fix: utils: add missing warn.rs

lib/utils/src/warn.rs

@@ -0,0 +1,15 @@
+// Taken from https://internals.rust-lang.org/t/pre-rfc-add-compile-warning-macro/9370/7
+
+#[macro_export]
+macro_rules! compile_warning {
+    ($name:ident, $message:expr $(,)*) => {
+        mod $name {
+            #[must_use = $message]
+            struct compile_warning;
+            #[allow(dead_code)]
+            fn trigger_warning() {
+                compile_warning;
+            }
+        }
+    };
+}