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

src/merkle/smt/full/mod.rs

@@ -285,6 +285,13 @@ impl SparseMerkleTree<SMT_DEPTH> for Smt {
         }
     }
 
+    fn is_leaf_empty(leaf: &SmtLeaf) -> bool {
+        match leaf {
+            SmtLeaf::Empty(_) => true,
+            _ => false,
+        }
+    }
+
     fn key_to_leaf_index(key: &RpoDigest) -> LeafIndex<SMT_DEPTH> {
         let most_significant_felt = key[3];
         LeafIndex::new_max_depth(most_significant_felt.as_int())

src/merkle/smt/mod.rs

@@ -167,6 +167,13 @@ 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 what `existing_leaf` would look like if `key` and `value` WERE inserted into the
+    /// tree, without mutating the tree itself.
+    ///
     /// `existing_leaf` must have the same index as the key, or the result will be meaningless. To
     /// get a prospective leaf based on the current state of the tree, use `self.get_leaf(key)` as
     /// the argument for `existing_leaf`. The return value from this function can be chained back

src/merkle/smt/simple/mod.rs

@@ -311,6 +311,10 @@ impl<const DEPTH: u8> SparseMerkleTree<DEPTH> for SimpleSmt<DEPTH> {
         *value
     }
 
+    fn is_leaf_empty(leaf: &Word) -> bool {
+        *leaf == Self::EMPTY_VALUE
+    }
+
     fn key_to_leaf_index(key: &LeafIndex<DEPTH>) -> LeafIndex<DEPTH> {
         *key
     }