Add ExactSizeIterator implementation to Chain

Author: mendess

library/core/src/iter/adapters/chain.rs

@@ -273,6 +273,16 @@ where
 {
 }
 
+impl<A, B> ExactSizeIterator for Chain<A, B>
+where
+    A: ExactSizeIterator,
+    B: ExactSizeIterator,
+{
+    fn len(&self) -> usize {
+        self.a.len() + self.b.len()
+    }
+}
+
 #[stable(feature = "default_iters", since = "1.70.0")]
 impl<A: Default, B: Default> Default for Chain<A, B> {
     /// Creates a `Chain` from the default values for `A` and `B`.