Make Step::extend_with must_use.

examples/simulation.rs

@@ -439,7 +439,7 @@ fn main() {
             .build_with_transactions(txs.clone(), rng)
             .expect("instantiate QueueingHoneyBadger");
         let (sq, mut step) = SenderQueue::builder(qhb, peer_ids.into_iter()).build(our_id);
-        step.extend_with(qhb_step, Message::from);
+        assert!(step.extend_with(qhb_step, Message::from).is_empty());
         (sq, step)
     };
 

src/queueing_honey_badger/mod.rs

@@ -117,10 +117,7 @@ where
     }
 
     /// Creates a new Queueing Honey Badger instance with an empty buffer.
-    pub fn build<R: Rng>(self, rng: &mut R) -> Result<QueueingHoneyBadgerWithStep<T, N, Q>>
-    where
-        R: 'static + Rng + Send + Sync,
-    {
+    pub fn build<R: Rng>(self, rng: &mut R) -> Result<QueueingHoneyBadgerWithStep<T, N, Q>> {
         self.build_with_transactions(None, rng)
     }
 

src/traits.rs

@@ -114,6 +114,7 @@ impl<M, O, N> Step<M, O, N> {
     }
 
     /// Extends `self` with `other`s messages and fault logs, and returns `other.output`.
+    #[must_use]
     pub fn extend_with<M2, O2, FM>(&mut self, other: Step<M2, O2, N>, f_msg: FM) -> Vec<O2>
     where
         FM: Fn(M2) -> M,

tests/queueing_honey_badger.rs

@@ -164,9 +164,8 @@ where
     let mut rng = rand::thread_rng().gen::<Isaac64Rng>();
     let (qhb, qhb_step) =
         QueueingHoneyBadger::builder_joining(our_id, secret_key, join_plan, &mut rng)
-            .expect("failed to rebuild the node with a join plan")
-            .batch_size(3)
-            .build(&mut rng);
+            .and_then(|builder| builder.batch_size(3).build(&mut rng))
+            .expect("failed to rebuild the node with a join plan");
     let (sq, mut sq_step) = SenderQueue::builder(qhb, peer_ids.into_iter()).build(our_id);
     *node.instance_mut() = sq;
     sq_step.extend(qhb_step.map(|output| output, Message::from));
@@ -193,7 +192,7 @@ fn new_queueing_hb(
         .build(&mut rng)
         .expect("failed to build QueueingHoneyBadger");
     let (sq, mut step) = SenderQueue::builder(qhb, peer_ids).build(our_id);
-    step.extend(qhb_step.map(|output| output, Message::from));
+    assert!(step.extend_with(qhb_step, Message::from).is_empty());
     (sq, step)
 }