feat(smt): make leaf-emptiness knowable at the trait level

src/merkle/smt/full/mod.rs

@@ -263,6 +263,13 @@ impl SparseMerkleTree<SMT_DEPTH> for Smt {
         leaf.hash()
     }
 
+    fn is_leaf_empty(leaf: &SmtLeaf) -> bool {
+        match leaf {
+            SmtLeaf::Empty(_) => true,
+            _ => false,
+        }
+    }
+
     fn hash_prospective_leaf(&self, key: &RpoDigest, value: &Word) -> RpoDigest {
         // Future work could avoid cloning the leaf by mirroring some of the insertion logic and
         // hashing without an intermediate leaf, but cloning is only expensive for multi-leaves,

src/merkle/smt/mod.rs

@@ -167,6 +167,10 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
     /// Returns the hash of a leaf
     fn hash_leaf(leaf: &Self::Leaf) -> RpoDigest;
 
+    /// Returns true if a leaf is completely sparse and does not hold any key-value pairs.
+    #[allow(dead_code)]
+    fn is_leaf_empty(leaf: &Self::Leaf) -> bool;
+
     /// Returns the hash of a leaf if the leaf WERE inserted into the tree,
     /// without performing any insertion or other mutation.
     ///

src/merkle/smt/simple/mod.rs

@@ -302,6 +302,10 @@ impl<const DEPTH: u8> SparseMerkleTree<DEPTH> for SimpleSmt<DEPTH> {
         leaf.into()
     }
 
+    fn is_leaf_empty(leaf: &Word) -> bool {
+        *leaf == Self::EMPTY_VALUE
+    }
+
     fn hash_prospective_leaf(&self, _key: &LeafIndex<DEPTH>, value: &Word) -> RpoDigest {
         Self::hash_leaf(value)
     }