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Temporary Values get Killed After the end of a Statement

October 11, 2025

Rust

Contents

1. The Problem

Consider the following function where self.cache is of type HashMap<String, BlockChain>:

    async fn get_index(&self) -> HttpResponse {
        let blockchain = self.cache.lock().unwrap().get("blockchain").unwrap();
        let first_block = blockchain[0].clone();
        let block_json = serde_json::to_string(&first_block).unwrap();
        debug!("block_json {:?}", block_json);
        HttpResponse::Ok().json(block_json)
    }

Problem:

The reason is that:

  1. self.cache.lock() is a MutexGuard object with life time 'a
  2. self.cache.lock().get has signature: 
    impl<K, V> HashMap<K, V> {
    pub fn get<'a, Q>(&'a self, k: &Q) -> Option<&'a V>
  3. self.cache.lock().get("key").unwrap() is a &BlockChain object, i.e., a pointer
  4. Pointer can never be moved, it will be copied whenever we assign it to another variable. Now blockchain = self.cache.lock().get("key").unwrap() is a copy of a pointer
  5. self.cache.lock() is a temporary value, now it gets dropped at the end of the statement after ;
  6. self.cache.lock().get is now borrowing a dropped value, hence crashed

Remark. By dropped we mean that the value has executed its drop method implemented for the Drop trait. Most of the time it is a cleanup process for freeing the memory, but it is not always true.

2. Solution

We simply move the temporary data to a local variable which has longer life span (so that the .get() makes sense):

    async fn get_index(&self) -> HttpResponse {
        let unlocked_cache = self.cache.lock().unwrap();
        let blockchain = unlocked_cache.get("blockchain").unwrap();
        let first_block = blockchain[0].clone();
        let block_json = serde_json::to_string(&first_block).unwrap();
        debug!("block_json {:?}", block_json);
        HttpResponse::Ok().json(block_json)
    }