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

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Message-ID: <8lCE3SyChVVb2HphigkwKsxv7etgmS0N3AzcDyYtCEoqcFEKvY-5ILkOrWUr_vnWrvsWrAHXVfwcWARfsiMHC8Yc03sND-PuuK-2z9j4z6I=@protonmail.com>
Date:   Sun, 25 Jun 2023 14:17:44 +0000
From:   Björn Roy Baron <bjorn3_gh@...tonmail.com>
To:     Benno Lossin <benno.lossin@...ton.me>
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 Sunday, June 25th, 2023 at 15:07, Benno Lossin <benno.lossin@...ton.me> wrote:
> 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:

$(let $this = unsafe { ::core::ptr::NonNull::new_unchecked(slot) };)? comes 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.

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