rust: static_assert: add optional message

Add an optional panic message to the `static_assert!` macro.

The panic message doesn't support argument formatting, because the
`assert!` macro only supports formatting in non-const contexts.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: Rust-for-Linux#1149
Signed-off-by: Altan Ozlu <altan@ozlu.eu>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Link: https://lore.kernel.org/r/20250326202520.1176162-2-altan@ozlu.eu
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: altanozlu

rust/kernel/static_assert.rs

@@ -6,6 +6,10 @@
 ///
 /// Similar to C11 [`_Static_assert`] and C++11 [`static_assert`].
 ///
+/// An optional panic message can be supplied after the expression.
+/// Currently only a string literal without formatting is supported
+/// due to constness limitations of the [`assert!`] macro.
+///
 /// The feature may be added to Rust in the future: see [RFC 2790].
 ///
 /// [`_Static_assert`]: https://en.cppreference.com/w/c/language/_Static_assert
@@ -25,10 +29,11 @@
 ///     x + 2
 /// }
 /// static_assert!(f(40) == 42);
+/// static_assert!(f(40) == 42, "f(x) must add 2 to the given input.");
 /// ```
 #[macro_export]
 macro_rules! static_assert {
-    ($condition:expr) => {
-        const _: () = core::assert!($condition);
+    ($condition:expr $(,$arg:literal)?) => {
+        const _: () = core::assert!($condition $(,$arg)?);
     };
 }