Implications of Arc<K>: Borrow<Q>

[tatsuya6502]

Discussed in #163

^{Originally posted by JayKickliter July 9, 2022}
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]

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]

I am closing this issue.

• Resolved for the sync and future caches via Change the trait bound Arc<K>: Borrow<Q> to K: Borrow<Q> (sync and future caches) #167:
  • I published Moka v0.9.1 with the this fix.
• Won't fix for the unsync and experimental dash caches:
  • I could not find a way to fix it for them without modifying the source codes of std::collections::HashMap (for the unsync cache) and dashmap::DashMap (for the dash cache)