impl Hash for Value

[edwardycl]

IndexMap is the only type that is used within Value that does not implement Hash, and it is by design: indexmap-rs/indexmap#288 (comment)

If "preserve_order" feature is not set, Value::Object uses BTreeMap instead of IndexMap so it is possible to auto-impl Hash for Value.

[dtolnay]

Cargo features are required to be additive, so enabling a feature cannot result in an impl being removed.

[edwardycl]

Now that I think about it, since the type of key is fixed to String, IndexMap can also be hashed by sorting its key-value in a consistent order. It doesn't break the Eq-Hash relationship, and we can implement Hash regardless of the "preserve_order" feature. What do you think?

[dtolnay]

Thanks!

For the cfg(feature = "preserve_order") case, maybe a better performing implementation would be to hash each entry individually and xor them together. This would make the hash insensitive to order.

use std::hash::{BuildHasher, RandomState};
use std::sync::OnceLock;

static RANDOM_STATE: OnceLock<RandomState> = OnceLock::new();
let random_state = RANDOM_STATE.get_or_init(RandomState::new);
self.map
    .iter()
    .map(|entry| random_state.hash_one(entry))
    .fold(0, |xor, next| xor ^ next)
    .hash(state);

I would be interested to see a benchmark comparing this approach.