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/htmlcollection.rs

@@ -30,12 +30,14 @@ impl<S: Encoder<E>, E> Encodable<S, E> for Box<CollectionFilter> {
 }
 
 #[deriving(Encodable)]
+#[must_root]
 pub enum CollectionTypeId {
     Static(Vec<JS<Element>>),
     Live(JS<Node>, Box<CollectionFilter>)
 }
 
 #[deriving(Encodable)]
+#[must_root]
 pub struct HTMLCollection {
     collection: CollectionTypeId,
     reflector_: Reflector,