参考下面的代码：

use std::collections::HashMap;
use std::sync::RwLock;
    
#[derive(Debug, PartialEq, Eq)]
pub enum RespType<'a> {
    SimpleString(&'a str),
    RespError((&'a str, &'a str)),
    Integer(i32),
    BulkString(Option<&'a str>),
    Array(Vec<RespType<'a>>),
}
    
type Db<'a> = RwLock<HashMap<&'a str, RespType<'a>>>;
pub trait KeyValueStore<'a> {
    fn get_value<'b>(&'a self, key: &'b str, callback: &'a dyn Fn(&'a RespType<'a>));
    fn set_value(&'a mut self, key: &'a str, value: RespType<'a>) -> &'a RespType<'a>;
}
    
impl<'a> KeyValueStore<'a> for Db<'a> {
    fn get_value<'b>(&'a self, key: &'b str, callback: &'a dyn Fn(&'a RespType<'a>)) {
        static null_bulk_string:RespType = RespType::BulkString(None);
        let guard = self.read().unwrap();
        let result = guard.get(key);
        match result{
            Some(v) => {callback(v)}
            None => {callback(&null_bulk_string)}
        };
    }
    
    fn set_value(&'a mut self, key: &'a str, value: RespType<'a>) -> &'static RespType<'static> {
        static ok: RespType = RespType::SimpleString("OK");
        (*self.get_mut().unwrap()).insert(key, value).unwrap();
        &ok
    }
}
    
fn main(){
    
}

错误消息如下所示：

error[E0597]: `guard` does not live long enough
  --> src/main.rs:23:22
   |
19 | impl<'a> KeyValueStore<'a> for Db<'a> {
   |      -- lifetime `'a` defined here
...
23 |         let result = guard.get(key);
   |                      ^^^^^^^^^^^^^^
   |                      |
   |                      borrowed value does not live long enough
   |                      argument requires that `guard` is borrowed for `'a`
...
28 |     }
   |     - `guard` dropped here while still borrowed

读锁在get_value结束时被丢弃，但我似乎无法将其传递给callback，callback显然没有超过读锁的生存期。这是怎么回事？我也不明白错误消息想说什么。有人能解释一下吗？