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Date:   Sun, 25 Jun 2023 14:17:44 +0000
To:     Benno Lossin <benno.lossin@proton.me>
From:   =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
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 Sunday, June 25th, 2023 at 15:07, Benno Lossin <benno.lossin@proton.me> =
wrote:
> On 25.06.23 14:56, Bj=C3=B6rn Roy Baron wrote:
> > On Saturday, June 24th, 2023 at 23:14, Benno Lossin <benno.lossin@proto=
n.me> wrote:
>=20
> >>>> +                        // Ensure that the struct is indeed `Zeroab=
le`.
> >>>> +                        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 cons=
ume 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 w=
ith {} 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
>=20
> 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:

$(let $this =3D unsafe { ::core::ptr::NonNull::new_unchecked(slot) };)? com=
es after
this code in the same block that contains struct __InitOk;. And after that =
another
$crate::__init_internal!() invocation. That is why I don't get that this is=
 allowed
at all.

>=20
> ```
> macro_rules! foo {
>      ($($a:expr)?) =3D> {{
>          $(
>              bar();
>              $a
>          )?
>      }};
> }
>=20
> fn bar() {}
>=20
> fn main() {
>      foo!(());
>      foo!();
> }
> ```
>=20
> it expands to this:
> ```
> fn main() {
>      {
>          bar();
>          ()
>      };
>      {};
> }
> ```
>=20
> --
> Cheers,
> Benno
>