[PATCH v4 1/2] rust: kernel: create `overflow_assert!` macro

[Antonio Hickey <contact@antoniohickey.com>]

This commit creates a macro for overflow assertions, the use of this
macro will avoid local `#ifdef`s by encapsulating the conditional
behavior (like `#[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]`) to the macro.

In addition this macro allows us to document the intent of the assertion
more clearly.

Co-developed-by: Daniel Cote <danielstonecote@gmail.com>
Signed-off-by: Daniel Cote <danielstonecote@gmail.com>
Signed-off-by: Antonio Hickey <contact@antoniohickey.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1159
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/lib.rs             |  1 +
 rust/kernel/overflow_assert.rs | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 rust/kernel/overflow_assert.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 6b4774b2b1c3..e395adb6b293 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -92,6 +92,7 @@
 pub mod of;
 #[cfg(CONFIG_PM_OPP)]
 pub mod opp;
+pub mod overflow_assert;
 pub mod page;
 #[cfg(CONFIG_PCI)]
 pub mod pci;
diff --git a/rust/kernel/overflow_assert.rs b/rust/kernel/overflow_assert.rs
new file mode 100644
index 000000000000..5164339813cb
--- /dev/null
+++ b/rust/kernel/overflow_assert.rs
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Overflow assertion.
+
+/// Verifies at runtime that an expression is within an expected bound.
+///
+/// This macro is only active when `CONFIG_RUST_OVERFLOW_CHECKS` is enabled.
+///
+/// # Examples
+///
+/// ```
+/// overflow_assert!(3 <= 10);
+/// overflow_assert!(5 <= 5);
+///
+/// const X: u8 = 5;
+/// overflow_assert!(X + 3 < 10);
+///
+/// const MAX: i32 = 42;
+/// const fn f(x: i32) -> i32 {
+///     x + 1
+/// }
+/// overflow_assert!(f(40) < MAX);
+/// overflow_assert!(f(40) < MAX, "f(x) must not overflow the max value.");
+/// ```
+#[macro_export]
+macro_rules! overflow_assert {
+    ($cond:expr $(,$arg:literal)?) => {
+        if cfg!(CONFIG_RUST_OVERFLOW_CHECKS) {
+            ::core::assert!($cond $(,$arg)?);
+        }
+    };
+}
-- 
2.50.0