Initialize rooted dictionaries to a stable value before setting fields.

components/script/dom/bindings/js.rs

@@ -31,7 +31,8 @@ use dom::bindings::trace::JSTraceable;
 use dom::bindings::trace::trace_reflector;
 use dom::node::Node;
 use heapsize::HeapSizeOf;
-use js::jsapi::{JSObject, JSTracer};
+use js::jsapi::{JSObject, JSTracer, Heap};
+use js::rust::GCMethods;
 use mitochondria::OnceCell;
 use script_layout_interface::TrustedNodeAddress;
 use script_thread::STACK_ROOTS;
@@ -654,3 +655,29 @@ unsafe impl<T: DomObject> JSTraceable for Root<T> {
         // Already traced.
     }
 }
+
+/// Helper trait for safer manipulations of Option<Heap<T>> values.
+pub trait OptionalHeapSetter {
+    type Value;
+    /// Update this optional heap value with a new value.
+    fn set(&mut self, v: Option<Self::Value>);
+}
+
+impl<T: GCMethods + Copy> OptionalHeapSetter for Option<Heap<T>> where Heap<T>: Default {
+    type Value = T;
+    fn set(&mut self, v: Option<T>) {
+        let v = match v {
+            None => {
+                *self = None;
+                return;
+            }
+            Some(v) => v,
+        };
+
+        if self.is_none() {
+            *self = Some(Heap::default());
+        }
+
+        self.as_ref().unwrap().set(v);
+    }
+}