rust: move `static_assert` into `build_assert`

Conceptually, `static_assert` is also a build-time assertion that occurs
earlier in the pipeline. Consolidate the implementation so that we can use
this as the canonical place to add more useful build-time assertions.

Reviewed-by: Yury Norov <ynorov@nvidia.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260319121653.2975748-2-gary@kernel.org
[ Used kernel vertical style. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 37 insertions, 3 deletions

diff --git a/rust/kernel/build_assert.rs b/rust/kernel/build_assert.rs
index f8124dbc663f..d464494d430a 100644
--- a/rust/kernel/build_assert.rs
+++ b/rust/kernel/build_assert.rs
@@ -1,10 +1,46 @@
 // SPDX-License-Identifier: GPL-2.0
 
-//! Build-time assert.
+//! Various assertions that happen during build-time.
 
 #[doc(hidden)]
 pub use build_error::build_error;
 
+/// Static assert (i.e. compile-time assert).
+///
+/// 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
+/// [`static_assert`]: https://en.cppreference.com/w/cpp/language/static_assert
+/// [RFC 2790]: https://github.com/rust-lang/rfcs/issues/2790
+///
+/// # Examples
+///
+/// ```
+/// static_assert!(42 > 24);
+/// static_assert!(core::mem::size_of::<u8>() == 1);
+///
+/// const X: &[u8] = b"bar";
+/// static_assert!(X[1] == b'a');
+///
+/// const fn f(x: i32) -> i32 {
+///     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 $(,$arg:literal)?) => {
+        const _: () = ::core::assert!($condition $(,$arg)?);
+    };
+}
+
 /// Fails the build if the code path calling `build_error!` can possibly be executed.
 ///
 /// If the macro is executed in const context, `build_error!` will panic.
@@ -74,8 +110,6 @@ macro_rules! build_error {
 ///     assert!(n > 1); // Run-time check
 /// }
 /// ```
-///
-/// [`static_assert!`]: crate::static_assert!
 #[macro_export]
 macro_rules! build_assert {
     ($cond:expr $(,)?) => {{