handle empty containers on race for vec and array

Author: matheus-consoli

src/future/race/array.rs

@@ -70,6 +70,7 @@ where
     type Future = Race<Fut::IntoFuture, N>;
 
     fn race(self) -> Self::Future {
+        assert!(N > 0, "race requires at least one future");
         Race {
             futures: self.map(|fut| fut.into_future()),
             indexer: Indexer::new(N),
@@ -83,6 +84,13 @@ mod test {
     use super::*;
     use core::future;
 
+    #[test]
+    #[should_panic(expected = "race requires at least one future")]
+    fn empty_array() {
+        let futs: [future::Ready<()>; 0] = [];
+        let _ = futs.race();
+    }
+
     // NOTE: we should probably poll in random order.
     #[test]
     fn no_fairness() {

src/future/race/mod.rs

@@ -22,5 +22,10 @@ pub trait Race {
     /// other futures are cancelled.
     ///
     /// This function returns a new future which polls all futures concurrently.
+    ///
+    /// # Panics
+    ///
+    /// This method will panic if the collection is empty (for `Vec` and array
+    /// implementations) since there would be no future to race.
     fn race(self) -> Self::Future;
 }

src/future/race/vec.rs

@@ -73,6 +73,7 @@ where
     type Future = Race<Fut::IntoFuture>;
 
     fn race(self) -> Self::Future {
+        assert!(!self.is_empty(), "race requires at least one future");
         Race {
             indexer: Indexer::new(self.len()),
             futures: self.into_iter().map(|fut| fut.into_future()).collect(),
@@ -87,6 +88,15 @@ mod test {
     use alloc::vec;
     use core::future;
 
+    #[test]
+    #[should_panic(expected = "race requires at least one future")]
+    fn empty_vec() {
+        futures_lite::future::block_on(async {
+            let futs: Vec<future::Ready<()>> = vec![];
+            let _ = futs.race().await;
+        });
+    }
+
     // NOTE: we should probably poll in random order.
     #[test]
     fn no_fairness() {