rust: static_assert: add `static_assert!` macro

Add the `static_assert!` macro, which is a compile-time assert, similar
to the C11 `_Static_assert` and C++11 `static_assert` declarations [1,2].
Do so in a new module, called `static_assert`.

For instance:

    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);

Link: https://en.cppreference.com/w/c/language/_Static_assert [1]
Link: https://en.cppreference.com/w/cpp/language/static_assert [2]
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/lib.rs

@@ -26,6 +26,7 @@
 pub mod error;
 pub mod prelude;
 pub mod print;
+mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
 pub mod str;

rust/kernel/prelude.rs

@@ -19,6 +19,8 @@
 
 pub use super::{dbg, pr_alert, pr_crit, pr_debug, pr_emerg, pr_err, pr_info, pr_notice, pr_warn};
 
+pub use super::static_assert;
+
 pub use super::error::{code::*, Error, Result};
 
 pub use super::{str::CStr, ThisModule};

rust/kernel/static_assert.rs

+// SPDX-License-Identifier: GPL-2.0
+
+//! Static assert.
+
+/// Static assert (i.e. compile-time assert).
+///
+/// Similar to C11 [`_Static_assert`] and C++11 [`static_assert`].
+///
+/// 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);
+/// ```
+#[macro_export]
+macro_rules! static_assert {
+    ($condition:expr) => {
+        const _: () = core::assert!($condition);
+    };
+}