iter 23.1.2 fixup: align workspace-root idiom + add collision test for ReservedModuleName

crates/ailang-core/src/workspace.rs

@@ -900,8 +900,9 @@ mod tests {
         // it. Loading any well-formed workspace must result in
         // `ws.modules["prelude"]` being present with the Ordering
         // type def.
+        let manifest_dir = env!("CARGO_MANIFEST_DIR");
         let workspace_root =
-            std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../..");
+            Path::new(manifest_dir).parent().unwrap().parent().unwrap();
         let entry = workspace_root.join("examples").join("ws_main.ail.json");
 
         let ws = load_workspace(&entry).expect("load workspace");
@@ -921,6 +922,27 @@ mod tests {
         );
     }
 
+    #[test]
+    fn user_module_named_prelude_is_rejected() {
+        // Iter 23.1: the loader auto-injects a module named "prelude",
+        // so a user module that also claims that name would collide
+        // silently. The collision is caught explicitly with
+        // `ReservedModuleName`.
+        let dir = tmp_dir("prelude_collision");
+        write_module(&dir, "prelude", &[]);
+        let entry = dir.join("prelude.ail.json");
+
+        let err = load_workspace(&entry).expect_err("must reject user prelude");
+        match err {
+            WorkspaceLoadError::ReservedModuleName { name } => {
+                assert_eq!(name, "prelude");
+            }
+            other => panic!(
+                "expected ReservedModuleName, got: {other:?}"
+            ),
+        }
+    }
+
     #[test]
     fn detects_import_cycle() {
         let dir = tmp_dir("cycle");