linux-kernel - Re: [PATCH v4] rust: kernel: introduce `unsafe_precondition

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Message-ID: <aJuxfHaslGAlBJTR@fedora>
Date: Wed, 13 Aug 2025 02:56:20 +0530
From: Ritvik Gupta <ritvikfoss@...il.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v4] rust: kernel: introduce `unsafe_precondition_assert!`
 macro

> +/// # Examples
> +///
> +/// ```no_run
> +/// # use kernel::unsafe_precondition_assert;
> +/// # use kernel::cpu::{nr_cpu_ids, CpuId};
> +/// /// Creates a [`CpuId`] from the given `id` without bound checks.
> +/// ///
> +/// /// # Safety
> +/// ///
> +/// /// The caller must ensure that `id` is a valid CPU ID (i.e, `0 <= id < nr_cpu_ids()`).
> +/// unsafe fn new_cpu_id_unchecked(id: i32) -> CpuId {
> +///     let max_cpus = nr_cpu_ids();
> +///
> +///     unsafe_precondition_assert!(id >= 0, "id ({}) must be positive", id);
> +///
> +///     unsafe_precondition_assert!(
> +///         id < max_cpus, "id ({}) must be less than total CPUs ({})", id, max_cpus
> +///     );
> +///
> +///     CpuId(id)
> +/// }
> +/// ```

The example I've included is incorrect because `CpuId` inner field is private,
so it can't be constructed this way.

`CpuId::from_i32_unchecked()` exists, using it here would make the example a
trivial wrapper around that method, that would be unnecessary and confusing.
Also, I wasn't sure about the doctest config when I wrote it.

I'll send v5 with correct and much better example.
Plz ignore it. Sorry for the noise.