Crate indexmap

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IndexMap is a hash table where the iteration order of the key-value pairs is independent of the hash values of the keys.

IndexSet is a corresponding hash set using the same implementation and with similar properties.


IndexMap and IndexSet are drop-in compatible with the std HashMap and HashSet, but they also have some features of note:

  • The ordering semantics (see their documentation for details)
  • Sorting methods and the .pop() methods.
  • The Equivalent trait, which offers more flexible equality definitions between borrowed and owned versions of keys.
  • The MutableKeys trait, which gives opt-in mutable access to hash map keys.

Feature Flags

To reduce the amount of compiled code in the crate by default, certain features are gated behind feature flags. These allow you to opt in to (or out of) functionality. Below is a list of the features available in this crate.

Note: only the std feature is enabled by default.

Alternate Hashers

IndexMap and IndexSet have a default hasher type S = RandomState, just like the standard HashMap and HashSet, which is resistant to HashDoS attacks but not the most performant. Type aliases can make it easier to use alternate hashers:

use fnv::FnvBuildHasher;
use fxhash::FxBuildHasher;
use indexmap::{IndexMap, IndexSet};

type FnvIndexMap<K, V> = IndexMap<K, V, FnvBuildHasher>;
type FnvIndexSet<T> = IndexSet<T, FnvBuildHasher>;

type FxIndexMap<K, V> = IndexMap<K, V, FxBuildHasher>;
type FxIndexSet<T> = IndexSet<T, FxBuildHasher>;

let std: IndexSet<i32> = (0..100).collect();
let fnv: FnvIndexSet<i32> = (0..100).collect();
let fx: FxIndexSet<i32> = (0..100).collect();
assert_eq!(std, fnv);
assert_eq!(std, fx);

Rust Version

This version of indexmap requires Rust 1.64 or later.

The indexmap 2.x release series will use a carefully considered version upgrade policy, where in a later 2.x version, we will raise the minimum required Rust version.

No Standard Library Targets

This crate supports being built without std, requiring alloc instead. This is chosen by disabling the default “std” cargo feature, by adding default-features = false to your dependency specification.



  • IndexMap is a hash table where the iteration order of the key-value pairs is independent of the hash values of the keys.
  • A hash set implemented using IndexMap


  • Create an IndexMap from a list of key-value pairs
  • Create an IndexSet from a list of values