Auto merge of #8060 - nox:deref-js, r=Ms2ger

Implement Deref<Target=T> for JS<T> where T: Reflectable

We can only borrow `JS<T>` from rooted things, so it's safe to deref it.
The only types that provide mutable `JS<T>` things are `MutHeap<JS<T>>` and
`MutNullableHeap<JS<T>>`, which don't actually expose that they contain
`JS<T>` values.

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components/script/dom/bindings/js.rs

@@ -96,6 +96,16 @@ impl<T: Reflectable> JS<T> {
     }
 }
 
+impl<T: Reflectable> Deref for JS<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // We can only have &JS<T> from a rooted thing, so it's safe to deref
+        // it to &T.
+        unsafe { &**self.ptr }
+    }
+}
+
 impl<T: Reflectable> JSTraceable for JS<T> {
     fn trace(&self, trc: *mut JSTracer) {
         trace_reflector(trc, "", unsafe { (**self.ptr).reflector() });