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 
#[cfg(has_std)]
#[macro_export]
/// Create an `IndexMap` from a list of key-value pairs
///
/// ## Example
///
/// ```
/// use indexmap::indexmap;
///
/// let map = indexmap!{
///     "a" => 1,
///     "b" => 2,
/// };
/// assert_eq!(map["a"], 1);
/// assert_eq!(map["b"], 2);
/// assert_eq!(map.get("c"), None);
///
/// // "a" is the first key
/// assert_eq!(map.keys().next(), Some(&"a"));
/// ```
macro_rules! indexmap {
    (@single $($x:tt)*) => (());
    (@count $($rest:expr),*) => (<[()]>::len(&[$($crate::indexmap!(@single $rest)),*]));

    ($($key:expr => $value:expr,)+) => { $crate::indexmap!($($key => $value),+) };
    ($($key:expr => $value:expr),*) => {
        {
            let _cap = $crate::indexmap!(@count $($key),*);
            let mut _map = $crate::IndexMap::with_capacity(_cap);
            $(
                _map.insert($key, $value);
            )*
            _map
        }
    };
}

#[cfg(has_std)]
#[macro_export]
/// Create an `IndexSet` from a list of values
///
/// ## Example
///
/// ```
/// use indexmap::indexset;
///
/// let set = indexset!{
///     "a",
///     "b",
/// };
/// assert!(set.contains("a"));
/// assert!(set.contains("b"));
/// assert!(!set.contains("c"));
///
/// // "a" is the first value
/// assert_eq!(set.iter().next(), Some(&"a"));
/// ```
macro_rules! indexset {
    (@single $($x:tt)*) => (());
    (@count $($rest:expr),*) => (<[()]>::len(&[$($crate::indexset!(@single $rest)),*]));

    ($($value:expr,)+) => { $crate::indexset!($($value),+) };
    ($($value:expr),*) => {
        {
            let _cap = $crate::indexset!(@count $($value),*);
            let mut _set = $crate::IndexSet::with_capacity(_cap);
            $(
                _set.insert($value);
            )*
            _set
        }
    };
}

// generate all the Iterator methods by just forwarding to the underlying
// self.iter and mapping its element.
macro_rules! iterator_methods {
    // $map_elt is the mapping function from the underlying iterator's element
    // same mapping function for both options and iterators
    ($map_elt:expr) => {
        fn next(&mut self) -> Option<Self::Item> {
            self.iter.next().map($map_elt)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }

        fn count(self) -> usize {
            self.iter.len()
        }

        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            self.iter.nth(n).map($map_elt)
        }

        fn last(mut self) -> Option<Self::Item> {
            self.next_back()
        }

        fn collect<C>(self) -> C
        where
            C: FromIterator<Self::Item>,
        {
            // NB: forwarding this directly to standard iterators will
            // allow it to leverage unstable traits like `TrustedLen`.
            self.iter.map($map_elt).collect()
        }
    };
}

macro_rules! double_ended_iterator_methods {
    // $map_elt is the mapping function from the underlying iterator's element
    // same mapping function for both options and iterators
    ($map_elt:expr) => {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.iter.next_back().map($map_elt)
        }
    };
}