From 6999c421ef7a80104c0799ce8d800a98d0ae85c0 Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Fri, 2 Oct 2015 18:41:06 -0400
Subject: [PATCH] libcore: Chain must exhaust a before b.

part of #28810
---
 src/libcore/iter.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/libcore/iter.rs b/src/libcore/iter.rs
index abf50250dc2ef..bd24cd4609e4b 100644
--- a/src/libcore/iter.rs
+++ b/src/libcore/iter.rs
@@ -1559,7 +1559,12 @@ impl<A, B> Iterator for Chain<A, B> where
     #[inline]
     fn last(self) -> Option<A::Item> {
         match self.state {
-            ChainState::Both => self.b.last().or(self.a.last()),
+            ChainState::Both => {
+                // Must exhaust a before b.
+                let a_last = self.a.last();
+                let b_last = self.b.last();
+                b_last.or(a_last)
+            },
             ChainState::Front => self.a.last(),
             ChainState::Back => self.b.last()
         }