Mark JSTraceable and its method as unsafe

components/jstraceable_derive/lib.rs

@@ -41,10 +41,11 @@ fn expand_string(input: &str) -> String {
     }
 
     let tokens = quote! {
-        impl #impl_generics ::dom::bindings::trace::JSTraceable for #name #ty_generics #where_clause {
+        #[allow(unsafe_code)]
+        unsafe impl #impl_generics ::dom::bindings::trace::JSTraceable for #name #ty_generics #where_clause {
             #[inline]
             #[allow(unused_variables, unused_imports)]
-            fn trace(&self, tracer: *mut ::js::jsapi::JSTracer) {
+            unsafe fn trace(&self, tracer: *mut ::js::jsapi::JSTracer) {
                 use ::dom::bindings::trace::JSTraceable;
                 match *self {
                     #match_body

components/script/docs/JS-Servos-only-GC.md

@@ -122,8 +122,8 @@ which has an area, and the trait provides a way to get that object's area.
 Now let's look at the `JSTraceable` trait, which we use for tracing:
 
 ```rust
-pub trait JSTraceable {
-    fn trace(&self, trc: *mut JSTracer);
+pub unsafe trait JSTraceable {
+    unsafe fn trace(&self, trc: *mut JSTracer);
 }
 ```
 
@@ -182,7 +182,7 @@ pub fn trace_reflector(tracer: *mut JSTracer, description: &str, reflector: &Ref
 }
 
 impl<T: Reflectable> JSTraceable for JS<T> {
-    fn trace(&self, trc: *mut JSTracer) {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
         trace_reflector(trc, "", unsafe { (**self.ptr).reflector() });
     }
 }

components/script/dom/bindings/js.rs

@@ -105,18 +105,18 @@ impl<T: Reflectable> Deref for JS<T> {
     }
 }
 
-impl<T: Reflectable> JSTraceable for JS<T> {
-    fn trace(&self, trc: *mut JSTracer) {
+unsafe impl<T: Reflectable> JSTraceable for JS<T> {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
         #[cfg(debug_assertions)]
-        let trace_str = format!("for {} on heap", unsafe { type_name::<T>() });
+        let trace_str = format!("for {} on heap", type_name::<T>());
         #[cfg(debug_assertions)]
         let trace_info = &trace_str[..];
         #[cfg(not(debug_assertions))]
         let trace_info = "for DOM object on heap";
 
         trace_reflector(trc,
                         trace_info,
-                        unsafe { (**self.ptr).reflector() });
+                        (**self.ptr).reflector());
     }
 }