Fix tyvar_behind_raw_pointer warning

As tracked at rust-lang/rust#46906, checked
with rustc 1.26.0-nightly (9cb18a92a 2018-03-02) here and using patched
mozjs in Servo (rustc 1.25.0-nightly (15a1e2844 2018-01-20)).

Author: Xanewok

src/rust.rs

@@ -1477,7 +1477,7 @@ impl<'a> CapturedJSStack<'a> {
     pub fn as_string(&self, indent: Option<usize>) -> Option<String> {
         unsafe {
             let stack_handle = self.stack.handle();
-            rooted!(in(self.cx) let mut js_string = ptr::null_mut());
+            rooted!(in(self.cx) let mut js_string = ptr::null_mut::<JSString>());
             let string_handle = js_string.handle_mut();
 
             if !IsSavedFrame(stack_handle.get()) {

tests/typedarray.rs

@@ -7,6 +7,7 @@ extern crate mozjs;
 
 use mozjs::jsapi::CompartmentOptions;
 use mozjs::jsapi::JSAutoCompartment;
+use mozjs::jsapi::JSObject;
 use mozjs::jsapi::JS_NewGlobalObject;
 use mozjs::jsapi::OnNewGlobalHookOption;
 use mozjs::jsapi::Type;
@@ -44,7 +45,7 @@ fn typedarray() {
         typedarray!(in(cx) let view: ArrayBufferView = rval.to_object());
         assert_eq!(view.unwrap().get_array_type(), Type::Uint8);
 
-        rooted!(in(cx) let mut rval = ptr::null_mut());
+        rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
         assert!(Uint32Array::create(cx, CreateWith::Slice(&[1, 3, 5]), rval.handle_mut()).is_ok());
 
         typedarray!(in(cx) let array: Uint32Array = rval.get());
@@ -54,11 +55,11 @@ fn typedarray() {
         array.as_mut().unwrap().update(&[2, 4, 6]);
         assert_eq!(array.unwrap().as_slice(), &[2, 4, 6][..]);
 
-        rooted!(in(cx) let rval = ptr::null_mut());
+        rooted!(in(cx) let rval = ptr::null_mut::<JSObject>());
         typedarray!(in(cx) let array: Uint8Array = rval.get());
         assert!(array.is_err());
 
-        rooted!(in(cx) let mut rval = ptr::null_mut());
+        rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
         assert!(Uint32Array::create(cx, CreateWith::Length(5), rval.handle_mut()).is_ok());
 
         typedarray!(in(cx) let array: Uint32Array = rval.get());
@@ -87,7 +88,7 @@ fn typedarray_update_panic() {
         );
 
         let _ac = JSAutoCompartment::new(cx, global.get());
-        rooted!(in(cx) let mut rval = ptr::null_mut());
+        rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
         let _ = Uint32Array::create(cx, CreateWith::Slice(&[1, 2, 3, 4, 5]), rval.handle_mut());
         typedarray!(in(cx) let mut array: Uint32Array = rval.get());
         array.as_mut().unwrap().update(&[0, 2, 4, 6, 8, 10]);