指定返回的闭包引用的生命周期

Question

鉴于此结构：

struct Token<'a> {
     r: &'a Regex,
     f: &'a Fn(String) -> Symbol
}

从fn：

返回时，我无法找出正确的语法来指定闭包生命周期

fn get_tokens<'a>() -> Vec<Token<'a>> {
    static ref NUMBER: Regex = Regex::new(r"$(\d+|\d*\.\d+)").unwrap(); // TODO avoid "000"?
    let num_token = Token{ r: &*NUMBER, f: &|s| Symbol::Number(s) };
    let tokens = vec![num_token];

    tokens
}

如果没有生命周期参数，我会得到借来的值不够长的错误：

error: borrowed value does not live long enough
   --> src\syntax\lex.rs:49:45
   |
49 |     let num_token = Token{ r: &*NUMBER, f: &|s| Symbol::Number(s) };
   |                                             ^^^^^^^^^^^^^^^^^^^^^ temporary value created here
...
53 | }
   | - temporary value only lives until here
   |
note: borrowed value must be valid for the lifetime 'a as defined on the block at 36:38...
     --> src\syntax\lex.rs:36:39
   |
36 | fn get_tokens<'a>() -> Vec<Token<'a>> {
   |                                       ^

我找不到任何显示为这样的闭包指定生命周期参数的示例。