transfer-contract: add last_pos contract call

contracts/transfer/src/lib.rs

@@ -104,6 +104,11 @@ unsafe fn existing_nullifiers(arg_len: u32) -> u32 {
     })
 }
 
+#[no_mangle]
+unsafe fn last_pos(arg_len: u32) -> u32 {
+    rusk_abi::wrap_call(arg_len, |_: ()| STATE.last_pos())
+}
+
 // "Feeder" queries
 
 #[no_mangle]

contracts/transfer/src/state.rs

@@ -434,6 +434,11 @@ impl TransferState {
         self.tree.root()
     }
 
+    /// Get the position of the last note in the tree.
+    pub fn last_pos(&self) -> u64 {
+        self.tree.leaves_len()
+    }
+
     /// Get the opening
     pub fn opening(
         &self,

contracts/transfer/src/tree.rs

@@ -97,4 +97,8 @@ impl Tree {
     ) -> Option<PoseidonOpening<(), TRANSFER_TREE_DEPTH, A>> {
         self.tree.opening(pos)
     }
+
+    pub fn leaves_len(&self) -> u64 {
+        self.leaves.len() as u64
+    }
 }