Re: [PATCH v1 1/2] rust: Add support for calling a function exactly once

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Nov 05, 2025 at 02:47:30PM +0900, FUJITA Tomonori wrote:
> Add the Rust equivalent of the kernel's `DO_ONCE_LITE` macro. While it
> would be possible to implement the feature entirely as a Rust macro,
> the functionality that can be implemented as regular functions has
> been extracted and implemented as the `OnceLite` struct for better
> code maintainability.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

The OnceLite implementation LGTM, but I have suggestions for the macro.

> +/// Run the given function exactly once.
> +///
> +/// This is equivalent to the kernel's `DO_ONCE_LITE` macro.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// kernel::do_once_lite!(|| {
> +///     kernel::pr_info!("This will be printed only once\n");
> +/// });
> +/// ```
> +#[macro_export]
> +macro_rules! do_once_lite {
> +    ($e:expr) => {{
> +        #[link_section = ".data..once"]
> +        static ONCE: $crate::once_lite::OnceLite = $crate::once_lite::OnceLite::new();
> +        ONCE.call_once($e)
> +    }};
> +}

Wouldn't it be nicer if the macro syntax looks like this?

	kernel::do_once_lite! {
	    kernel::pr_info!("This will be printed only once\n");
	}

You might implement it like this:

	macro_rules! do_once_lite {
	    { $($e:tt)* } => {{
	        #[link_section = ".data..once"]
	        static ONCE: $crate::once_lite::OnceLite = $crate::once_lite::OnceLite::new();
	        ONCE.call_once(|| {
		    $($e)*
		})
	    }};
	}

Alice