Borrow problem in Rust

gistfile1.rs

  /// Add a new UTXO to the set
  pub fn add_utxo(&mut self, txo: TxOut, txid: Sha256dHash, vout: uint) -> bool {
    let txid = txid.as_bitv();
    // Construct node if needed
    let node = match self.tree.lookup_mut(&txid) {
      Some(node) => node,
      None => {
        self.tree.insert(&txid, UtxoNode { out: vec![] });
        // Note: If this fails, it indicates a bug in our program and/or an OOM
        // condition. But maybe we should do something better than task failure
        // anyway?
        self.tree.lookup_mut(&txid).unwrap()
      }
    };
    // Insert the output
    node.out.grow_set(vout, &None, Some(txo));
    // Return success
    return true;
  }