Add tests for Extend<(T, U)> for (ExtendT, ExtendU)

ordering of side-effects to `coretest`.

Author: steffahn

library/coretests/tests/iter/traits/collect.rs

@@ -0,0 +1,34 @@
+#[test]
+fn test_extend_tuple_side_effects() {
+    use std::cell::RefCell;
+    use std::iter::zip;
+    
+    struct TrackingExtender<'a, T>(
+        &'static str,
+        &'a RefCell<Vec<(&'static str, Vec<T>)>>,
+        Vec<T>,
+    );
+    impl<T: Clone> Extend<T> for TrackingExtender<'_, T> {
+        fn extend<I: IntoIterator<Item = T>>(&mut self, i: I) {
+            let items = Vec::from_iter(i);
+            self.1.borrow_mut().push((self.0, items.clone()));
+            self.2.extend(items);
+        }
+    }
+
+    let effects = RefCell::new(vec![]);
+    let l = TrackingExtender("l", &effects, vec![]);
+    let r = TrackingExtender("r", &effects, vec![]);
+    let mut p = ((l, r), ());
+    p.extend(zip([(1, 2), (3, 4)], [(), ()]));
+    let effects = effects.into_inner();
+    assert_eq!(
+        effects,
+        [
+            ("l", vec![1]),
+            ("r", vec![2]),
+            ("l", vec![3]),
+            ("r", vec![4]),
+        ],
+    );
+}

library/coretests/tests/iter/traits/mod.rs

@@ -1,4 +1,5 @@
 mod accum;
+mod collect;
 mod double_ended;
 mod iterator;
 mod step;