Implications of Arc<K>: Borrow<Q>

[JayKickliter]

For my particular use case I'm finding moka::sync::Cache requirement of Arc<K>: Borrow<Q> very restrictive, as my keytype is somewhat expensive to create. Is this an absolute requirement for this kind of cache or an implementation detail?

Expected behavior

Using HashMap as an example, I'd expect to be able to querey using a &[u8] when the key type is Vec<u8>.

    use std::collections::HashMap;
    let mut map: HashMap<Vec<u8>, ()> = HashMap::new();
    map.insert(vec![1_u8], ());
    map.contains_key([1_u8].as_slice());

Failing example, simplified

    use moka::sync::Cache;
    let cache: Cache<Vec<u8>, ()> = Cache::new(1);
    cache.insert(vec![1_u8], ());
    cache.contains_key([1_u8].as_slice());

error[E0277]: the trait bound `Arc<Vec<u8>>: Borrow<[u8]>` is not satisfied
   --> src/main.rs:5:11
    |
5   |     cache.contains_key([1_u8].as_slice());
    |           ^^^^^^^^^^^^ the trait `Borrow<[u8]>` is not implemented for `Arc<Vec<u8>>`
    |
    = help: the trait `Borrow<T>` is implemented for `Arc<T>`
note: required by a bound in `moka::sync::Cache::<K, V, S>::contains_key`
   --> /Users/jay/.cargo/registry/src/github.com-1ecc6299db9ec823/moka-0.9.0/src/sync/cache.rs:880:17
    |
880 |         Arc<K>: Borrow<Q>,
    |                 ^^^^^^^^^ required by this bound in `moka::sync::Cache::<K, V, S>::contains_key`

EDIT: I now wonder if this could be considered a bug after noticing the following.

This also seems to imply that the docs contains_key are incorrect:

The key may be any borrowed form of the cache’s key type,

[tatsuya6502]

@JayKickliter — Thank you for reporting. I have converted this into a GitHub issue #166 and let me try to fix it.

Is this an absolute requirement for this kind of cache or an implementation detail?

I would say it is an implementation detail.

Moka cache wraps K by Arc when storing K to the internal storage, and the trait bound Arc<K>: Borrow<Q> does not work when K is Vec<u8> and Q is &[u8]. (Of course, it works when Q is &Vec<u8>). The same thing happens when K is String and Q is &str.

I tried briefly and could not fix. But there has got to be a way.

[tatsuya6502]

Hi @JayKickliter,

I found a solution for sync and future caches. I am working on it here:

• Change the trait bound Arc<K>: Borrow<Q> to K: Borrow<Q> (sync and future caches) #167
  • Added unit test

If it goes well, I will publish Moka v0.9.1 with the solution.

[tatsuya6502]

Hi @JayKickliter,

I have merged the above pull request, and published Moka v0.9.1 to crates.io. I believe it will fix your issue. Thanks!

[JayKickliter]

I tried briefly and could not fix

Me too, but the code base is much more expansive than I realized and I got lost.