it: test subsurface positioning

src/it/test_utils/test_object_ext.rs

@@ -0,0 +1,17 @@
+use {
+    crate::{it::test_error::TestError, object::Object},
+    std::rc::Rc,
+};
+
+pub trait TestObjectExt {
+    fn downcast<T: 'static>(self: Rc<Self>) -> Result<Rc<T>, TestError>;
+}
+
+impl TestObjectExt for dyn Object {
+    fn downcast<T: 'static>(self: Rc<Self>) -> Result<Rc<T>, TestError> {
+        match self.into_any().downcast() {
+            Ok(t) => Ok(t),
+            _ => bail!("Object has an incompatible type id"),
+        }
+    }
+}