Make Instance.new only work for instances that exist

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Remodel Changelog
 
 ## Unreleased Changes
+* **Breaking:** `Instance.new` now only works for instances that actually exist.
 * Added `Instance:Clone()` for copying instances all over the place, as is Roblox tradition. ([#12](https://github.com/rojo-rbx/remodel/issues/12))
 * Added `DataModel:GetService()` for finding services and creating them if they don't exist, like Roblox does.
 * Improved error messages in preparation for [#7](https://github.com/rojo-rbx/remodel/issues/7) to be fixed upstream.

src/roblox_api/mod.rs

@@ -20,6 +20,13 @@ struct Instance;
 impl UserData for Instance {
     fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
         methods.add_function("new", |context, class_name: String| {
+            if rbx_reflection::get_class_descriptor(&class_name).is_none() {
+                return Err(rlua::Error::external(format!(
+                    "'{}' is not a valid class of Instance.",
+                    class_name,
+                )));
+            }
+
             let master_tree = RemodelContext::get(context)?.master_tree;
             let mut master_handle = master_tree.lock().unwrap();