[PATCH v2 1/2] rust: make unsafe_precondition_assert! const compatible

[JX]

The no-message form of unsafe_precondition_assert! routes the
condition through the formatting machinery. This prevents the macro from
being used in const unsafe functions.

Use a fixed diagnostic literal instead. This allows the no-message form
in const contexts without interpreting tokens from the condition as
format string syntax. Keep the custom-message form unchanged since it
intentionally supports runtime formatting.

Suggested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Link: https://github.com/Rust-for-Linux/linux/issues/1232
Signed-off-by: JX <1239989762@xxxxxx>
---
 rust/kernel/safety.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/safety.rs b/rust/kernel/safety.rs
index c1c6bd0fa2..f3c498c242 100644
--- a/rust/kernel/safety.rs
+++ b/rust/kernel/safety.rs
@@ -7,6 +7,15 @@
 /// The check is enabled at runtime if debug assertions (`CONFIG_RUST_DEBUG_ASSERTIONS`)
 /// are enabled. Otherwise, this macro is a no-op.
 ///
+/// The form without a custom message can be used in const contexts.
+///
+/// ```
+/// # use kernel::unsafe_precondition_assert;
+/// const fn check(value: usize) {
+///     unsafe_precondition_assert!({ value < 4 });
+/// }
+/// ```
+///
 /// # Examples
 ///
 /// ```no_run
@@ -40,7 +49,7 @@
 #[macro_export]
 macro_rules! unsafe_precondition_assert {
     ($cond:expr $(,)?) => {
-        $crate::unsafe_precondition_assert!(@inner $cond, ::core::stringify!($cond))
+        ::core::debug_assert!($cond, "unsafe precondition violated")
     };
 
     ($cond:expr, $($arg:tt)+) => {
-- 
2.54.0