implement CachedFrozenArray (#34145)

* extract code into CachedFrozenArray

Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

* fix borrow crash

Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

* fix already borrowed error

using an else will cause the borrow to live more than it needs
Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

* restore return statement

Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

---------

Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

components/script/dom/bindings/frozenarray.rs

@@ -0,0 +1,52 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use js::conversions::ToJSValConvertible;
+use js::jsapi::Heap;
+use js::jsval::JSVal;
+use js::rust::MutableHandleValue;
+
+use crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::utils::to_frozen_array;
+use crate::script_runtime::JSContext;
+
+#[derive(JSTraceable)]
+pub struct CachedFrozenArray {
+    frozen_value: DomRefCell<Option<Heap<JSVal>>>,
+}
+
+impl CachedFrozenArray {
+    pub fn new() -> CachedFrozenArray {
+        CachedFrozenArray {
+            frozen_value: DomRefCell::new(None),
+        }
+    }
+
+    pub fn get_or_init<F: FnOnce() -> Vec<T>, T: ToJSValConvertible>(
+        &self,
+        f: F,
+        cx: JSContext,
+        mut retval: MutableHandleValue,
+    ) {
+        if let Some(inner) = &*self.frozen_value.borrow() {
+            retval.set(inner.get());
+            return;
+        }
+
+        let array = f();
+        to_frozen_array(array.as_slice(), cx, retval);
+
+        // Safety: need to create the Heap value in its final memory location before setting it.
+        *self.frozen_value.borrow_mut() = Some(Heap::default());
+        self.frozen_value
+            .borrow()
+            .as_ref()
+            .unwrap()
+            .set(retval.get());
+    }
+
+    pub fn clear(&self) {
+        *self.frozen_value.borrow_mut() = None;
+    }
+}

components/script/dom/bindings/mod.rs

@@ -142,6 +142,7 @@ pub mod constructor;
 pub mod conversions;
 pub mod error;
 pub mod finalize;
+pub mod frozenarray;
 pub mod guard;
 pub mod import;
 pub mod inheritance;