servo: Merge #18059 - Using OnceCell<T> from Mitochondria (from sendilkumarn:mito); r=nox

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13402  (github issue number if applicable).

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Source-Repo: https://github.com/servo/servo
Source-Revision: 5c797d194346d5be3c55da51d82bfcd1031a51f5

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 6936fce573771660b0e310ebccadd5c2fa542602

servo/Cargo.lock

@@ -1874,6 +1874,11 @@ dependencies = [
  "ws2_32-sys 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "mitochondria"
+version = "1.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "mozjs_sys"
 version = "0.0.0"
@@ -2597,6 +2602,7 @@ dependencies = [
  "metrics 0.0.1",
  "mime 0.2.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "mime_guess 1.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mitochondria 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "msg 0.0.1",
  "net_traits 0.0.1",
  "num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3911,6 +3917,7 @@ dependencies = [
 "checksum miniz-sys 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)" = "28eaee17666671fa872e567547e8428e83308ebe5808cdf6a0e28397dbe2c726"
 "checksum mio 0.6.9 (registry+https://github.com/rust-lang/crates.io-index)" = "9e965267d4d58496fc4f740e9861118367f13570cadf66316ed2c3f2f14d87c7"
 "checksum miow 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "8c1f2f3b1cf331de6896aabf6e9d55dca90356cc9960cca7eaaf408a355ae919"
+"checksum mitochondria 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)" = "9de3eca27871df31c33b807f834b94ef7d000956f57aa25c5aed9c5f0aae8f6f"
 "checksum mozjs_sys 0.0.0 (git+https://github.com/servo/mozjs)" = "<none>"
 "checksum mp3-metadata 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "2f61cf32f7fc3cec83a15a255ac60bceb6cac59a7ce190cb824ca25c0fce0feb"
 "checksum mp4parse 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7b81651f9ede53d59281b54c7eb51ae50a868ac4765dd3bdfbbc79ce3d8aca7a"

servo/components/script/Cargo.toml

@@ -56,6 +56,7 @@ lazy_static = "0.2"
 libc = "0.2"
 log = "0.3.5"
 metrics = {path = "../metrics"}
+mitochondria = "1.1.2"
 mime = "0.2.1"
 mime_guess = "1.8.0"
 msg = {path = "../msg"}

servo/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

servo/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;
@@ -64,7 +64,7 @@ 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>>>,
 }
@@ -166,10 +166,6 @@ 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>
@@ -220,11 +216,11 @@ 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

servo/components/script/lib.rs

@@ -70,6 +70,7 @@ extern crate metrics;
 #[macro_use]
 extern crate mime;
 extern crate mime_guess;
+extern crate mitochondria;
 extern crate msg;
 extern crate net_traits;
 extern crate num_traits;