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Date:   Sun, 25 Jun 2023 13:07:05 +0000
To:     =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <nmi@metaspace.dk>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Asahi Lina <lina@asahilina.net>
Subject: Re: [PATCH 5/7] rust: init: add `..Zeroable::zeroed()` syntax for zeroing all missing fields
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On 25.06.23 14:56, Bj=C3=B6rn 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 consum=
e 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 wit=
h {} 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)?) =3D> {
>              $($a)?
>              bar();
>          }
>      }
>
>      fn main() {
>          foo!(());
>      }
>
> Is there something I'm missing?
>
> Cheers,
> Bj=C3=B6rn

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)?) =3D> {{
         $(
             bar();
             $a
         )?
     }};
}

fn bar() {}

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

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

--
Cheers,
Benno