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A HashMap<K, V> in Rust is a dynamically sized, heap-allocated collection that stores key-value pairs, providing average O(1)O(1) time complexity for lookups, insertions, and deletions. Under the hood, Rust’s standard library implements a SwissTable—an open-addressing hash table utilizing quadratic probing (specifically a triangular probing sequence) and SIMD-accelerated metadata lookups (ported from Google’s Abseil C++ library).

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Trait Requirements

For a type K to be used as a key in a HashMap, it must implement two specific traits from the standard library:
  1. std::cmp::Eq: Ensures strict equivalence relations. This is required to resolve hash collisions. Because floating-point types (f32, f64) possess NaN values that do not equal themselves, they only implement PartialEq, not Eq, and cannot be used directly as keys.
  2. std::hash::Hash: Allows the type to feed its internal state into a Hasher to compute a deterministic hash value.

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Default Hashing Algorithm

By default, HashMap utilizes SipHash 1-3. This algorithm is cryptographically resistant to HashDoS (Denial of Service) attacks, ensuring that malicious actors cannot intentionally trigger worst-case O(n)O(n) collision performance. Because SipHash prioritizes security over raw throughput, developers often override the default BuildHasher with faster, non-cryptographic algorithms (like ahash or FxHash) when operating on trusted keys.

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Initialization and Memory Allocation

A HashMap does not allocate heap memory until the first element is inserted unless explicitly instructed. 
use std::collections::HashMap;

// Zero-allocation initialization. Explicit type annotations satisfy the compiler.
let mut map: HashMap<String, i32> = HashMap::new();

// Pre-allocating memory for a known lower bound to prevent reallocation overhead.
let mut preallocated_map: HashMap<&str, i32> = HashMap::with_capacity(100);

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Ownership and Move Semantics

HashMap takes ownership of the keys and values inserted into it. If types do not implement the Copy trait, they are moved into the map, rendering the original variables inaccessible. 
let key = String::from("k1");
let val = String::from("v1");

let mut map: HashMap<String, String> = HashMap::new();
map.insert(key, val); // `key` and `val` are moved here.

// println!("{}", key); // COMPILER ERROR: value borrowed here after move
To store references, both the key and value must have lifetimes that outlive the HashMap: 
let key = String::from("k1");
let mut map: HashMap<&str, i32> = HashMap::new();

// Storing a string slice (reference) instead of an owned String
map.insert(&key, 42);

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Retrieval and Mutation

Because a key might not exist, retrieval methods return an Option enum containing a reference to the value, preventing null pointer dereferencing. When the key type K is a reference (e.g., &str), the get method requires a reference to that key type (&&str) due to how the Borrow trait is implemented. 
// Returns Option<&V>
if let Some(value) = map.get(&"k1") {
    println!("Found: {}", value);
}

// Returns Option<&mut V> for in-place mutation
if let Some(value_mut) = map.get_mut(&"k1") {
    *value_mut += 10;
}

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The Entry API

Rust provides the Entry API to handle conditional insertion and mutation without requiring multiple hash lookups. It returns an enum (Occupied or Vacant) representing the state of the bucket. Unlike get, the entry method takes the key by value. 
let mut map: HashMap<&str, i32> = HashMap::new();

// Inserts 100 if "k1" does not exist. Returns a mutable reference to the value.
map.entry("k1").or_insert(100);

// In-place mutation using a closure if the key exists, otherwise inserts a default.
map.entry("k1")
    .and_modify(|e| *e += 1)
    .or_insert(1);
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