From 3eddc743f2ca3dbd5a5a41e3c2477ca9fbc7b97c Mon Sep 17 00:00:00 2001
From: Konrad Borowski <konrad@borowski.pw>
Date: Wed, 5 Dec 2018 16:44:10 +0100
Subject: [PATCH 1/2] Use inner iterator may_have_side_effect for Cloned

Previous implementation wasn't correct, as an inner iterator
could have had side effects.
---
 src/libcore/iter/mod.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 62e1f9fcb640c..de7ab8843daed 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -602,7 +602,9 @@ unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
     }
 
     #[inline]
-    fn may_have_side_effect() -> bool { false }
+    fn may_have_side_effect() -> bool {
+        I::may_have_side_effect()
+    }
 }
 
 #[unstable(feature = "trusted_len", issue = "37572")]

From a96430799987541b02aa9605f091dfc8368fa668 Mon Sep 17 00:00:00 2001
From: Konrad Borowski <konrad@borowski.pw>
Date: Wed, 5 Dec 2018 17:53:34 +0100
Subject: [PATCH 2/2] Add a test for cloned side effects

---
 src/libcore/tests/iter.rs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index 495483db5551c..19be1a07c5baa 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1249,6 +1249,23 @@ fn test_cloned() {
     assert_eq!(it.next_back(), None);
 }
 
+#[test]
+fn test_cloned_side_effects() {
+    let mut count = 0;
+    {
+        let iter = [1, 2, 3]
+            .iter()
+            .map(|x| {
+                count += 1;
+                x
+            })
+            .cloned()
+            .zip(&[1]);
+        for _ in iter {}
+    }
+    assert_eq!(count, 2);
+}
+
 #[test]
 fn test_double_ended_map() {
     let xs = [1, 2, 3, 4, 5, 6];