Impl `Hash` for `IndexMap`.

[xuxiaocheng0201]

Sometimes, IndexMap is passed as a parameter. But many cache implementations are essentially a large hashmap. So if I want to cache the return value of a function which contains a parameter of type IndexMap, IndexMap must implements Hash. But currently is not.

I wrote a simple wrapper and it works fine:

use std::hash::{Hash, Hasher};

use derive_more::{Deref, DerefMut};
use indexmap::IndexMap;
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize, Clone, Eq, PartialEq, Deref, DerefMut)]
pub struct HashIndexMap<K: Hash + Eq, V>(pub IndexMap<K, V>);

impl<K: Hash + Eq, V: Hash> Hash for HashIndexMap<K, V> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        state.write_usize(self.0.len());
        for (k, v) in &self.0 {
            k.hash(state);
            v.hash(state);
        }
    }
}

[cuviper]

The problem is inconsistency between Eq and Hash -- see #288. However, we have since published the ordermap crate, which is a thin wrapper around indexmap that does use map order for both Eq and Hash, and Ord as well.

Your own wrapper can work too, but you should re-implement PartialEq using iterator order, rather than deriving it.