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

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Message-ID: <6f4e1d68-f828-8990-4859-8ab24907fa46@proton.me>
Date:   Sun, 25 Jun 2023 13:07:05 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Björn Roy Baron <bjorn3_gh@...tonmail.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Andreas Hindborg <nmi@...aspace.dk>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Asahi Lina <lina@...hilina.net>
Subject: Re: [PATCH 5/7] rust: init: add `..Zeroable::zeroed()` syntax for zeroing all missing fields

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