Update twenty-first/src/util_types/merkle_tree.rs

Co-authored-by: Jan Ferdinand Sauer <ferdinand@neptune.cash>

Author: aszepieniec

twenty-first/src/util_types/merkle_tree.rs

@@ -316,10 +316,14 @@ impl MerkleTree {
 
     /// All leafs of the Merkle tree.
     pub fn leafs(&self) -> impl Iterator<Item = &Digest> {
-        self.nodes.iter().skip(
-            (self.num_nodes() / 2)
-                .try_into()
-                .expect("MerkleTreeNodeIndex to usize conversion error"),
+        // This conversion can only fail if the number of leafs is larger than
+        // usize::MAX. This implies that the number of nodes is larger than
+        // usize::MAX. Since the nodes are stored in a Vec, the number of nodes
+        // can never exceed usize::MAX.
+        // This proof by contradiction shows that unwrapping is fine.
+        let num_leafs = usize::try_from(self.num_leafs()).unwrap();
+
+        self.nodes.iter().skip(num_leafs)
         )
     }