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

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Date:   Wed, 05 Jul 2023 21:44:16 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Gary Guo <gary@...yguo.net>
Cc:     Boqun Feng <boqun.feng@...il.com>, 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 05.07.23 19:48, Gary Guo wrote:
> 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.

Yes this is a mistake in the example, will fix.

-- 
Cheers,
Benno

> 
>>
>> 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
>>>
>>>
>