Struct im::HashMap

pub struct HashMap<K, V, S = RandomState> where
    K: Clone,
    V: Clone,  { /* fields omitted */ }

An unordered map.

An immutable hash map using hash array mapped tries.

Most operations on this map are O(log_x n) for a suitably high x that it should be nearly O(1) for most maps. Because of this, it's a great choice for a generic map as long as you don't mind that keys will need to implement Hash and Eq.

Map entries will have a predictable order based on the hasher being used. Unless otherwise specified, all maps will share an instance of the default RandomState hasher, which will produce consistent hashes for the duration of its lifetime, but not between restarts of your program.

Methods

impl<K, V> HashMap<K, V, RandomState> where     K: Hash + Eq + Clone,     V: Clone,

pub fn new() -> Self

Construct an empty hash map.

pub fn singleton(k: K, v: V) -> HashMap<K, V>

Construct a hash map with a single mapping.

Examples

let map = HashMap::singleton(123, "onetwothree");
assert_eq!(
  map.get(&123),
  Some(&"onetwothree")
);

impl<K, V, S> HashMap<K, V, S> where     K: Clone,     V: Clone,

pub fn is_empty(&self) -> bool

Test whether a hash map is empty.

Time: O(1)

Examples

assert!(
  !hashmap!{1 => 2}.is_empty()
);
assert!(
  HashMap::<i32, i32>::new().is_empty()
);

pub fn len(&self) -> usize

Get the size of a hash map.

Time: O(1)

Examples

assert_eq!(3, hashmap!{
  1 => 11,
  2 => 22,
  3 => 33
}.len());

impl<K, V, S> HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher,

pub fn with_hasher<RS>(hasher: RS) -> Self where     Arc<S>: From<RS>,

Construct an empty hash map using the provided hasher.

pub fn hasher(&self) -> &Arc<S>

Get a reference to the map's BuildHasher.

pub fn new_from<K1, V1>(&self) -> HashMap<K1, V1, S> where     K1: Hash + Eq + Clone,     V1: Clone,

Construct an empty hash map using the same hasher as the current hash map.

Important traits for Iter<'a, K, V>

impl<'a, K, V> Iterator for Iter<'a, K, V> type Item = &'a (K, V);

pub fn iter(&self) -> Iter<K, V>

Get an iterator over the key/value pairs of a hash map.

Please note that the order is consistent between maps using the same hasher, but no other ordering guarantee is offered. Items will not come out in insertion order or sort order. They will, however, come out in the same order every time for the same map.

Important traits for IterMut<'a, K, V>

impl<'a, K, V> Iterator for IterMut<'a, K, V> where
    K: Clone,
    V: Clone,  type Item = &'a mut V;

pub fn iter_mut(&mut self) -> IterMut<K, V>

Get a mutable iterator over the values of a hash map.

Please note that the order is consistent between maps using the same hasher, but no other ordering guarantee is offered. Items will not come out in insertion order or sort order. They will, however, come out in the same order every time for the same map.

Important traits for Keys<'a, K, V>

impl<'a, K, V> Iterator for Keys<'a, K, V> type Item = &'a K;

pub fn keys(&self) -> Keys<K, V>

Get an iterator over a hash map's keys.

Please note that the order is consistent between maps using the same hasher, but no other ordering guarantee is offered. Items will not come out in insertion order or sort order. They will, however, come out in the same order every time for the same map.

Important traits for Values<'a, K, V>

impl<'a, K, V> Iterator for Values<'a, K, V> type Item = &'a V;

pub fn values(&self) -> Values<K, V>

Get an iterator over a hash map's values.

Please note that the order is consistent between maps using the same hasher, but no other ordering guarantee is offered. Items will not come out in insertion order or sort order. They will, however, come out in the same order every time for the same map.

pub fn get<BK: ?Sized>(&self, key: &BK) -> Option<&V> where     BK: Hash + Eq,     K: Borrow<BK>,

Get the value for a key from a hash map.

Time: O(log n)

Examples

let map = hashmap!{123 => "lol"};
assert_eq!(
  map.get(&123),
  Some(&"lol")
);

pub fn get_mut<BK: ?Sized>(&mut self, key: &BK) -> Option<&mut V> where     BK: Hash + Eq,     K: Borrow<BK>,

Get a mutable reference to the value for a key from a hash map.

Time: O(log n)

Examples

let map = hashmap!{123 => "lol"};
assert_eq!(
  map.get(&123),
  Some(&"lol")
);

pub fn contains_key<BK: ?Sized>(&self, k: &BK) -> bool where     BK: Hash + Eq,     K: Borrow<BK>,

Test for the presence of a key in a hash map.

Time: O(log n)

Examples

let map = hashmap!{123 => "lol"};
assert!(
  map.contains_key(&123)
);
assert!(
  !map.contains_key(&321)
);

pub fn insert(&mut self, k: K, v: V) -> Option<V>

Insert a key/value mapping into a map.

If the map already has a mapping for the given key, the previous value is overwritten.

Time: O(log n)

Examples

let mut map = hashmap!{};
map.insert(123, "123");
map.insert(456, "456");
assert_eq!(
  map,
  hashmap!{123 => "123", 456 => "456"}
);

pub fn remove<BK: ?Sized>(&mut self, k: &BK) -> Option<V> where     BK: Hash + Eq,     K: Borrow<BK>,

Remove a key/value pair from a map, if it exists, and return the removed value.

This is a copy-on-write operation, so that the parts of the set's structure which are shared with other sets will be safely copied before mutating.

Time: O(log n)

Examples

let mut map = hashmap!{123 => "123", 456 => "456"};
assert_eq!(Some("123"), map.remove(&123));
assert_eq!(Some("456"), map.remove(&456));
assert_eq!(None, map.remove(&789));
assert!(map.is_empty());

pub fn remove_with_key<BK: ?Sized>(&mut self, k: &BK) -> Option<(K, V)> where     BK: Hash + Eq,     K: Borrow<BK>,

Remove a key/value pair from a map, if it exists, and return the removed key and value.

Time: O(log n)

Examples

let mut map = hashmap!{123 => "123", 456 => "456"};
assert_eq!(Some((123, "123")), map.remove_with_key(&123));
assert_eq!(Some((456, "456")), map.remove_with_key(&456));
assert_eq!(None, map.remove_with_key(&789));
assert!(map.is_empty());

pub fn clear(&mut self)

Discard all elements from the map.

This leaves you with an empty map, and all elements that were previously inside it are dropped.

Time: O(n)

Examples

let mut map = hashmap![1=>1, 2=>2, 3=>3];
map.clear();
assert!(map.is_empty());

pub fn entry(&mut self, key: K) -> Entry<K, V, S>

Get the Entry for a key in the map for in-place manipulation.

Time: O(log n)

pub fn update(&self, k: K, v: V) -> Self

Construct a new hash map by inserting a key/value mapping into a map.

If the map already has a mapping for the given key, the previous value is overwritten.

Time: O(log n)

Examples

let map = hashmap!{};
assert_eq!(
  map.update(123, "123"),
  hashmap!{123 => "123"}
);

pub fn update_with<F>(&self, k: K, v: V, f: F) -> Self where     F: FnOnce(V, V) -> V,

Construct a new hash map by inserting a key/value mapping into a map.

If the map already has a mapping for the given key, we call the provided function with the old value and the new value, and insert the result as the new value.

Time: O(log n)

pub fn update_with_key<F>(&self, k: K, v: V, f: F) -> Self where     F: FnOnce(&K, V, V) -> V,

Construct a new map by inserting a key/value mapping into a map.

If the map already has a mapping for the given key, we call the provided function with the key, the old value and the new value, and insert the result as the new value.

Time: O(log n)

pub fn update_lookup_with_key<F>(&self, k: K, v: V, f: F) -> (Option<V>, Self) where     F: FnOnce(&K, &V, V) -> V,

Construct a new map by inserting a key/value mapping into a map, returning the old value for the key as well as the new map.

If the map already has a mapping for the given key, we call the provided function with the key, the old value and the new value, and insert the result as the new value.

Time: O(log n)

pub fn alter<F>(&self, f: F, k: K) -> Self where     F: FnOnce(Option<V>) -> Option<V>,

Update the value for a given key by calling a function with the current value and overwriting it with the function's return value.

The function gets an Option<V> and returns the same, so that it can decide to delete a mapping instead of updating the value, and decide what to do if the key isn't in the map.

Time: O(log n)

pub fn without<BK: ?Sized>(&self, k: &BK) -> Self where     BK: Hash + Eq,     K: Borrow<BK>,

Construct a new map without the given key.

Construct a map that's a copy of the current map, absent the mapping for key if it's present.

Time: O(log n)

pub fn extract<BK: ?Sized>(&self, k: &BK) -> Option<(V, Self)> where     BK: Hash + Eq,     K: Borrow<BK>,

Remove a key/value pair from a map, if it exists, and return the removed value as well as the updated map.

Time: O(log n)

pub fn extract_with_key<BK: ?Sized>(&self, k: &BK) -> Option<(K, V, Self)> where     BK: Hash + Eq,     K: Borrow<BK>,

Remove a key/value pair from a map, if it exists, and return the removed key and value as well as the updated list.

Time: O(log n)

pub fn union(self, other: Self) -> Self

Construct the union of two maps, keeping the values in the current map when keys exist in both maps.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 3 => 3};
let map2 = hashmap!{2 => 2, 3 => 4};
let expected = hashmap!{1 => 1, 2 => 2, 3 => 3};
assert_eq!(expected, map1.union(map2));

pub fn union_with<F>(self, other: Self, f: F) -> Self where     F: FnMut(V, V) -> V,

Construct the union of two maps, using a function to decide what to do with the value when a key is in both maps.

The function is called when a value exists in both maps, and receives the value from the current map as its first argument, and the value from the other map as the second. It should return the value to be inserted in the resulting map.

Time: O(n log n)

pub fn union_with_key<F>(self, other: Self, f: F) -> Self where     F: FnMut(&K, V, V) -> V,

Construct the union of two maps, using a function to decide what to do with the value when a key is in both maps.

The function is called when a value exists in both maps, and receives a reference to the key as its first argument, the value from the current map as the second argument, and the value from the other map as the third argument. It should return the value to be inserted in the resulting map.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 3 => 4};
let map2 = hashmap!{2 => 2, 3 => 5};
let expected = hashmap!{1 => 1, 2 => 2, 3 => 9};
assert_eq!(expected, map1.union_with_key(
    map2,
    |key, left, right| left + right
));

pub fn unions<I>(i: I) -> Self where     S: Default,     I: IntoIterator<Item = Self>,

Construct the union of a sequence of maps, selecting the value of the leftmost when a key appears in more than one map.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 3 => 3};
let map2 = hashmap!{2 => 2};
let expected = hashmap!{1 => 1, 2 => 2, 3 => 3};
assert_eq!(expected, HashMap::unions(vec![map1, map2]));

pub fn unions_with<I, F>(i: I, f: F) -> Self where     S: Default,     I: IntoIterator<Item = Self>,     F: Fn(V, V) -> V,

Construct the union of a sequence of maps, using a function to decide what to do with the value when a key is in more than one map.

The function is called when a value exists in multiple maps, and receives the value from the current map as its first argument, and the value from the next map as the second. It should return the value to be inserted in the resulting map.

Time: O(n log n)

pub fn unions_with_key<I, F>(i: I, f: F) -> Self where     S: Default,     I: IntoIterator<Item = Self>,     F: Fn(&K, V, V) -> V,

Construct the union of a sequence of maps, using a function to decide what to do with the value when a key is in more than one map.

The function is called when a value exists in multiple maps, and receives a reference to the key as its first argument, the value from the current map as the second argument, and the value from the next map as the third argument. It should return the value to be inserted in the resulting map.

Time: O(n log n)

pub fn difference(self, other: Self) -> Self

Construct the difference between two maps by discarding keys which occur in both maps.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 3 => 4};
let map2 = hashmap!{2 => 2, 3 => 5};
let expected = hashmap!{1 => 1, 2 => 2};
assert_eq!(expected, map1.difference(map2));

pub fn difference_with<F>(self, other: Self, f: F) -> Self where     F: FnMut(V, V) -> Option<V>,

Construct the difference between two maps by using a function to decide what to do if a key occurs in both.

Time: O(n log n)

pub fn difference_with_key<F>(self, other: Self, f: F) -> Self where     F: FnMut(&K, V, V) -> Option<V>,

Construct the difference between two maps by using a function to decide what to do if a key occurs in both. The function receives the key as well as both values.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 3 => 4};
let map2 = hashmap!{2 => 2, 3 => 5};
let expected = hashmap!{1 => 1, 2 => 2, 3 => 9};
assert_eq!(expected, map1.difference_with_key(
    map2,
    |key, left, right| Some(left + right)
));

pub fn intersection(self, other: Self) -> Self

Construct the intersection of two maps, keeping the values from the current map.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 2 => 2};
let map2 = hashmap!{2 => 3, 3 => 4};
let expected = hashmap!{2 => 2};
assert_eq!(expected, map1.intersection(map2));

pub fn intersection_with<B, C, F>(     self,     other: HashMap<K, B, S>,     f: F ) -> HashMap<K, C, S> where     B: Clone,     C: Clone,     F: FnMut(V, B) -> C,

Construct the intersection of two maps, calling a function with both values for each key and using the result as the value for the key.

Time: O(n log n)

pub fn intersection_with_key<B, C, F>(     self,     other: HashMap<K, B, S>,     f: F ) -> HashMap<K, C, S> where     B: Clone,     C: Clone,     F: FnMut(&K, V, B) -> C,

Construct the intersection of two maps, calling a function with the key and both values for each key and using the result as the value for the key.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 2 => 2};
let map2 = hashmap!{2 => 3, 3 => 4};
let expected = hashmap!{2 => 5};
assert_eq!(expected, map1.intersection_with_key(
    map2,
    |key, left, right| left + right
));

pub fn is_submap_by<B, RM, F>(&self, other: RM, cmp: F) -> bool where     B: Clone,     F: FnMut(&V, &B) -> bool,     RM: Borrow<HashMap<K, B, S>>,

Test whether a map is a submap of another map, meaning that all keys in our map must also be in the other map, with the same values.

Use the provided function to decide whether values are equal.

Time: O(n log n)

pub fn is_proper_submap_by<B, RM, F>(&self, other: RM, cmp: F) -> bool where     B: Clone,     F: FnMut(&V, &B) -> bool,     RM: Borrow<HashMap<K, B, S>>,

Test whether a map is a proper submap of another map, meaning that all keys in our map must also be in the other map, with the same values. To be a proper submap, ours must also contain fewer keys than the other map.

Use the provided function to decide whether values are equal.

Time: O(n log n)

pub fn is_submap<RM>(&self, other: RM) -> bool where     V: PartialEq,     RM: Borrow<Self>,

Test whether a map is a submap of another map, meaning that all keys in our map must also be in the other map, with the same values.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 2 => 2};
let map2 = hashmap!{1 => 1, 2 => 2, 3 => 3};
assert!(map1.is_submap(map2));

pub fn is_proper_submap<RM>(&self, other: RM) -> bool where     V: PartialEq,     RM: Borrow<Self>,

Test whether a map is a proper submap of another map, meaning that all keys in our map must also be in the other map, with the same values. To be a proper submap, ours must also contain fewer keys than the other map.

Time: O(n log n)

Examples

let map1 = hashmap!{1 => 1, 2 => 2};
let map2 = hashmap!{1 => 1, 2 => 2, 3 => 3};
assert!(map1.is_proper_submap(map2));

let map3 = hashmap!{1 => 1, 2 => 2};
let map4 = hashmap!{1 => 1, 2 => 2};
assert!(!map3.is_proper_submap(map4));

Trait Implementations

impl<K: Ord + Hash + Eq + Clone, V: Clone, S: BuildHasher> From<HashMap<K, V, S>> for OrdMap<K, V>

fn from(m: HashMap<K, V, S>) -> Self

Performs the conversion.

impl<'a, K: Ord + Hash + Eq + Clone, V: Clone, S: BuildHasher> From<&'a HashMap<K, V, S>> for OrdMap<K, V>

fn from(m: &'a HashMap<K, V, S>) -> Self

Performs the conversion.

impl<K, V, S> Clone for HashMap<K, V, S> where     K: Clone,     V: Clone,

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<K, V, S> PartialEq for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: PartialEq + Clone,     S: BuildHasher,

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl<K, V, S> Eq for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Eq + Clone,     S: BuildHasher,

impl<K, V, S> PartialOrd for HashMap<K, V, S> where     K: Hash + Eq + Clone + PartialOrd,     V: PartialOrd + Clone,     S: BuildHasher,

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<K, V, S> Ord for HashMap<K, V, S> where     K: Hash + Eq + Ord + Clone,     V: Ord + Clone,     S: BuildHasher,

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

impl<K, V, S> Hash for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Hash + Clone,     S: BuildHasher,

fn hash<H>(&self, state: &mut H) where     H: Hasher,

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where     H: Hasher,

Feeds a slice of this type into the given [Hasher].

impl<K, V, S> Default for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn default() -> Self

Returns the "default value" for a type.

impl<K, V, S> Add for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher,

type Output = HashMap<K, V, S>

The resulting type after applying the + operator.

fn add(self, other: Self) -> Self::Output

Performs the + operation.

impl<'a, K, V, S> Add for &'a HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher,

type Output = HashMap<K, V, S>

The resulting type after applying the + operator.

fn add(self, other: Self) -> Self::Output

Performs the + operation.

impl<K, V, S> Sum for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn sum<I>(it: I) -> Self where     I: Iterator<Item = Self>,

Method which takes an iterator and generates Self from the elements by "summing up" the items.

impl<K, V, S, RK, RV> Extend<(RK, RV)> for HashMap<K, V, S> where     K: Hash + Eq + Clone + From<RK>,     V: Clone + From<RV>,     S: BuildHasher,

fn extend<I>(&mut self, iter: I) where     I: IntoIterator<Item = (RK, RV)>,

Extends a collection with the contents of an iterator.

impl<'a, BK: ?Sized, K, V, S> Index<&'a BK> for HashMap<K, V, S> where     BK: Hash + Eq,     K: Hash + Eq + Clone + Borrow<BK>,     V: Clone,     S: BuildHasher,

type Output = V

The returned type after indexing.

fn index(&self, key: &BK) -> &Self::Output

Performs the indexing (container[index]) operation.

impl<'a, BK: ?Sized, K, V, S> IndexMut<&'a BK> for HashMap<K, V, S> where     BK: Hash + Eq,     K: Hash + Eq + Clone + Borrow<BK>,     V: Clone,     S: BuildHasher,

fn index_mut(&mut self, key: &BK) -> &mut Self::Output

Performs the mutable indexing (container[index]) operation.

impl<K, V, S> Debug for HashMap<K, V, S> where     K: Hash + Eq + Clone + Debug,     V: Debug + Clone,     S: BuildHasher,

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher,

type Item = &'a (K, V)

The type of the elements being iterated over.

type IntoIter = Iter<'a, K, V>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<K, V, S> IntoIterator for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher,

type Item = (K, V)

The type of the elements being iterated over.

type IntoIter = ConsumingIter<(K, V)>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from_iter<T>(i: T) -> Self where     T: IntoIterator<Item = (K, V)>,

Creates a value from an iterator.

impl<K, V, S> AsRef<HashMap<K, V, S>> for HashMap<K, V, S> where     K: Clone,     V: Clone,

fn as_ref(&self) -> &Self

Performs the conversion.

impl<'m, 'k, 'v, K: ?Sized, V: ?Sized, OK, OV, SA, SB> From<&'m HashMap<&'k K, &'v V, SA>> for HashMap<OK, OV, SB> where     K: Hash + Eq + ToOwned<Owned = OK>,     V: ToOwned<Owned = OV>,     OK: Hash + Eq + Clone + Borrow<K>,     OV: Borrow<V> + Clone,     SA: BuildHasher,     SB: BuildHasher + Default,

fn from(m: &HashMap<&K, &V, SA>) -> Self

Performs the conversion.

impl<'a, K, V, S> From<&'a [(K, V)]> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: &'a [(K, V)]) -> Self

Performs the conversion.

impl<K, V, S> From<Vec<(K, V)>> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: Vec<(K, V)>) -> Self

Performs the conversion.

impl<'a, K, V, S> From<&'a Vec<(K, V)>> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: &'a Vec<(K, V)>) -> Self

Performs the conversion.

impl<K, V, S> From<HashMap<K, V>> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: HashMap<K, V>) -> Self

Performs the conversion.

impl<'a, K, V, S> From<&'a HashMap<K, V>> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: &'a HashMap<K, V>) -> Self

Performs the conversion.

impl<K, V, S> From<BTreeMap<K, V>> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: BTreeMap<K, V>) -> Self

Performs the conversion.

impl<'a, K, V, S> From<&'a BTreeMap<K, V>> for HashMap<K, V, S> where     K: Hash + Eq + Clone,     V: Clone,     S: BuildHasher + Default,

fn from(m: &'a BTreeMap<K, V>) -> Self

Performs the conversion.