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

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Message-ID: <20230705184836.363c8c8b.gary@garyguo.net>
Date:   Wed, 5 Jul 2023 18:48:36 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Benno Lossin <benno.lossin@...ton.me>,
        Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        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 Mon, 3 Jul 2023 11:15:55 -0700
Boqun Feng <boqun.feng@...il.com> wrote:

> On Sat, Jun 24, 2023 at 09:25:19AM +0000, Benno Lossin wrote:
> [...]
> (this is `init_slot`)
> > @@ -1064,7 +1152,7 @@ macro_rules! __init_internal {
> >          @data($data:ident),
> >          @slot($slot:ident),
> >          @guards($($guards:ident,)*),
> > -        @munch_fields($(,)?),
> > +        @munch_fields($(..Zeroable::zeroed())? $(,)?),  
> 
> since you append an unconditional comma ',' to init_slot and
> make_initializer when "calling" them in with_update_parsed, shouldn't
> this be:
> 
> +        @munch_fields($(..Zeroable::zeroed(),)? $(,)?),
> 
> , and..
> 
> >      ) => {
> >          // Endpoint of munching, no fields are left. If execution reaches this point, all fields
> >          // have been initialized. Therefore we can now dismiss the guards by forgetting them.
> > @@ -1157,6 +1245,30 @@ macro_rules! __init_internal {
> >              @munch_fields($($rest)*),
> >          );
> >      };
> > +    (make_initializer:
> > +        @slot($slot:ident),
> > +        @type_name($t:ident),
> > +        @munch_fields(..Zeroable::zeroed() $(,)?),  
> 
> this should be:
> 
> +        @munch_fields(..Zeroable::zeroed() , $(,)?),
> 
> Otherwise the example before `pin_init!()` wouldn't compile:
> 
> 	/// pin_init!(Buf {
> 	///     buf: [1; 64],
> 	///     ..Zeroable::zeroed(),
> 	/// });

Comma is not allowed after base struct.

> 
> Regards,
> Boqun
> 
> > +        @acc($($acc:tt)*),
> > +    ) => {
> > +        // Endpoint, nothing more to munch, create the initializer. Since the users specified
> > +        // `..Zeroable::zeroed()`, the slot will already have been zeroed and all field that have
> > +        // not been overwritten are thus zero and initialized. We still check that all fields are
> > +        // actually accessible by using the struct update syntax ourselves.
> > +        // Since we are in the `if false` branch, this will never get executed. We abuse `slot` to
> > +        // get the correct type inference here:
> > +        unsafe {
> > +            let mut zeroed = ::core::mem::zeroed();
> > +            // We have to use type inference her to make zeroed have the correct type. This does
> > +            // not get executed, so it has no effect.
> > +            ::core::ptr::write($slot, zeroed);
> > +            zeroed = ::core::mem::zeroed();
> > +            ::core::ptr::write($slot, $t {
> > +                $($acc)*
> > +                ..zeroed
> > +            });
> > +        }
> > +    };
> >      (make_initializer:
> >          @slot($slot:ident),
> >          @type_name($t:ident),
> > --
> > 2.41.0
> > 
> >