1 files changed, 26 insertions, 0 deletions
diff --git a/compiler/rustc_index/src/bit_set/tests.rs b/compiler/rustc_index/src/bit_set/tests.rs
index 0350740aa81..eaa4aafe721 100644
--- a/compiler/rustc_index/src/bit_set/tests.rs
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -76,6 +76,32 @@ fn union_two_sets() {
 }
 
 #[test]
+fn union_not() {
+    let mut a = DenseBitSet::<usize>::new_empty(100);
+    let mut b = DenseBitSet::<usize>::new_empty(100);
+
+    a.insert(3);
+    a.insert(5);
+    a.insert(80);
+    a.insert(81);
+
+    b.insert(5); // Already in `a`.
+    b.insert(7);
+    b.insert(63);
+    b.insert(81); // Already in `a`.
+    b.insert(90);
+
+    a.union_not(&b);
+
+    // After union-not, `a` should contain all values in the domain, except for
+    // the ones that are in `b` and were _not_ already in `a`.
+    assert_eq!(
+        a.iter().collect::<Vec<_>>(),
+        (0usize..100).filter(|&x| !matches!(x, 7 | 63 | 90)).collect::<Vec<_>>(),
+    );
+}
+
+#[test]
 fn chunked_bitset() {
     let mut b0 = ChunkedBitSet::<usize>::new_empty(0);
     let b0b = b0.clone();