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/bindings/codegen/CodegenRust.py

@@ -2868,7 +2868,10 @@ class CGUnionStruct(CGThing):
         enumConversions = [
             "            e%s(ref inner) => inner.to_jsval(cx)," % v["name"] for v in templateVars
         ]
-        return ("""pub enum %s {
+        # XXXManishearth The following should be #[must_root],
+        # however we currently allow it till #2661 is fixed
+        return ("""#[allow(unrooted_must_root)]
+pub enum %s {
 %s
 }
 

components/script/dom/bindings/global.rs

@@ -34,6 +34,7 @@ pub enum GlobalRoot<'a, 'b> {
 /// A traced reference to a global object, for use in fields of traced Rust
 /// structures.
 #[deriving(Encodable)]
+#[must_root]
 pub enum GlobalField {
     WindowField(JS<Window>),
     WorkerField(JS<WorkerGlobalScope>),

components/script/dom/bindings/js.rs

@@ -61,6 +61,7 @@ use std::mem;
 /// Importantly, it requires explicit rooting in order to interact with the inner value.
 /// Can be assigned into JS-owned member fields (i.e. `JS<T>` types) safely via the
 /// `JS<T>::assign` method or `OptionalSettable::assign` (for `Option<JS<T>>` fields).
+#[allow(unrooted_must_root)]
 pub struct Temporary<T> {
     inner: JS<T>,
     /// On-stack JS pointer to assuage conservative stack scanner
@@ -106,6 +107,7 @@ impl<T: Reflectable> Temporary<T> {
 }
 
 /// A rooted, JS-owned value. Must only be used as a field in other JS-owned types.
+#[must_root]
 pub struct JS<T> {
     ptr: *const T
 }