Auto merge of #451 - longwusha:master, r=jdm

[#411] Add some tests for default-initialized roots

1. At the current stage, I only completed the test code for JSVal; I need to know how to implement several other types of default values in order to complete all the test code.
2. The default value of JSVal's current implementation is undefined, so we should use the is_undefined() method to verify the result.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-mozjs/451)
<!-- Reviewable:end -->

src/rust.rs

@@ -400,11 +400,11 @@ macro_rules! rooted {
     };
     (in($cx:expr) let $name:ident: $type:ty) => {
         let mut __root = $crate::jsapi::Rooted::new_unrooted();
-        let $name = $crate::rust::RootedGuard::new($cx, &mut __root, $type::default());
+        let $name = $crate::rust::RootedGuard::new($cx, &mut __root, <$type as $crate::rust::GCMethods>::initial());
     };
     (in($cx:expr) let mut $name:ident: $type:ty) => {
         let mut __root = $crate::jsapi::Rooted::new_unrooted();
-        let mut $name = $crate::rust::RootedGuard::new($cx, &mut __root, $type::default());
+        let mut $name = $crate::rust::RootedGuard::new($cx, &mut __root, <$type as $crate::rust::GCMethods>::initial());
     };
 }
 

tests/rooting.rs

@@ -21,6 +21,8 @@ use mozjs::jsapi::JSPROP_ENUMERATE;
 use mozjs::jsapi::JS_SetGCZeal;
 use mozjs::jsapi::OnNewGlobalHookOption;
 use mozjs::jsapi::Value;
+use mozjs::jsapi::{JSObject, JSString, JSFunction};
+use mozjs::jsval::JSVal;
 use mozjs::rust::{Runtime, SIMPLE_GLOBAL_CLASS, define_methods};
 use std::ptr;
 
@@ -45,6 +47,18 @@ fn rooting() {
                                                               &CLASS as *const _,
                                                               prototype_proto.handle().into()));
         define_methods(cx, proto.handle(), METHODS).unwrap();
+
+        rooted!(in(cx) let root : JSVal);
+        assert_eq!(root.get().is_undefined(), true);
+
+        rooted!(in(cx) let root : *mut JSObject);
+        assert_eq!(root.get().is_null(), true);
+
+        rooted!(in(cx) let root : *mut JSString);
+        assert_eq!(root.get().is_null(), true);
+
+        rooted!(in(cx) let root : *mut JSFunction);
+        assert_eq!(root.get().is_null(), true);
     }
 }