[PATCH] rust: assertions: add const_assert

[Paolo Bonzini <pbonzini@redhat.com>]

Add a new assertion that is similar to "const { assert!(...) }" but can be used
outside functions and with older versions of Rust.  Similar in concept (but
not in code) to the homonymous macro of the "static_assertions" crate.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/qemu-api/src/assertions.rs | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/rust/qemu-api/src/assertions.rs b/rust/qemu-api/src/assertions.rs
index 104dec39774..1fa3955d867 100644
--- a/rust/qemu-api/src/assertions.rs
+++ b/rust/qemu-api/src/assertions.rs
@@ -120,3 +120,25 @@ macro_rules! assert_match {
         );
     };
 }
+
+/// Assert at compile time that an expression is true.  This is similar
+/// to `const { assert!(...); }` but it works outside functions, as well as
+/// on versions of Rust before 1.79.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::const_assert;
+/// const_assert!("abc".len() == 3);
+/// ```
+///
+/// ```compile_fail
+/// # use qemu_api::const_assert;
+/// const_assert!("abc".len() == 2); // does not compile
+/// ```
+#[macro_export]
+macro_rules! const_assert {
+    ($x:expr) => {
+        const _: [(); { const VALUE__: bool = $x; VALUE__ as usize }] = [()];
+    };
+}
-- 
2.48.1