[PATCH v4] rust: kernel: introduce `unsafe_precondition_assert!` macro

[Ritvik Gupta <ritvikfoss@gmail.com>]

Introduce a new `safety` module containing `unsafe_precondition_assert!`
macro. It is a wrapper around `debug_assert!`, intended for validating
pre-conditions of unsafe function.

When `CONFIG_RUST_DEBUG_ASSERTIONS` flag is enabled, this macro performs
runtime checks to ensure that the preconditions for unsafe function hold.
Otherwise, the macro is a no-op.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1162
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library/topic/.60unsafe_precondition_assert.60.20macro/with/528457452
Signed-off-by: Ritvik Gupta <ritvikfoss@gmail.com>
---

Changes in v4:
 - Change doc example
 - Add `no_run` attribute to the doc example
 - Link to v3: https://lore.kernel.org/rust-for-linux/20250731111234.28602-1-ritvikfoss@gmail.com/

Changes in v3:
 - Change doc example
 - Link to v2: https://lore.kernel.org/all/20250730181420.6979b4f1@eugeo/T/#m9cd35a8fc02a18bd03934c7ecdcffe8667b5fbbd

Changes in v2:
 - Wrap `debug_assert!` internally instead of using `pr_err!` with `assert!` + `cfg!(debug_assertions)
 - Print “unsafe precondition(s) violated” only on assertion failure (no longer always printed)
 - Use `# Safety` section instead of comment in the example
 - Rename module-level doc
 - Link to v1: https://lore.kernel.org/rust-for-linux/20250716045957.39732-1-ritvikfoss@gmail.com/

---
 rust/kernel/lib.rs    |  1 +
 rust/kernel/safety.rs | 48 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)
 create mode 100644 rust/kernel/safety.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index ed53169e795c..5eb301685ce8 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -113,6 +113,7 @@
 pub mod rbtree;
 pub mod regulator;
 pub mod revocable;
+pub mod safety;
 pub mod security;
 pub mod seq_file;
 pub mod sizes;
diff --git a/rust/kernel/safety.rs b/rust/kernel/safety.rs
new file mode 100644
index 000000000000..880307a5fbf4
--- /dev/null
+++ b/rust/kernel/safety.rs
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Safety related APIs.
+
+/// Checks that preconditions of an unsafe function are followed.
+///
+/// The check is enabled at runtime if debug assertions (`CONFIG_RUST_DEBUG_ASSERTIONS`)
+/// are enabled. Otherwise, this macro is no-op.
+///
+/// # 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)
+/// }
+/// ```
+///
+/// # Panics
+///
+/// Panics if the expression is evaluated to `false` at runtime.
+#[macro_export]
+macro_rules! unsafe_precondition_assert {
+    ($cond:expr $(,)?) => {
+        $crate::unsafe_precondition_assert!(@inner $cond, ::core::stringify!($cond))
+    };
+
+    ($cond:expr, $($arg:tt)+) => {
+        $crate::unsafe_precondition_assert!(@inner $cond, ::core::format_args!($($arg)+))
+    };
+
+    (@inner $cond:expr, $msg:expr) => {
+        ::core::debug_assert!($cond, "unsafe precondition(s) violated: {}", $msg) };
+}

base-commit: 37816488247ddddbc3de113c78c83572274b1e2e
-- 
2.50.1