这个工作的Rust代码重复本身有些：

fn append_column(tree: &TreeView, attribute: &str, id: i32) {
    let column = TreeViewColumn::new();
    match attribute {
        "text" => {
            let cell = CellRendererText::new();    
            TreeViewColumnExt::pack_start(&column, &cell, true);
            TreeViewColumnExt::add_attribute(&column, &cell, attribute, id);
        }
        "pixbuf" => {
            let cell = CellRendererPixbuf::new();
            TreeViewColumnExt::pack_start(&column, &cell, true);
            TreeViewColumnExt::add_attribute(&column, &cell, attribute, id);
        }
        _ => { panic!("Unexpected attribute") }
    }
    tree.append_column(&column);
}

我尝试使用downcast()来简化它，如下所示：

fn append_column(tree: &TreeView, attribute: &str, id: i32) {
    let column = TreeViewColumn::new();
    let cell : CellRenderer;
    match attribute {
        "text" => { cell = CellRendererText::new().downcast::<CellRenderer>().unwrap() }
        "pixbuf" => { cell = CellRendererPixbuf::new().downcast::<CellRenderer>().unwrap() }
        _ => { panic!("Unexpected attribute") }
    }
    TreeViewColumnExt::pack_start(&column, &cell, true);
    TreeViewColumnExt::add_attribute(&column, &cell, attribute, id);
    tree.append_column(&column);
}

但我得到了以下错误：

error[E0277]: the trait bound `CellRenderer: IsA<CellRendererText>` is not satisfied
   --> src/main.rs:23:52
    |
23  |         "text" => { cell = CellRendererText::new().downcast::<CellRenderer>().unwrap() }
    |                                                    ^^^^^^^^ the trait `IsA<CellRendererText>` is not implemented for `CellRenderer`

我能去掉重复的吗？