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

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Message-ID: <ZKMQW4UGPQZ8yF39@boqun-archlinux>
Date:   Mon, 3 Jul 2023 11:15:55 -0700
From:   Boqun Feng <boqun.feng@...il.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>,
        Gary Guo <gary@...yguo.net>,
        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 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(),
	/// });

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
> 
>