Re: [PATCH] rust: add `assert_sync` function

["Benno Lossin" <lossin@kernel.org>]

On Sat Jun 7, 2025 at 3:02 PM CEST, Christian Schrefl wrote:
> Adds a new file `compile_assert.rs` for asserts during compile time and
> add the `assert_sync` function to this file.
>
> This will be used in `miscdevice` to avoid regression in case a `: Send`
> bound falsely gets dropped in the future.
>
> Suggested-by: Benno Lossin <lossin@kernel.org>
> Link: https://lore.kernel.org/rust-for-linux/20250530-b4-rust_miscdevice_registrationdata-v4-0-d313aafd7e59@gmail.com/T/#mdf3328834ce1d136daf836c9e089b5a8627a6d53
> Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>

Thanks!

> ---
> For now I've only added the function.
>
> Some things that might make sense to do as well:
> - Move `static_assert` into `compile_assert.rs`.

Sounds reasonable.

> - Add `assert_sync` to prelude.

I don't think we need to do that. At least not yet.

> - Add `assert_send` as well.

Sounds like a good idea.

> - Use these asserts in various places around the kernel. (I'm not sure
> where it would make sense)
> ---
>  rust/kernel/compile_assert.rs | 24 ++++++++++++++++++++++++
>  rust/kernel/lib.rs            |  1 +
>  2 files changed, 25 insertions(+)
>
> diff --git a/rust/kernel/compile_assert.rs b/rust/kernel/compile_assert.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..2a99de1ba919dc3952d7a1585869567a44106b44
> --- /dev/null
> +++ b/rust/kernel/compile_assert.rs
> @@ -0,0 +1,24 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Compile-time asserts.
> +
> +/// Asserts that the given type is [`Sync`]. This check is done at compile time and does nothing
> +/// at runtime.
> +///
> +/// Note that this is only intended to avoid regressions and for sanity checks.
> +///
> +/// # Examples
> +/// ```
> +/// # use kernel::compile_assert::assert_sync;
> +/// # use kernel::types::NotThreadSafe;
> +///
> +///
> +/// // Do the assertion in a const block to make sure it won't be executed at runtime.
> +/// const _:() = {

s/_:()/_: ()/

> +///     assert_sync::<i32>(); // Succeeds because `i32` is Sync
> +///     // assert_sync::<NotThreadSafe>(); // Fails because `NotThreadSafe` is not `Sync`.

Can you split this into two examples and mark the failing one with
`compile_fail`?

We also could provide a macro similar to [1].

[1]: https://docs.rs/static_assertions/latest/static_assertions/

---
Cheers,
Benno

> +/// };
> +///
> +/// ```
> +#[inline(always)]
> +pub const fn assert_sync<T: ?Sized + Sync>() {}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 7e227b52b4d8ff23d39d5cc9ad3ab57132c448d0..e1630e5079b2436eda6f8b71225bd5371af337b4 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -56,6 +56,7 @@
>  pub mod block;
>  #[doc(hidden)]
>  pub mod build_assert;
> +pub mod compile_assert;
>  pub mod cred;
>  pub mod device;
>  pub mod device_id;
>
> ---
> base-commit: 7a17bbc1d952057898cb0739e60665908fbb8c72
> change-id: 20250607-assert_sync-62fddc5a0adc
>
> Best regards,