indexmap 1.6.2

A hash table with consistent order and fast iteration. The indexmap is a hash table where the iteration order of the key-value pairs is independent of the hash values of the keys. It has the usual hash table functionality, it preserves insertion order except after removals, and it allows lookup of its elements by either hash table key or numerical index. A corresponding hash set type is also provided. This crate was initially published under the name ordermap, but it was renamed to indexmap.
Documentation 
//! Functions to serialize and deserialize an `IndexMap` as an ordered sequence.
//!
//! The default `serde` implementation serializes `IndexMap` as a normal map,
//! but there is no guarantee that serialization formats will preserve the order
//! of the key-value pairs. This module serializes `IndexMap` as a sequence of
//! `(key, value)` elements instead, in order.
//!
//! This module may be used in a field attribute for derived implementations:
//!
//! ```
//! # use indexmap::IndexMap;
//! # use serde_derive::{Deserialize, Serialize};
//! #[derive(Deserialize, Serialize)]
//! struct Data {
//!     #[serde(with = "indexmap::serde_seq")]
//!     map: IndexMap<i32, u64>,
//!     // ...
//! }
//! ```
//!
//! Requires crate feature `"serde"` or `"serde-1"`

use serde::de::{Deserialize, Deserializer, SeqAccess, Visitor};
use serde::ser::{Serialize, Serializer};

use core::fmt::{self, Formatter};
use core::hash::{BuildHasher, Hash};
use core::marker::PhantomData;

use crate::IndexMap;

/// Serializes an `IndexMap` as an ordered sequence.
///
/// This function may be used in a field attribute for deriving `Serialize`:
///
/// ```
/// # use indexmap::IndexMap;
/// # use serde_derive::Serialize;
/// #[derive(Serialize)]
/// struct Data {
///     #[serde(serialize_with = "indexmap::serde_seq::serialize")]
///     map: IndexMap<i32, u64>,
///     // ...
/// }
/// ```
///
/// Requires crate feature `"serde"` or `"serde-1"`
pub fn serialize<K, V, S, T>(map: &IndexMap<K, V, S>, serializer: T) -> Result<T::Ok, T::Error>
where
    K: Serialize + Hash + Eq,
    V: Serialize,
    S: BuildHasher,
    T: Serializer,
{
    serializer.collect_seq(map)
}

/// Visitor to deserialize a *sequenced* `IndexMap`
struct SeqVisitor<K, V, S>(PhantomData<(K, V, S)>);

impl<'de, K, V, S> Visitor<'de> for SeqVisitor<K, V, S>
where
    K: Deserialize<'de> + Eq + Hash,
    V: Deserialize<'de>,
    S: Default + BuildHasher,
{
    type Value = IndexMap<K, V, S>;

    fn expecting(&self, formatter: &mut Formatter<'_>) -> fmt::Result {
        write!(formatter, "a sequenced map")
    }

    fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
    where
        A: SeqAccess<'de>,
    {
        let capacity = seq.size_hint().unwrap_or(0);
        let mut map = IndexMap::with_capacity_and_hasher(capacity, S::default());

        while let Some((key, value)) = seq.next_element()? {
            map.insert(key, value);
        }

        Ok(map)
    }
}

/// Deserializes an `IndexMap` from an ordered sequence.
///
/// This function may be used in a field attribute for deriving `Deserialize`:
///
/// ```
/// # use indexmap::IndexMap;
/// # use serde_derive::Deserialize;
/// #[derive(Deserialize)]
/// struct Data {
///     #[serde(deserialize_with = "indexmap::serde_seq::deserialize")]
///     map: IndexMap<i32, u64>,
///     // ...
/// }
/// ```
///
/// Requires crate feature `"serde"` or `"serde-1"`
pub fn deserialize<'de, D, K, V, S>(deserializer: D) -> Result<IndexMap<K, V, S>, D::Error>
where
    D: Deserializer<'de>,
    K: Deserialize<'de> + Eq + Hash,
    V: Deserialize<'de>,
    S: Default + BuildHasher,
{
    deserializer.deserialize_seq(SeqVisitor(PhantomData))
}