“借来的数据不能存储在其关闭之外”是什么意思?

fn main() {
    let mut fields = Vec::new();
    let pusher = &mut |a: &str| {
        fields.push(a);
    };
}

error: borrowed data cannot be stored outside of its closure
 --> src/main.rs:4:21
  |
3 |     let pusher = &mut |a: &str| {
  |         ------        --------- ...because it cannot outlive this closure
  |         |
  |         borrowed data cannot be stored into here...
4 |         fields.push(a);
  |                     ^ cannot be stored outside of its closure

这个错误是什么意思?我如何修复我的代码?