moving to oncecell

adding oncecell for JS references

removing option<JS<T>> to <JS<T>>

changing return types

removing get method and refactoring the function

changing getElements method

lint fixes

moving to default

simplifying return

ordering

Removing elements

linting

components/script/dom/bindings/js.rs

@@ -32,6 +32,7 @@ use dom::bindings::trace::trace_reflector;
 use dom::node::Node;
 use heapsize::HeapSizeOf;
 use js::jsapi::{JSObject, JSTracer};
+use mitochondria::OnceCell;
 use script_layout_interface::TrustedNodeAddress;
 use script_thread::STACK_ROOTS;
 use std::cell::UnsafeCell;
@@ -391,6 +392,55 @@ impl<T: DomObject> HeapSizeOf for MutNullableJS<T> {
     }
 }
 
+/// A holder that allows to lazily initialize the value only once
+/// `JS<T>`, using OnceCell
+/// Essentially a `OnceCell<JS<T>>`.
+///
+/// This should only be used as a field in other DOM objects; see warning
+/// on `JS<T>`.
+#[must_root]
+pub struct OnceCellJS<T: DomObject> {
+    ptr: OnceCell<JS<T>>,
+}
+
+impl<T: DomObject> OnceCellJS<T> {
+    /// Retrieve a copy of the current inner value. If it is `None`, it is
+    /// initialized with the result of `cb` first.
+    #[allow(unrooted_must_root)]
+    pub fn init_once<F>(&self, cb: F) -> &T
+        where F: FnOnce() -> Root<T>
+    {
+        debug_assert!(thread_state::get().is_script());
+        &self.ptr.init_once(|| JS::from_ref(&cb()))
+    }
+}
+
+impl<T: DomObject> Default for OnceCellJS<T> {
+    #[allow(unrooted_must_root)]
+    fn default() -> OnceCellJS<T> {
+        debug_assert!(thread_state::get().is_script());
+        OnceCellJS {
+            ptr: OnceCell::new(),
+        }
+    }
+}
+
+impl<T: DomObject> HeapSizeOf for OnceCellJS<T> {
+    fn heap_size_of_children(&self) -> usize {
+        // See comment on HeapSizeOf for JS<T>.
+        0
+    }
+}
+
+#[allow(unrooted_must_root)]
+unsafe impl<T: DomObject> JSTraceable for OnceCellJS<T> {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
+        if let Some(ptr) = self.ptr.as_ref() {
+            ptr.trace(trc);
+        }
+    }
+}
+
 impl<T: DomObject> LayoutJS<T> {
     /// Returns an unsafe pointer to the interior of this JS object. This is
     /// the only method that be safely accessed from layout. (The fact that

components/script/dom/htmlformelement.rs

@@ -13,7 +13,7 @@ use dom::bindings::codegen::Bindings::HTMLFormElementBinding::HTMLFormElementMet
 use dom::bindings::codegen::Bindings::HTMLInputElementBinding::HTMLInputElementMethods;
 use dom::bindings::codegen::Bindings::HTMLTextAreaElementBinding::HTMLTextAreaElementMethods;
 use dom::bindings::inheritance::{Castable, ElementTypeId, HTMLElementTypeId, NodeTypeId};
-use dom::bindings::js::{JS, MutNullableJS, Root, RootedReference};
+use dom::bindings::js::{JS, OnceCellJS, Root, RootedReference};
 use dom::bindings::refcounted::Trusted;
 use dom::bindings::reflector::DomObject;
 use dom::bindings::str::DOMString;
@@ -66,7 +66,7 @@ pub struct GenerationId(u32);
 pub struct HTMLFormElement {
     htmlelement: HTMLElement,
     marked_for_reset: Cell<bool>,
-    elements: MutNullableJS<HTMLFormControlsCollection>,
+    elements: OnceCellJS<HTMLFormControlsCollection>,
     generation_id: Cell<GenerationId>,
     controls: DOMRefCell<Vec<JS<Element>>>,
 }
@@ -168,10 +168,6 @@ impl HTMLFormElementMethods for HTMLFormElement {
 
     // https://html.spec.whatwg.org/multipage/#dom-form-elements
     fn Elements(&self) -> Root<HTMLFormControlsCollection> {
-        if let Some(elements) = self.elements.get() {
-            return elements;
-        }
-
         #[derive(HeapSizeOf, JSTraceable)]
         struct ElementsFilter {
             form: Root<HTMLFormElement>
@@ -222,11 +218,11 @@ impl HTMLFormElementMethods for HTMLFormElement {
                 }
             }
         }
-        let filter = box ElementsFilter { form: Root::from_ref(self) };
-        let window = window_from_node(self);
-        let elements = HTMLFormControlsCollection::new(&window, self.upcast(), filter);
-        self.elements.set(Some(&elements));
-        elements
+        Root::from_ref(self.elements.init_once(|| {
+            let filter = box ElementsFilter { form: Root::from_ref(self) };
+            let window = window_from_node(self);
+            HTMLFormControlsCollection::new(&window, self.upcast(), filter)
+        }))
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-form-length