Add __type to Error's userdata metatable.

Close #585

src/util/error.rs

@@ -349,7 +349,11 @@ pub(crate) unsafe fn init_error_registry(state: *mut ffi::lua_State) -> Result<(
         state,
         Some(|state| {
             ffi::lua_pushcfunction(state, error_tostring);
-            rawset_field(state, -2, "__tostring")
+            ffi::lua_setfield(state, -2, cstr!("__tostring"));
+
+            // This is mostly for Luau typeof() function
+            ffi::lua_pushstring(state, cstr!("error"));
+            ffi::lua_setfield(state, -2, cstr!("__type"));
         }),
     )?;
 

src/util/userdata.rs

@@ -47,7 +47,7 @@ pub(crate) unsafe fn get_internal_metatable<T: TypeKey>(state: *mut ffi::lua_Sta
 // Uses 6 stack spaces and calls checkstack.
 pub(crate) unsafe fn init_internal_metatable<T: TypeKey>(
     state: *mut ffi::lua_State,
-    customize_fn: Option<fn(*mut ffi::lua_State) -> Result<()>>,
+    customize_fn: Option<fn(*mut ffi::lua_State)>,
 ) -> Result<()> {
     check_stack(state, 6)?;
 
@@ -62,11 +62,11 @@ pub(crate) unsafe fn init_internal_metatable<T: TypeKey>(
     ffi::lua_pushboolean(state, 0);
     rawset_field(state, -2, "__metatable")?;
 
-    if let Some(f) = customize_fn {
-        f(state)?;
-    }
-
     protect_lua!(state, 1, 0, |state| {
+        if let Some(f) = customize_fn {
+            f(state);
+        }
+
         ffi::lua_rawsetp(state, ffi::LUA_REGISTRYINDEX, T::type_key());
     })?;
 

tests/luau.rs

@@ -436,5 +436,16 @@ fn test_loadstring() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_typeof_error() -> Result<()> {
+    let lua = Lua::new();
+
+    let err = Error::runtime("just a test error");
+    let res = lua.load("return typeof(...)").call::<String>(err)?;
+    assert_eq!(res, "error");
+
+    Ok(())
+}
+
 #[path = "luau/require.rs"]
 mod require;