cxx-qt-lib: Add const_assert for asserting at compile-time

This is needed to ensure the Rust-side QMessageLogContext struct has the
expected size.

crates/cxx-qt-lib/src/core/qtlogging.rs

@@ -79,6 +79,10 @@ pub struct QMessageLogContext<'a> {
     _phantom: PhantomData<&'a c_char>,
 }
 
+const_assert!(
+    size_of::<QMessageLogContext>() == (size_of::<i32>() * 2) + (size_of::<*const c_char>() * 3)
+);
+
 impl<'a> QMessageLogContext<'a> {
     pub fn new(
         file: &'a CStr,
@@ -125,5 +129,6 @@ unsafe impl ExternType for QMessageLogContext<'_> {
     type Kind = cxx::kind::Trivial;
 }
 
+use crate::const_assert;
 use crate::core::qtlogging::ffi::category;
 pub use ffi::{qt_message_output, QtMsgType};

crates/cxx-qt-lib/src/lib.rs

@@ -25,3 +25,5 @@ pub use crate::qml::*;
 mod quickcontrols;
 #[cfg(feature = "qt_quickcontrols")]
 pub use crate::quickcontrols::*;
+
+mod util;

crates/cxx-qt-lib/src/util.rs

@@ -0,0 +1,18 @@
+// SPDX-FileCopyrightText: 2025 Klarälvdalens Datakonsult AB, a KDAB Group company <[email protected]>
+// SPDX-FileContributor: Joshua Goins <[email protected]>
+//
+// SPDX-License-Identifier: MIT OR Apache-2.0
+
+/// Asserts that a boolean expression is true at compile time.
+///
+/// See [`core::assert!`] for more information.
+///
+/// ```compile_fail
+/// const_assert!(5 == 4);
+/// ```
+#[macro_export]
+macro_rules! const_assert {
+    ($x:expr $(,)?) => {
+        const _: () = ::core::assert!($x);
+    };
+}