Add failing leak detection test

Author: CAD97

crates/slice-dst/tests/leak.rs

@@ -0,0 +1,61 @@
+use {
+    slice_dst::*,
+    std::sync::atomic::{AtomicUsize, Ordering::SeqCst},
+};
+
+struct DropTracking<'a> {
+    place: &'a AtomicUsize,
+}
+
+impl<'a> DropTracking<'a> {
+    fn new(place: &'a AtomicUsize) -> Self {
+        place.fetch_add(1, SeqCst);
+        DropTracking { place }
+    }
+}
+
+impl Drop for DropTracking<'_> {
+    fn drop(&mut self) {
+        self.place.fetch_sub(1, SeqCst);
+    }
+}
+
+#[test]
+fn bad_exactsizeiterator() {
+    struct Iter<'a> {
+        counter: &'a AtomicUsize,
+        len: usize,
+    }
+
+    impl ExactSizeIterator for Iter<'_> {
+        fn len(&self) -> usize {
+            self.len
+        }
+    }
+
+    impl<'a> Iterator for Iter<'a> {
+        type Item = DropTracking<'a>;
+
+        fn next(&mut self) -> Option<Self::Item> {
+            match self.len {
+                0 | 1 => None,
+                _ => {
+                    self.len -= 1;
+                    Some(DropTracking::new(self.counter))
+                }
+            }
+        }
+    }
+
+    let mut counter = AtomicUsize::new(0);
+    let _ = std::panic::catch_unwind(|| {
+        let _: Box<_> = SliceWithHeader::new::<Box<_>, _>(
+            (),
+            Iter {
+                counter: &counter,
+                len: 5,
+            },
+        );
+    });
+    assert_eq!(*counter.get_mut(), 0);
+}