From d8ceb2ddf82455a8bdc82d9733e86d8c86195fb5 Mon Sep 17 00:00:00 2001
From: pmendes <pmendes@cloudflare.com>
Date: Thu, 18 Apr 2024 14:26:28 +0100
Subject: [PATCH] Add ExactSizeIterator implementation to Chain

---
 library/core/src/iter/adapters/chain.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/library/core/src/iter/adapters/chain.rs b/library/core/src/iter/adapters/chain.rs
index bcaac2f42cf04..aae80d6db1214 100644
--- a/library/core/src/iter/adapters/chain.rs
+++ b/library/core/src/iter/adapters/chain.rs
@@ -273,6 +273,18 @@ where
 {
 }
 
+#[stable(feature = "impl_exact_size_iterator_for_chain", since = "CURRENT_RUSTC_VERSION")]
+impl<A, B> ExactSizeIterator for Chain<A, B>
+where
+    A: ExactSizeIterator,
+    B: ExactSizeIterator<Item = A::Item>,
+{
+    fn len(&self) -> usize {
+        self.a.as_ref().map(|i| i.len()).unwrap_or_default()
+            + self.b.as_ref().map(|i| i.len()).unwrap_or_default()
+    }
+}
+
 #[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`.