shardtree: Fix pruning of annotated `Parent` nodes with `Nil` children.

The current behavior of `unite` incorrectly discards annotation data.

shardtree/CHANGELOG.md

@@ -24,6 +24,9 @@ and this project adheres to Rust's notion of
 - Fixes an error that could occur if an inserted `Frontier` node was
   interpreted as a node that had actually had its value observed as though it
   had been inserted using the ordinary tree insertion methods.
+- Fixes an error in an internal method that could result in subtree root
+  annotation data being discarded when pruning a `Parent` node having
+  `Nil` nodes for both its left and right children.
 
 ## [0.3.1] - 2024-04-03
 

shardtree/src/prunable.rs

@@ -312,7 +312,7 @@ impl<H: Hashable + Clone + PartialEq> PrunableTree<H> {
     /// `level` must be the level of the two nodes that are being joined.
     pub(crate) fn unite(level: Level, ann: Option<Arc<H>>, left: Self, right: Self) -> Self {
         match (left, right) {
-            (Tree(Node::Nil), Tree(Node::Nil)) => Tree::empty(),
+            (Tree(Node::Nil), Tree(Node::Nil)) if ann.is_none() => Tree::empty(),
             (Tree(Node::Leaf { value: lv }), Tree(Node::Leaf { value: rv }))
                 // we can prune right-hand leaves that are not marked or reference leaves; if a
                 // leaf is a checkpoint then that information will be propagated to the replacement