Replace memo queue with append-only vector (#767)

* replace memo queue with append-only vector

* reuse `DeletedEntries` list across revisions

Author: ibraheemdev

Diff for: src/function.rs

@@ -296,7 +296,8 @@ where
                 self.memo_ingredient_indices.get(ingredient_index),
             )
         });
-        std::mem::take(&mut self.deleted_entries);
+
+        self.deleted_entries.clear();
     }
 
     fn fmt_index(&self, index: crate::Id, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {

Diff for: src/function/delete.rs

@@ -1,15 +1,13 @@
 use std::ptr::NonNull;
 
-use crossbeam_queue::SegQueue;
-
 use super::memo::Memo;
 use super::Configuration;
 
 /// Stores the list of memos that have been deleted so they can be freed
 /// once the next revision starts. See the comment on the field
 /// `deleted_entries` of [`FunctionIngredient`][] for more details.
 pub(super) struct DeletedEntries<C: Configuration> {
-    seg_queue: SegQueue<SharedBox<Memo<C::Output<'static>>>>,
+    memos: boxcar::Vec<SharedBox<Memo<C::Output<'static>>>>,
 }
 
 unsafe impl<T: Send> Send for SharedBox<T> {}
@@ -18,30 +16,32 @@ unsafe impl<T: Sync> Sync for SharedBox<T> {}
 impl<C: Configuration> Default for DeletedEntries<C> {
     fn default() -> Self {
         Self {
-            seg_queue: Default::default(),
+            memos: Default::default(),
         }
     }
 }
 
 impl<C: Configuration> DeletedEntries<C> {
     /// # Safety
     ///
-    /// The memo must be valid and safe to free when the `DeletedEntries` list is dropped.
+    /// The memo must be valid and safe to free when the `DeletedEntries` list is cleared or dropped.
     pub(super) unsafe fn push(&self, memo: NonNull<Memo<C::Output<'_>>>) {
         let memo = unsafe {
             std::mem::transmute::<NonNull<Memo<C::Output<'_>>>, NonNull<Memo<C::Output<'static>>>>(
                 memo,
             )
         };
 
-        self.seg_queue.push(SharedBox(memo));
+        self.memos.push(SharedBox(memo));
+    }
+
+    /// Free all deleted memos, keeping the list available for reuse.
+    pub(super) fn clear(&mut self) {
+        self.memos.clear();
     }
 }
 
 /// A wrapper around `NonNull` that frees the allocation when it is dropped.
-///
-/// `crossbeam::SegQueue` does not expose mutable accessors so we have to create
-/// a wrapper to run code during `Drop`.
 struct SharedBox<T>(NonNull<T>);
 
 impl<T> Drop for SharedBox<T> {