Auto merge of #16861 - gterzian:use_microtask_to_await_stable_state, r=jdm

Use microtasks to await a stable state.

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@jdm @KiChjang First pass at using microtasks to await a stable state. I ran into all sorts of problems to get it to compile, I think it's mainly related to the fact that the microtasks are stored in a `Vec`, which meant the `Runnalbe.handler(self: Box<Self>)` couldn't be called while iterating over the Vec... It compiles now although I haven't run any tests. I'm assuming I'm missing something fundamental and was hoping my changes would highlight the problems I run into, and you had a better idea of how to implement this... Perhaps we shouldn't pass a `Runnable` to `await_stable_state` at all?

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15375 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/htmlmediaelement.rs

@@ -31,6 +31,7 @@ use dom_struct::dom_struct;
 use html5ever::{LocalName, Prefix};
 use ipc_channel::ipc;
 use ipc_channel::router::ROUTER;
+use microtask::{Microtask, MicrotaskRunnable};
 use net_traits::{FetchResponseListener, FetchMetadata, Metadata, NetworkError};
 use net_traits::request::{CredentialsMode, Destination, RequestInit, Type as RequestType};
 use network_listener::{NetworkListener, PreInvoke};
@@ -429,7 +430,11 @@ impl HTMLMediaElement {
 
         // Step 4
         let doc = document_from_node(self);
-        ScriptThread::await_stable_state(ResourceSelectionTask::new(self, doc.base_url()));
+        let task = MediaElementMicrotask::ResourceSelectionTask {
+            elem: Root::from_ref(self),
+            base_url: doc.base_url()
+        };
+        ScriptThread::await_stable_state(Microtask::MediaElement(task));
     }
 
     // https://html.spec.whatwg.org/multipage/#concept-media-load-algorithm
@@ -781,7 +786,36 @@ impl VirtualMethods for HTMLMediaElement {
         self.super_type().unwrap().unbind_from_tree(context);
 
         if context.tree_in_doc {
-            ScriptThread::await_stable_state(PauseIfNotInDocumentTask::new(self));
+            let task = MediaElementMicrotask::PauseIfNotInDocumentTask {
+                elem: Root::from_ref(self)
+            };
+            ScriptThread::await_stable_state(Microtask::MediaElement(task));
+        }
+    }
+}
+
+#[derive(JSTraceable, HeapSizeOf)]
+pub enum MediaElementMicrotask {
+    ResourceSelectionTask {
+        elem: Root<HTMLMediaElement>,
+        base_url: ServoUrl
+    },
+    PauseIfNotInDocumentTask {
+        elem: Root<HTMLMediaElement>,
+    }
+}
+
+impl MicrotaskRunnable for MediaElementMicrotask {
+    fn handler(&self) {
+        match self {
+            &MediaElementMicrotask::ResourceSelectionTask { ref elem, ref base_url } => {
+                elem.resource_selection_algorithm_sync(base_url.clone());
+            },
+            &MediaElementMicrotask::PauseIfNotInDocumentTask { ref elem } => {
+                if !elem.upcast::<Node>().is_in_doc() {
+                    elem.internal_pause_steps();
+                }
+            },
         }
     }
 }
@@ -809,28 +843,6 @@ impl Runnable for FireSimpleEventTask {
     }
 }
 
-struct ResourceSelectionTask {
-    elem: Trusted<HTMLMediaElement>,
-    base_url: ServoUrl,
-}
-
-impl ResourceSelectionTask {
-    fn new(elem: &HTMLMediaElement, url: ServoUrl) -> ResourceSelectionTask {
-        ResourceSelectionTask {
-            elem: Trusted::new(elem),
-            base_url: url,
-        }
-    }
-}
-
-impl Runnable for ResourceSelectionTask {
-    fn name(&self) -> &'static str { "ResourceSelectionTask" }
-
-    fn handler(self: Box<ResourceSelectionTask>) {
-        self.elem.root().resource_selection_algorithm_sync(self.base_url);
-    }
-}
-
 struct DedicatedMediaSourceFailureTask {
     elem: Trusted<HTMLMediaElement>,
 }
@@ -851,29 +863,6 @@ impl Runnable for DedicatedMediaSourceFailureTask {
     }
 }
 
-struct PauseIfNotInDocumentTask {
-    elem: Trusted<HTMLMediaElement>,
-}
-
-impl PauseIfNotInDocumentTask {
-    fn new(elem: &HTMLMediaElement) -> PauseIfNotInDocumentTask {
-        PauseIfNotInDocumentTask {
-            elem: Trusted::new(elem),
-        }
-    }
-}
-
-impl Runnable for PauseIfNotInDocumentTask {
-    fn name(&self) -> &'static str { "PauseIfNotInDocumentTask" }
-
-    fn handler(self: Box<PauseIfNotInDocumentTask>) {
-        let elem = self.elem.root();
-        if !elem.upcast::<Node>().is_in_doc() {
-            elem.internal_pause_steps();
-        }
-    }
-}
-
 enum ResourceSelectionMode {
     Object,
     Attribute(String),