wasm-bindgen-futures: use queueMicrotask for next tick runs (rust…

…wasm#3203)

but use `catch` to wrap it in a Result to be able to fall back to the
previous `Promsise.then` variant.

Signed-off-by: Dominik Csapak <[email protected]>

crates/futures/src/queue.rs

@@ -4,6 +4,12 @@ use std::collections::VecDeque;
 use std::rc::Rc;
 use wasm_bindgen::prelude::*;
 
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen(catch)]
+    fn queueMicrotask(closure: &Closure<dyn FnMut(JsValue)>) -> Result<(), JsValue>;
+}
+
 struct QueueState {
     // The queue of Tasks which are to be run in order. In practice this is all the
     // synchronous work of futures, and each `Task` represents calling `poll` on
@@ -48,10 +54,9 @@ impl Queue {
     // Schedule a task to run on the next tick
     pub(crate) fn schedule_task(&self, task: Rc<crate::task::Task>) {
         self.state.tasks.borrow_mut().push_back(task);
-        // Note that we currently use a promise and a closure to do this, but
-        // eventually we should probably use something like `queueMicrotask`:
-        // https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/queueMicrotask
-        if !self.state.is_scheduled.replace(true) {
+        // Use queueMicrotask to execute as soon as possible. If it does not exist
+        // fall back to the promise resolution
+        if !self.state.is_scheduled.replace(true) && queueMicrotask(&self.closure).is_err() {
             let _ = self.promise.then(&self.closure);
         }
     }