Implement Debug for JsFuture

crates/futures/src/lib.rs

@@ -104,6 +104,7 @@
 #![deny(missing_docs)]
 
 use std::cell::{Cell, RefCell};
+use std::fmt;
 use std::rc::Rc;
 use std::sync::Arc;
 
@@ -128,6 +129,12 @@ pub struct JsFuture {
     callbacks: Option<(Closure<FnMut(JsValue)>, Closure<FnMut(JsValue)>)>,
 }
 
+impl fmt::Debug for JsFuture {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "JsFuture {{ ... }}")
+    }
+}
+
 impl From<Promise> for JsFuture {
     fn from(js: Promise) -> JsFuture {
         // Use the `then` method to schedule two callbacks, one for the