---

yaml
---
r: 168883
b: refs/heads/snap-stage3
c: cd85f0a
h: refs/heads/master
i:
  168881: 0be9382
  168879: 94cb62d
v: v3

Author: steveklabnik

[refs]

@@ -1,7 +1,7 @@
 ---
 refs/heads/master: 5e21e17d9638d14af41e27e5ca9a21c8a1bc0170
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
-refs/heads/snap-stage3: 20fa7cbfc0b5143f6c88e288772fc0f65610c2ec
+refs/heads/snap-stage3: cd85f0a56a790f24bb1c1928fbc37c9d24dd2937
 refs/heads/try: 5204084bd2e46af7cc6e0147430e44dd0d657bbb
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/dist-snap: ba4081a5a8573875fed17545846f6f6902c8ba8d

branches/snap-stage3/src/doc/guide-testing.md

@@ -556,8 +556,12 @@ extern crate test;
 # fn main() {
 # struct X; impl X { fn iter<T>(&self, _: || -> T) {} } let b = X;
 b.iter(|| {
-    test::black_box(range(0u, 1000).fold(0, |old, new| old ^ new));
-});
+    let mut n = 1000_u32;
+
+    test::black_box(&mut n); // pretend to modify `n`
+
+    range(0, n).fold(0, |a, b| a ^ b)
+})
 # }
 ```
 
@@ -573,3 +577,6 @@ test bench_xor_1000_ints ... bench:       1 ns/iter (+/- 0)
 
 test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
 ```
+
+However, the optimizer can still modify a testcase in an undesirable manner
+even when using either of the above.