Remove usage of drain_filter (#30742)

This is a step on the way toward supporting stable rust.

components/script/dom/eventtarget.rs

@@ -448,7 +448,7 @@ impl EventTarget {
 
         let listener = EventListenerType::Additive(listener.clone());
         if let Some(entries) = handlers.get_mut(ty) {
-            entries.drain_filter(|e| e.listener == listener && e.once);
+            entries.retain(|e| e.listener != listener || !e.once)
         }
     }
 

components/script/lib.rs

@@ -2,7 +2,6 @@
  * 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/. */
 
-#![feature(drain_filter)]
 #![feature(once_cell)]
 #![feature(plugin)]
 #![feature(register_tool)]

components/script/task_queue.rs

@@ -120,23 +120,29 @@ impl<T: QueuedTaskConversion> TaskQueue<T> {
         }
 
         // 4. Filter tasks from non-priority task-sources.
-        let to_be_throttled: Vec<T> = incoming
+        // TODO: This can use `extract_if` once that is stabilized.
-            .drain_filter(|msg| {
+        let mut to_be_throttled = Vec::new();
-                let task_source = match msg.task_source_name() {
+        let mut index = 0;
+        while index != incoming.len() {
+            index += 1; // By default we go to the next index of the vector.
+
+            let task_source = match incoming[index - 1].task_source_name() {
                 Some(task_source) => task_source,
-                    None => return false,
+                None => continue,
             };
+
             match task_source {
-                    TaskSourceName::PerformanceTimeline => return true,
+                TaskSourceName::PerformanceTimeline => {
+                    to_be_throttled.push(incoming.remove(index - 1));
+                    index -= 1; // We've removed an element, so the next has the same index.
+                },
                 _ => {
                     // A task that will not be throttled, start counting "business"
                     self.taken_task_counter
                         .set(self.taken_task_counter.get() + 1);
-                        return false;
                 },
             }
-            })
+        }
-            .collect();
 
         for msg in incoming {
             if let Some(pipeline_id) = msg.pipeline_id() {