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

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Message-ID: <ada8307d-5177-2094-683f-bce619f1ea44@ryhl.io>
Date:   Thu, 30 Mar 2023 17:00:48 +0200
From:   Alice Ryhl <alice@...l.io>
To:     y86-dev@...tonmail.com
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 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?

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

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