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

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

On 25.07.23 13:57, Alice Ryhl wrote:
> Benno Lossin <benno.lossin@proton.me> writes:
>> 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,
>> }
>> ```
> 
> Yeah. The way that these macros usually expand is by adding `where T:
> Zeroable` to the impl for each generic parameter, and failing
> compilation if that is not enough to ensure that all of the fields are
> `Zeroable`.
> 
> You might want to consider this expansion instead:
> ```
> impl<T: Zeroable> Zeroable for Foo<T> {}
> const _: () = {
>       fn assert_zeroable<T: Zeroable>(arg: &T) {}
>       fn ensure_zeroable<T: Zeroable>(arg: &Foo<T>) {
>           assert_zeroable(&arg.field1);
>           assert_zeroable(&arg.field2);
>       }
> };
> ```

Is there a specific reason you think that I should us references here
instead of the expansion from above (where I just use the types and
not the fields themselves)?

-- 
Cheers,
Benno