in
general. I thought before that this would be a good idea, but I forgot that
if the bounds are never satisfied it would silently compile.

Do you think that we should have this expanded output instead?
```
impl<T: Zeroable> Zeroable for Foo<T> {}
const _: () = {
     fn assert_zeroable<T: Zeroable>() {}
     fn ensure_zeroable<T: Zeroable>() {
         assert_zeroable::<Field1>();
         assert_zeroable::<Field2>();
     }
};
```
If the input was
```
#[derive(Zeroable)]
struct Foo<T> {
     field1: Field1,
     field2: Field2,
}
```

> I suggest you instead add Opaque to the `impl_zeroable!` macro in
> `rust/kernel/init.rs`.

We would have to do this when using the alternative approach from
above, since we do not want the `Zeroable` bound on `T` for `Opaque`.
I wanted to avoid the `SAFETY` comment, since that is needed for the
`impl_zeroable` macro (as it has an unsafe block inside).

-- 
Cheers,
Benno