[PATCH v6] 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 v6:
 - Move variable names into format arguments in the doc example.
 - Move closing brace ('}') to new line in the macro.
 - Link to v5: https://lore.kernel.org/rust-for-linux/20250827060013.6874-1-ritvikfoss@gmail.com/

Changes in v5:
 - Change doc example
 - Use re-exported `kernel::prelude::fmt!` instead of `core::format_args!`
 - Link to v4: https://lore.kernel.org/rust-for-linux/20250808192005.209188-1-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 | 53 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)
 create mode 100644 rust/kernel/safety.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 3dd7bebe7888..40c82d227314 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -124,6 +124,7 @@
 pub mod rbtree;
 pub mod regulator;
 pub mod revocable;
+pub mod safety;
 pub mod scatterlist;
 pub mod security;
 pub mod seq_file;
diff --git a/rust/kernel/safety.rs b/rust/kernel/safety.rs
new file mode 100644
index 000000000000..3ff78cef2e02
--- /dev/null
+++ b/rust/kernel/safety.rs
@@ -0,0 +1,53 @@
+// 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;
+///
+/// struct RawBuffer<T: Copy, const N: usize> {
+///     data: [T; N],
+/// }
+///
+/// impl<T: Copy, const N: usize> RawBuffer<T, N> {
+///     /// # Safety
+///     ///
+///     /// The caller must ensure that `index` is less than `N`
+///     unsafe fn set_unchecked(&mut self, index: usize, value: T) {
+///         unsafe_precondition_assert!(
+///             index < N,
+///             "RawBuffer::set_unchecked requires index ({index}) < N ({N})"
+///         );
+///
+///         // SAFETY: By the safety requirements of this function, `index` is valid
+///         unsafe {
+///             *self.data.get_unchecked_mut(index) = value;
+///         }
+///     }
+/// }
+/// ```
+///
+/// # 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, $crate::prelude::fmt!($($arg)+))
+    };
+
+    (@inner $cond:expr, $msg:expr) => {
+        ::core::debug_assert!($cond, "unsafe precondition(s) violated: {}", $msg)
+    };
+}

base-commit: c746c3b5169831d7fb032a1051d8b45592ae8d78
-- 
2.51.0