1 files changed, 29 insertions, 0 deletions
diff --git a/library/alloc/tests/btree_set_hash.rs b/library/alloc/tests/btree_set_hash.rs
new file mode 100644
index 000000000..ab275ac43
--- /dev/null
+++ b/library/alloc/tests/btree_set_hash.rs
@@ -0,0 +1,29 @@
+use crate::hash;
+use std::collections::BTreeSet;
+
+#[test]
+fn test_hash() {
+    let mut x = BTreeSet::new();
+    let mut y = BTreeSet::new();
+
+    x.insert(1);
+    x.insert(2);
+    x.insert(3);
+
+    y.insert(3);
+    y.insert(2);
+    y.insert(1);
+
+    assert_eq!(hash(&x), hash(&y));
+}
+
+#[test]
+fn test_prefix_free() {
+    let x = BTreeSet::from([1, 2, 3]);
+    let y = BTreeSet::<i32>::new();
+
+    // If hashed by iteration alone, `(x, y)` and `(y, x)` would visit the same
+    // order of elements, resulting in the same hash. But now that we also hash
+    // the length, they get distinct sequences of hashed data.
+    assert_ne!(hash(&(&x, &y)), hash(&(&y, &x)));
+}