Mark js_sys::Promise as #[must_use] (#1927)

* Mark js_sys::Promise as #[must_use]

* Fix js_sys::Promise #[must_use] warnings

Author: silvanshade

crates/futures/src/lib.rs

@@ -158,7 +158,7 @@ impl From<Promise> for JsFuture {
             Closure::once(move |val| finish(&state, Err(val)))
         };
 
-        js.then2(&resolve, &reject);
+        let _ = js.then2(&resolve, &reject);
 
         state.borrow_mut().callbacks = Some((resolve, reject));
 

crates/futures/src/queue.rs

@@ -56,7 +56,7 @@ impl Queue {
         // eventually we should probably use something like `queueMicrotask`:
         // https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/queueMicrotask
         if !self.state.is_spinning.replace(true) {
-            self.promise.then(&self.closure);
+            let _ = self.promise.then(&self.closure);
         }
     }
 }

crates/js-sys/src/lib.rs

@@ -4567,6 +4567,7 @@ extern "C" {
     /// an asynchronous operation, and its resulting value.
     ///
     /// [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
+    #[must_use]
     #[wasm_bindgen(extends = Object)]
     #[derive(Clone, Debug)]
     pub type Promise;