Re: [PATCH 5/7] rust: init: add `..Zeroable::zeroed()` syntax for zeroing all missing fields

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

On 25.06.23 14:56, Björn Roy Baron wrote:
> On Saturday, June 24th, 2023 at 23:14, Benno Lossin <benno.lossin@proton.me> wrote:

>>>> +                        // Ensure that the struct is indeed `Zeroable`.
>>>> +                        is_zeroable(slot);
>>>> +                        // SAFETY:  The type implements `Zeroable` by the check above.
>>>> +                        unsafe { ::core::ptr::write_bytes(slot, 0, 1) };
>>>> +                        $init_zeroed // this will be `()` if set.
>>>
>>> How does this work? Shouldn't there be a ; after $init_zeroed to consume the () value?
>>
>> It is the last expression of a block and since it is `()` it is ok
>> (adding a ; would also be ok, but it is not necessary).
>
> I'm surprised it is considered the last expression of a block. Unlike with {} using $()? will still
> allow variables defined inside this as if they were outside of it. Also I can't reproduce this
> behavior with:
>
>      macro_rules! foo {
>          ($($a:expr)?) => {
>              $($a)?
>              bar();
>          }
>      }
>
>      fn main() {
>          foo!(());
>      }
>
> Is there something I'm missing?
>
> Cheers,
> Björn

Not sure what you mean with "allow variables defined inside this
as if they were outside of it". But note that in the macro `$init_zeroed`
is the last expression of a block. Here is a small example:

```
macro_rules! foo {
     ($($a:expr)?) => {{
         $(
             bar();
             $a
         )?
     }};
}

fn bar() {}

fn main() {
     foo!(());
     foo!();
}
```

it expands to this:
```
fn main() {
     {
         bar();
         ()
     };
     {};
}
```

--
Cheers,
Benno