fix(cordyceps): add Drop impl for List (#203)

When a linked list's `Handle` type owns the allocation for the item in
the list, it is necessary to drain all entries from the list and drop
them when the `List` instance is dropped. Otherwise, the entries may be
leaked.

This fixes a leak reported by Miri.

Fixes #165

BREAKING CHANGE:

The `List::new` constructor now requires a `T: Linked<list::Links<T>>`
bound.

cordyceps/src/list.rs

@@ -206,7 +206,7 @@ mod tests;
 ///
 /// [intrusive]: crate#intrusive-data-structures
 /// [`list::Links<T>`]: crate::list::Links
-pub struct List<T: ?Sized> {
+pub struct List<T: Linked<Links<T>> + ?Sized> {
     head: Link<T>,
     tail: Link<T>,
     len: usize,
@@ -248,7 +248,8 @@ struct LinksInner<T: ?Sized> {
 }
 
 // ==== impl List ====
-impl<T: ?Sized> List<T> {
+
+impl<T: Linked<Links<T>> + ?Sized> List<T> {
     /// Returns a new empty list.
     #[must_use]
     pub const fn new() -> List<T> {
@@ -286,9 +287,7 @@ impl<T: ?Sized> List<T> {
     pub fn len(&self) -> usize {
         self.len
     }
-}
 
-impl<T: Linked<Links<T>> + ?Sized> List<T> {
     /// Asserts as many of the linked list's invariants as possible.
     pub fn assert_valid(&self) {
         let head = match self.head {
@@ -513,6 +512,16 @@ impl<T: Linked<Links<T>> + ?Sized> fmt::Debug for List<T> {
     }
 }
 
+impl<T: Linked<Links<T>> + ?Sized> Drop for List<T> {
+    fn drop(&mut self) {
+        while let Some(node) = self.pop_front() {
+            drop(node);
+        }
+
+        debug_assert!(self.is_empty());
+    }
+}
+
 // ==== impl Links ====
 
 impl<T: ?Sized> Links<T> {

cordyceps/src/list/tests.rs

@@ -624,11 +624,10 @@ proptest::proptest! {
 fn run_fuzz(ops: Vec<Op>) {
     use std::collections::VecDeque;
 
+    let entries: Vec<_> = (0..ops.len()).map(|i| entry(i as i32)).collect();
     let mut ll = List::<Entry<'_>>::new();
     let mut reference = VecDeque::new();
 
-    let entries: Vec<_> = (0..ops.len()).map(|i| entry(i as i32)).collect();
-
     for (i, op) in ops.iter().enumerate() {
         let _span = tracing::info_span!("op", ?i, ?op).entered();
         tracing::info!(?op);