linux-kernel - Re: [PATCH v3 08/13] rust: init: add `stack_pin

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Message-ID: <4b048bfc-e4fe-8c2f-ebfe-9b6a410cd8b8@protonmail.com>
Date:   Thu, 30 Mar 2023 15:19:51 +0000
From:   Benno Lossin <y86-dev@...tonmail.com>
To:     Alice Ryhl <alice@...l.io>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Gary Guo <gary@...yguo.net>
Subject: Re: [PATCH v3 08/13] rust: init: add `stack_pin_init!` macro

On 30.03.23 17:00, Alice Ryhl wrote:
> On 3/30/23 00:33, y86-dev@...tonmail.com wrote:
>> From: Benno Lossin <y86-dev@...tonmail.com>
>>
>> The `stack_pin_init!` macro allows pin-initializing a value on the
>> stack. It accepts a `impl PinInit<T, E>` to initialize a `T`. It allows
>> propagating any errors via `?` or handling it normally via `match`.
>>
>> Signed-off-by: Benno Lossin <y86-dev@...tonmail.com>
>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>
>> ---
>> +#[macro_export]
>> +macro_rules! stack_pin_init {
>> +    (let $var:ident $(: $t:ty)? = $val:expr) => {
>> +        let mut $var = $crate::init::__internal::StackInit$(::<$t>)?::uninit();
>> +        let mut $var = {
>> +            let val = $val;
>> +            unsafe { $crate::init::__internal::StackInit::init(&mut $var, val) }
>> +        };
>> +    };
>> +    (let $var:ident $(: $t:ty)? =? $val:expr) => {
>> +        let mut $var = $crate::init::__internal::StackInit$(::<$t>)?::uninit();
>> +        let mut $var = {
>> +            let val = $val;
>> +            unsafe { $crate::init::__internal::StackInit::init(&mut $var, val)? }
>> +        };
>> +    };
>> +}
>
> This will be inconvenient to use if the initializer is infallible and is
> used inside an infallible function. However, I'm not sure what a better
> alternative would be. Perhaps we should have three variants?

That could be an option, any ideas for the syntax though? Or should it
be a different macro like `stack_pin_init!` and `try_stack_pin_init!`?

> Also, maybe a `<-` rather than `=` would be more consistent?

That is sadly not possible, since `<-` is not allowed after `ty` fragments.

> Anyway, I don't think this should block the PR. We can revisit it later
> if it becomes a problem.

Sure.

--
Cheers,
Benno