Prevent moving CanGc values between threads/tasks (#33902)

* Make CanGc non-sendable, and add documentation.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Update CanGc usage to fix usages that were moved between threads/tasks.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

10 files changed, 69 insertions, 89 deletions

diff --git a/components/script/dom/dedicatedworkerglobalscope.rs b/components/script/dom/dedicatedworkerglobalscope.rs
index 9c4f276c941..1b99d8771f2 100644
--- a/components/script/dom/dedicatedworkerglobalscope.rs
+++ b/components/script/dom/dedicatedworkerglobalscope.rs
@@ -333,7 +333,6 @@ impl DedicatedWorkerGlobalScope {
         gpu_id_hub: Arc<IdentityHub>,
         control_receiver: Receiver<DedicatedWorkerControlMsg>,
         context_sender: Sender<ThreadSafeJSContext>,
-        can_gc: CanGc,
     ) -> JoinHandle<()> {
         let serialized_worker_url = worker_url.to_string();
         let top_level_browsing_context_id = TopLevelBrowsingContextId::installed();
@@ -479,7 +478,7 @@ impl DedicatedWorkerGlobalScope {
                             // until the event loop is destroyed,
                             // which happens after the closing flag is set to true.
                             while !scope.is_closing() {
-                                run_worker_event_loop(&*global, Some(&worker), can_gc);
+                                run_worker_event_loop(&*global, Some(&worker), CanGc::note());
                             }
                         },
                         reporter_name,
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index 46a3c41ccc7..044d4f5bfb2 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -694,7 +694,7 @@ impl Document {
         self.activity.get() != DocumentActivity::Inactive
     }
 
-    pub fn set_activity(&self, activity: DocumentActivity, can_gc: CanGc) {
+    pub fn set_activity(&self, activity: DocumentActivity) {
         // This function should only be called on documents with a browsing context
         assert!(self.has_browsing_context);
         if activity == self.activity.get() {
@@ -752,7 +752,7 @@ impl Document {
                         false, // bubbles
                         false, // cancelable
                         true, // persisted
-                        can_gc,
+                        CanGc::note(),
                     );
                     let event = event.upcast::<Event>();
                     event.set_trusted(true);
@@ -2379,7 +2379,7 @@ impl Document {
     }
 
     // https://html.spec.whatwg.org/multipage/#the-end
-    pub fn maybe_queue_document_completion(&self, can_gc: CanGc) {
+    pub fn maybe_queue_document_completion(&self) {
         // https://html.spec.whatwg.org/multipage/#delaying-load-events-mode
         let is_in_delaying_load_events_mode = match self.window.undiscarded_window_proxy() {
             Some(window_proxy) => window_proxy.is_delaying_load_events_mode(),
@@ -2429,7 +2429,7 @@ impl Document {
                         atom!("load"),
                         EventBubbles::DoesNotBubble,
                         EventCancelable::NotCancelable,
-                        can_gc,
+                        CanGc::note(),
                     );
                     event.set_trusted(true);
 
@@ -2472,7 +2472,7 @@ impl Document {
                             false, // bubbles
                             false, // cancelable
                             false, // persisted
-                            can_gc,
+                            CanGc::note(),
                         );
                         let event = event.upcast::<Event>();
                         event.set_trusted(true);
diff --git a/components/script/dom/globalscope.rs b/components/script/dom/globalscope.rs
index fa2574cc252..74dca15fedd 100644
--- a/components/script/dom/globalscope.rs
+++ b/components/script/dom/globalscope.rs
@@ -3185,7 +3185,7 @@ impl GlobalScope {
         }
     }
 
-    pub fn handle_gamepad_event(&self, gamepad_event: GamepadEvent, can_gc: CanGc) {
+    pub fn handle_gamepad_event(&self, gamepad_event: GamepadEvent) {
         match gamepad_event {
             GamepadEvent::Connected(index, name, bounds, supported_haptic_effects) => {
                 self.handle_gamepad_connect(
@@ -3194,7 +3194,6 @@ impl GlobalScope {
                     bounds.axis_bounds,
                     bounds.button_bounds,
                     supported_haptic_effects,
-                    can_gc,
                 );
             },
             GamepadEvent::Disconnected(index) => {
@@ -3207,7 +3206,7 @@ impl GlobalScope {
     }
 
     /// <https://www.w3.org/TR/gamepad/#dfn-gamepadconnected>
-    pub fn handle_gamepad_connect(
+    fn handle_gamepad_connect(
         &self,
         // As the spec actually defines how to set the gamepad index, the GilRs index
         // is currently unused, though in practice it will almost always be the same.
@@ -3217,7 +3216,6 @@ impl GlobalScope {
         axis_bounds: (f64, f64),
         button_bounds: (f64, f64),
         supported_haptic_effects: GamepadSupportedHapticEffects,
-        can_gc: CanGc,
     ) {
         // TODO: 2. If document is not null and is not allowed to use the "gamepad" permission,
         //          then abort these steps.
@@ -3239,7 +3237,7 @@ impl GlobalScope {
                             button_bounds,
                             supported_haptic_effects,
                             false,
-                            can_gc,
+                            CanGc::note(),
                         );
                         navigator.set_gamepad(selected_index as usize, &gamepad);
                     }
diff --git a/components/script/dom/htmlimageelement.rs b/components/script/dom/htmlimageelement.rs
index 797ff8557fc..d2bfff7721d 100644
--- a/components/script/dom/htmlimageelement.rs
+++ b/components/script/dom/htmlimageelement.rs
@@ -516,12 +516,7 @@ impl HTMLImageElement {
                 });
                 self.pending_request.borrow_mut().final_url = Some(url);
                 self.pending_request.borrow_mut().image = Some(image);
-                self.finish_reacting_to_environment_change(
-                    src,
-                    generation,
-                    selected_pixel_density,
-                    can_gc,
-                );
+                self.finish_reacting_to_environment_change(src, generation, selected_pixel_density);
             },
             ImageResponse::MetadataLoaded(meta) => {
                 self.pending_request.borrow_mut().metadata = Some(meta);
@@ -1067,7 +1062,6 @@ impl HTMLImageElement {
             elem: &HTMLImageElement,
             selected_source: String,
             selected_pixel_density: f64,
-            can_gc: CanGc,
         ) -> IpcSender<PendingImageResponse> {
             let trusted_node = Trusted::new(elem);
             let (responder_sender, responder_receiver) = ipc::channel().unwrap();
@@ -1094,7 +1088,7 @@ impl HTMLImageElement {
                             if generation == element.generation.get() {
                                 element.process_image_response_for_environment_change(image.response,
                                     USVString::from(selected_source_clone), generation,
-                                    selected_pixel_density, can_gc);
+                                    selected_pixel_density, CanGc::note());
                             }
                         }),
                         &canceller,
@@ -1160,7 +1154,6 @@ impl HTMLImageElement {
             self,
             selected_source.0.clone(),
             selected_pixel_density,
-            can_gc,
         );
         let cache_result = image_cache.track_image(
             img_url.clone(),
@@ -1177,7 +1170,6 @@ impl HTMLImageElement {
                     selected_source,
                     generation,
                     selected_pixel_density,
-                    can_gc,
                 )
             },
             ImageCacheResult::Available(ImageOrMetadataAvailable::MetadataAvailable(m)) => {
@@ -1251,7 +1243,6 @@ impl HTMLImageElement {
         src: USVString,
         generation: u32,
         selected_pixel_density: f64,
-        can_gc: CanGc,
     ) {
         let this = Trusted::new(self);
         let window = window_from_node(self);
@@ -1262,7 +1253,7 @@ impl HTMLImageElement {
                 let relevant_mutation = this.generation.get() != generation;
                 // Step 15.1
                 if relevant_mutation {
-                    this.abort_request(State::Unavailable, ImageRequestPhase::Pending, can_gc);
+                    this.abort_request(State::Unavailable, ImageRequestPhase::Pending, CanGc::note());
                     return;
                 }
                 // Step 15.2
@@ -1280,7 +1271,7 @@ impl HTMLImageElement {
 
                     // Step 15.5
                     mem::swap(&mut this.current_request.borrow_mut(), &mut pending_request);
-                    this.abort_request(State::Unavailable, ImageRequestPhase::Pending, can_gc);
+                    this.abort_request(State::Unavailable, ImageRequestPhase::Pending, CanGc::note());
                 }
 
                 // Step 15.6
diff --git a/components/script/dom/htmlmediaelement.rs b/components/script/dom/htmlmediaelement.rs
index 08c63143ea3..7d162e6c848 100644
--- a/components/script/dom/htmlmediaelement.rs
+++ b/components/script/dom/htmlmediaelement.rs
@@ -611,7 +611,7 @@ impl HTMLMediaElement {
     }
 
     // https://html.spec.whatwg.org/multipage/#ready-states
-    fn change_ready_state(&self, ready_state: ReadyState, can_gc: CanGc) {
+    fn change_ready_state(&self, ready_state: ReadyState) {
         let old_ready_state = self.ready_state.get();
         self.ready_state.set(ready_state);
 
@@ -642,7 +642,7 @@ impl HTMLMediaElement {
                         task!(media_reached_current_data: move || {
                             let this = this.root();
                             this.upcast::<EventTarget>().fire_event(atom!("loadeddata"));
-                            this.delay_load_event(false, can_gc);
+                            this.delay_load_event(false, CanGc::note());
                         }),
                         window.upcast(),
                     );
@@ -796,12 +796,12 @@ impl HTMLMediaElement {
                 // Step 9.obj.3.
                 // Note that the resource fetch algorithm itself takes care
                 // of the cleanup in case of failure itself.
-                self.resource_fetch_algorithm(Resource::Object, can_gc);
+                self.resource_fetch_algorithm(Resource::Object);
             },
             Mode::Attribute(src) => {
                 // Step 9.attr.1.
                 if src.is_empty() {
-                    self.queue_dedicated_media_source_failure_steps(can_gc);
+                    self.queue_dedicated_media_source_failure_steps();
                     return;
                 }
 
@@ -809,7 +809,7 @@ impl HTMLMediaElement {
                 let url_record = match base_url.join(&src) {
                     Ok(url) => url,
                     Err(_) => {
-                        self.queue_dedicated_media_source_failure_steps(can_gc);
+                        self.queue_dedicated_media_source_failure_steps();
                         return;
                     },
                 };
@@ -820,7 +820,7 @@ impl HTMLMediaElement {
                 // Step 9.attr.4.
                 // Note that the resource fetch algorithm itself takes care
                 // of the cleanup in case of failure itself.
-                self.resource_fetch_algorithm(Resource::Url(url_record), can_gc);
+                self.resource_fetch_algorithm(Resource::Url(url_record));
             },
             // Step 9.children.
             Mode::Children(source) => {
@@ -830,7 +830,7 @@ impl HTMLMediaElement {
                 // Step 9.attr.2.
                 if src.is_empty() {
                     source.upcast::<EventTarget>().fire_event(atom!("error"));
-                    self.queue_dedicated_media_source_failure_steps(can_gc);
+                    self.queue_dedicated_media_source_failure_steps();
                     return;
                 }
                 // Step 9.attr.3.
@@ -838,20 +838,20 @@ impl HTMLMediaElement {
                     Ok(url) => url,
                     Err(_) => {
                         source.upcast::<EventTarget>().fire_event(atom!("error"));
-                        self.queue_dedicated_media_source_failure_steps(can_gc);
+                        self.queue_dedicated_media_source_failure_steps();
                         return;
                     },
                 };
                 // Step 9.attr.8.
-                self.resource_fetch_algorithm(Resource::Url(url_record), can_gc);
+                self.resource_fetch_algorithm(Resource::Url(url_record));
             },
         }
     }
 
-    fn fetch_request(&self, offset: Option<u64>, seek_lock: Option<SeekLock>, can_gc: CanGc) {
+    fn fetch_request(&self, offset: Option<u64>, seek_lock: Option<SeekLock>) {
         if self.resource_url.borrow().is_none() && self.blob_url.borrow().is_none() {
             eprintln!("Missing request url");
-            self.queue_dedicated_media_source_failure_steps(can_gc);
+            self.queue_dedicated_media_source_failure_steps();
             return;
         }
 
@@ -899,10 +899,10 @@ impl HTMLMediaElement {
     }
 
     // https://html.spec.whatwg.org/multipage/#concept-media-load-resource
-    fn resource_fetch_algorithm(&self, resource: Resource, can_gc: CanGc) {
-        if let Err(e) = self.setup_media_player(&resource, can_gc) {
+    fn resource_fetch_algorithm(&self, resource: Resource) {
+        if let Err(e) = self.setup_media_player(&resource) {
             eprintln!("Setup media player error {:?}", e);
-            self.queue_dedicated_media_source_failure_steps(can_gc);
+            self.queue_dedicated_media_source_failure_steps();
             return;
         }
 
@@ -935,7 +935,7 @@ impl HTMLMediaElement {
                         .media_element_task_source()
                         .queue(
                             task!(set_media_delay_load_event_flag_to_false: move || {
-                                this.root().delay_load_event(false, can_gc);
+                                this.root().delay_load_event(false, CanGc::note());
                             }),
                             window.upcast(),
                         )
@@ -954,7 +954,7 @@ impl HTMLMediaElement {
 
                 // Step 4.remote.2.
                 *self.resource_url.borrow_mut() = Some(url);
-                self.fetch_request(None, None, can_gc);
+                self.fetch_request(None, None);
             },
             Resource::Object => {
                 if let Some(ref src_object) = *self.src_object.borrow() {
@@ -963,7 +963,7 @@ impl HTMLMediaElement {
                             let blob_url = URL::CreateObjectURL(&self.global(), blob);
                             *self.blob_url.borrow_mut() =
                                 Some(ServoUrl::parse(&blob_url).expect("infallible"));
-                            self.fetch_request(None, None, can_gc);
+                            self.fetch_request(None, None);
                         },
                         SrcObject::MediaStream(ref stream) => {
                             let tracks = &*stream.get_tracks();
@@ -978,7 +978,7 @@ impl HTMLMediaElement {
                                     .set_stream(&track.id(), pos == tracks.len() - 1)
                                     .is_err()
                                 {
-                                    self.queue_dedicated_media_source_failure_steps(can_gc);
+                                    self.queue_dedicated_media_source_failure_steps();
                                 }
                             }
                         },
@@ -991,7 +991,7 @@ impl HTMLMediaElement {
     /// Queues a task to run the [dedicated media source failure steps][steps].
     ///
     /// [steps]: https://html.spec.whatwg.org/multipage/#dedicated-media-source-failure-steps
-    fn queue_dedicated_media_source_failure_steps(&self, can_gc: CanGc) {
+    fn queue_dedicated_media_source_failure_steps(&self) {
         let window = window_from_node(self);
         let this = Trusted::new(self);
         let generation_id = self.generation_id.get();
@@ -1036,7 +1036,7 @@ impl HTMLMediaElement {
                 });
 
                 // Step 7.
-                this.delay_load_event(false, can_gc);
+                this.delay_load_event(false, CanGc::note());
             }),
             window.upcast(),
         );
@@ -1123,7 +1123,7 @@ impl HTMLMediaElement {
 
             // Step 6.5.
             if self.ready_state.get() != ReadyState::HaveNothing {
-                self.change_ready_state(ReadyState::HaveNothing, can_gc);
+                self.change_ready_state(ReadyState::HaveNothing);
             }
 
             // Step 6.6.
@@ -1329,7 +1329,7 @@ impl HTMLMediaElement {
         }
     }
 
-    fn setup_media_player(&self, resource: &Resource, can_gc: CanGc) -> Result<(), ()> {
+    fn setup_media_player(&self, resource: &Resource) -> Result<(), ()> {
         let stream_type = match *resource {
             Resource::Object => {
                 if let Some(ref src_object) = *self.src_object.borrow() {
@@ -1380,7 +1380,7 @@ impl HTMLMediaElement {
                 let this = trusted_node.clone();
                 if let Err(err) = task_source.queue_with_canceller(
                     task!(handle_player_event: move || {
-                        this.root().handle_player_event(&event, can_gc);
+                        this.root().handle_player_event(&event, CanGc::note());
                     }),
                     &canceller,
                 ) {
@@ -1480,7 +1480,7 @@ impl HTMLMediaElement {
                 // => "If the media data can be fetched but is found by inspection to be in
                 //    an unsupported format, or can otherwise not be rendered at all"
                 if self.ready_state.get() < ReadyState::HaveMetadata {
-                    self.queue_dedicated_media_source_failure_steps(can_gc);
+                    self.queue_dedicated_media_source_failure_steps();
                 } else {
                     // https://html.spec.whatwg.org/multipage/#reaches-the-end
                     match self.direction_of_playback() {
@@ -1526,7 +1526,7 @@ impl HTMLMediaElement {
                                 );
 
                                 // https://html.spec.whatwg.org/multipage/#dom-media-have_current_data
-                                self.change_ready_state(ReadyState::HaveCurrentData, can_gc);
+                                self.change_ready_state(ReadyState::HaveCurrentData);
                             }
                         },
 
@@ -1730,7 +1730,7 @@ impl HTMLMediaElement {
                 }
 
                 // Step 6.
-                self.change_ready_state(ReadyState::HaveMetadata, can_gc);
+                self.change_ready_state(ReadyState::HaveMetadata);
 
                 // Step 7.
                 let mut jumped = false;
@@ -1800,7 +1800,7 @@ impl HTMLMediaElement {
                 }
             },
             PlayerEvent::EnoughData => {
-                self.change_ready_state(ReadyState::HaveEnoughData, can_gc);
+                self.change_ready_state(ReadyState::HaveEnoughData);
 
                 // The player has enough data and it is asking us to stop pushing
                 // bytes, so we cancel the ongoing fetch request iff we are able
@@ -1833,7 +1833,7 @@ impl HTMLMediaElement {
                 ));
             },
             PlayerEvent::SeekData(p, ref seek_lock) => {
-                self.fetch_request(Some(p), Some(seek_lock.clone()), can_gc);
+                self.fetch_request(Some(p), Some(seek_lock.clone()));
             },
             PlayerEvent::SeekDone(_) => {
                 // Continuation of
@@ -1852,7 +1852,7 @@ impl HTMLMediaElement {
                     PlaybackState::Paused => {
                         media_session_playback_state = MediaSessionPlaybackState::Paused;
                         if self.ready_state.get() == ReadyState::HaveMetadata {
-                            self.change_ready_state(ReadyState::HaveEnoughData, can_gc);
+                            self.change_ready_state(ReadyState::HaveEnoughData);
                         }
                     },
                     PlaybackState::Playing => {
@@ -2731,7 +2731,7 @@ impl FetchResponseListener for HTMLMediaElementFetchListener {
             if let Some(ref mut current_fetch_context) = *elem.current_fetch_context.borrow_mut() {
                 current_fetch_context.cancel(CancelReason::Error);
             }
-            elem.queue_dedicated_media_source_failure_steps(CanGc::note());
+            elem.queue_dedicated_media_source_failure_steps();
         }
     }
 
@@ -2867,7 +2867,7 @@ impl FetchResponseListener for HTMLMediaElementFetchListener {
             elem.upcast::<EventTarget>().fire_event(atom!("error"));
         } else {
             // => "If the media data cannot be fetched at all..."
-            elem.queue_dedicated_media_source_failure_steps(CanGc::note());
+            elem.queue_dedicated_media_source_failure_steps();
         }
     }
 
diff --git a/components/script/dom/serviceworkerglobalscope.rs b/components/script/dom/serviceworkerglobalscope.rs
index e4ce55ec3d5..3439b04dcdb 100644
--- a/components/script/dom/serviceworkerglobalscope.rs
+++ b/components/script/dom/serviceworkerglobalscope.rs
@@ -294,7 +294,6 @@ impl ServiceWorkerGlobalScope {
         control_receiver: Receiver<ServiceWorkerControlMsg>,
         context_sender: Sender<ThreadSafeJSContext>,
         closing: Arc<AtomicBool>,
-        can_gc: CanGc,
     ) -> JoinHandle<()> {
         let ScopeThings {
             script_url,
@@ -384,7 +383,7 @@ impl ServiceWorkerGlobalScope {
                     scope.execute_script(DOMString::from(source));
                 }
 
-                global.dispatch_activate(can_gc);
+                global.dispatch_activate(CanGc::note());
                 let reporter_name = format!("service-worker-reporter-{}", random::<u64>());
                 scope
                     .upcast::<GlobalScope>()
@@ -398,7 +397,7 @@ impl ServiceWorkerGlobalScope {
                             // which happens after the closing flag is set to true,
                             // or until the worker has run beyond its allocated time.
                             while !scope.is_closing() && !global.has_timed_out() {
-                                run_worker_event_loop(&*global, None, can_gc);
+                                run_worker_event_loop(&*global, None, CanGc::note());
                             }
                         },
                         reporter_name,
diff --git a/components/script/dom/worker.rs b/components/script/dom/worker.rs
index cfbec632303..48fe9cbda13 100644
--- a/components/script/dom/worker.rs
+++ b/components/script/dom/worker.rs
@@ -232,7 +232,6 @@ impl WorkerMethods for Worker {
             global.wgpu_id_hub(),
             control_receiver,
             context_sender,
-            can_gc,
         );
 
         let context = context_receiver
diff --git a/components/script/script_runtime.rs b/components/script/script_runtime.rs
index 04ff6b2eaf5..894e6d4a3fd 100644
--- a/components/script/script_runtime.rs
+++ b/components/script/script_runtime.rs
@@ -1115,10 +1115,17 @@ impl Runnable {
 }
 
 #[derive(Clone, Copy, Debug)]
-pub struct CanGc(());
+/// A compile-time marker that there are operations that could trigger a JS garbage collection
+/// operation within the current stack frame. It is trivially copyable, so it should be passed
+/// as a function argument and reused when calling other functions whenever possible. Since it
+/// is only meaningful within the current stack frame, it is impossible to move it to a different
+/// thread or into a task that will execute asynchronously.
+pub struct CanGc(std::marker::PhantomData<*mut ()>);
 
 impl CanGc {
+    /// Create a new CanGc value, representing that a GC operation is possible within the
+    /// current stack frame.
     pub fn note() -> CanGc {
-        CanGc(())
+        CanGc(std::marker::PhantomData)
     }
 }
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 01d8140fa79..ea6148325b9 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1637,7 +1637,7 @@ impl ScriptThread {
 
                 CompositorEvent::GamepadEvent(gamepad_event) => {
                     let global = window.upcast::<GlobalScope>();
-                    global.handle_gamepad_event(gamepad_event, can_gc);
+                    global.handle_gamepad_event(gamepad_event);
                 },
             }
         }
@@ -2034,7 +2034,7 @@ impl ScriptThread {
             let mut docs = self.docs_with_no_blocking_loads.borrow_mut();
             for document in docs.iter() {
                 let _realm = enter_realm(&**document);
-                document.maybe_queue_document_completion(can_gc);
+                document.maybe_queue_document_completion();
 
                 // Document load is a rendering opportunity.
                 ScriptThread::note_rendering_opportunity(document.window().pipeline_id());
@@ -2331,7 +2331,7 @@ impl ScriptThread {
                 self.handle_get_title_msg(pipeline_id)
             },
             ConstellationControlMsg::SetDocumentActivity(pipeline_id, activity) => {
-                self.handle_set_document_activity_msg(pipeline_id, activity, can_gc)
+                self.handle_set_document_activity_msg(pipeline_id, activity)
             },
             ConstellationControlMsg::SetThrottled(pipeline_id, throttled) => {
                 self.handle_set_throttled_msg(pipeline_id, throttled)
@@ -3003,12 +3003,7 @@ impl ScriptThread {
     }
 
     /// Handles activity change message
-    fn handle_set_document_activity_msg(
-        &self,
-        id: PipelineId,
-        activity: DocumentActivity,
-        can_gc: CanGc,
-    ) {
+    fn handle_set_document_activity_msg(&self, id: PipelineId, activity: DocumentActivity) {
         debug!(
             "Setting activity of {} to be {:?} in {:?}.",
             id,
@@ -3017,7 +3012,7 @@ impl ScriptThread {
         );
         let document = self.documents.borrow().find_document(id);
         if let Some(document) = document {
-            document.set_activity(activity, can_gc);
+            document.set_activity(activity);
             return;
         }
         let mut loads = self.incomplete_loads.borrow_mut();
diff --git a/components/script/serviceworker_manager.rs b/components/script/serviceworker_manager.rs
index 034da394310..3be5d3a9ef2 100644
--- a/components/script/serviceworker_manager.rs
+++ b/components/script/serviceworker_manager.rs
@@ -29,7 +29,7 @@ use crate::dom::serviceworkerglobalscope::{
     ServiceWorkerControlMsg, ServiceWorkerGlobalScope, ServiceWorkerScriptMsg,
 };
 use crate::dom::serviceworkerregistration::longest_prefix_match;
-use crate::script_runtime::{CanGc, ThreadSafeJSContext};
+use crate::script_runtime::ThreadSafeJSContext;
 
 enum Message {
     FromResource(CustomResponseMediator),
@@ -250,12 +250,10 @@ impl ServiceWorkerManager {
         None
     }
 
-    fn handle_message(&mut self, can_gc: CanGc) {
+    fn handle_message(&mut self) {
         while let Ok(message) = self.receive_message() {
             let should_continue = match message {
-                Message::FromConstellation(msg) => {
-                    self.handle_message_from_constellation(*msg, can_gc)
-                },
+                Message::FromConstellation(msg) => self.handle_message_from_constellation(*msg),
                 Message::FromResource(msg) => self.handle_message_from_resource(msg),
             };
             if !should_continue {
@@ -290,7 +288,7 @@ impl ServiceWorkerManager {
         }
     }
 
-    fn handle_message_from_constellation(&mut self, msg: ServiceWorkerMsg, can_gc: CanGc) -> bool {
+    fn handle_message_from_constellation(&mut self, msg: ServiceWorkerMsg) -> bool {
         match msg {
             ServiceWorkerMsg::Timeout(_scope) => {
                 // TODO: https://w3c.github.io/ServiceWorker/#terminate-service-worker
@@ -307,7 +305,7 @@ impl ServiceWorkerManager {
                     self.handle_register_job(job);
                 },
                 JobType::Update => {
-                    self.handle_update_job(job, can_gc);
+                    self.handle_update_job(job);
                 },
                 JobType::Unregister => {
                     // TODO: https://w3c.github.io/ServiceWorker/#unregister-algorithm
@@ -382,7 +380,7 @@ impl ServiceWorkerManager {
     }
 
     /// <https://w3c.github.io/ServiceWorker/#update>
-    fn handle_update_job(&mut self, job: Job, can_gc: CanGc) {
+    fn handle_update_job(&mut self, job: Job) {
         // Step 1: Get registation
         if let Some(registration) = self.registrations.get_mut(&job.scope_url) {
             // Step 3.
@@ -405,12 +403,8 @@ impl ServiceWorkerManager {
 
             // Very roughly steps 5 to 18.
             // TODO: implement all steps precisely.
-            let (new_worker, join_handle, control_sender, context, closing) = update_serviceworker(
-                self.own_sender.clone(),
-                job.scope_url.clone(),
-                scope_things,
-                can_gc,
-            );
+            let (new_worker, join_handle, control_sender, context, closing) =
+                update_serviceworker(self.own_sender.clone(), job.scope_url.clone(), scope_things);
 
             // Since we've just started the worker thread, ensure we can shut it down later.
             registration.note_worker_thread(join_handle, control_sender, context, closing);
@@ -449,7 +443,6 @@ fn update_serviceworker(
     own_sender: IpcSender<ServiceWorkerMsg>,
     scope_url: ServoUrl,
     scope_things: ScopeThings,
-    can_gc: CanGc,
 ) -> (
     ServiceWorker,
     JoinHandle<()>,
@@ -475,7 +468,6 @@ fn update_serviceworker(
         control_receiver,
         context_sender,
         closing.clone(),
-        can_gc,
     );
 
     let context = context_receiver
@@ -512,7 +504,7 @@ impl ServiceWorkerManagerFactory for ServiceWorkerManager {
                 resource_port,
                 constellation_sender,
             )
-            .handle_message(CanGc::note())
+            .handle_message()
         };
         if thread::Builder::new()
             .name("SvcWorkerManager".to_owned())