Add unrooted_must_root lint for enums and structs containing JS<T>, as well as functions with JS<T> in their parameter list

For safe wrappers over JS<T> (eg Temporary<T>) use #[allow(unrooted_must_root)].
For all other types containing a #[must_root] value, annotate the type with #[must_root] to ensure that it is never used unrooted

components/script/dom/domimplementation.rs

@@ -23,6 +23,7 @@ use dom::text::Text;
 use servo_util::str::DOMString;
 
 #[deriving(Encodable)]
+#[must_root]
 pub struct DOMImplementation {
     document: JS<Document>,
     reflector_: Reflector,