Trait Hash

pub trait Hash {
    // Required method
    fn hash<H: Hasher>(&self, state: &mut H);

    // Provided method
    fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)
       where Self: Sized { ... }
}

A hashable type.

Types implementing Hash are able to be hashed with an instance of Hasher.

Implementing Hash

You can derive Hash with #[derive(Hash)] if all fields implement Hash. The resulting hash will be the combination of the values from calling hash on each field.

#[derive(Hash)]
struct Rustacean {
    name: String,
    country: String,
}

If you need more control over how a value is hashed, you can of course implement the Hash trait yourself:

use std::hash::{Hash, Hasher};

struct Person {
    id: u32,
    name: String,
    phone: u64,
}

impl Hash for Person {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.id.hash(state);
        self.phone.hash(state);
    }
}

Hash and Eq

When implementing both Hash and Eq, it is important that the following property holds:

k1 == k2 -> hash(k1) == hash(k2)

In other words, if two keys are equal, their hashes must also be equal. HashMap and HashSet both rely on this behavior.

Thankfully, you won’t need to worry about upholding this property when deriving both Eq and Hash with #[derive(PartialEq, Eq, Hash)].

Violating this property is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on the correctness of these methods.

Prefix collisions

Implementations of hash should ensure that the data they pass to the Hasher are prefix-free. That is, values which are not equal should cause two different sequences of values to be written, and neither of the two sequences should be a prefix of the other.

For example, the standard implementation of Hash for &str passes an extra 0xFF byte to the Hasher so that the values ("ab", "c") and ("a", "bc") hash differently.

Portability

Due to differences in endianness and type sizes, data fed by Hash to a Hasher should not be considered portable across platforms. Additionally the data passed by most standard library types should not be considered stable between compiler versions.

This means tests shouldn’t probe hard-coded hash values or data fed to a Hasher and instead should check consistency with Eq.

Serialization formats intended to be portable between platforms or compiler versions should either avoid encoding hashes or only rely on Hash and Hasher implementations that provide additional guarantees.

Required Methods

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

Feeds this value into the given Hasher.

Examples

use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
7920.hash(&mut hasher);
println!("Hash is {:x}!", hasher.finish());

Provided Methods

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

where Self: Sized,

Feeds a slice of this type into the given Hasher.

This method is meant as a convenience, but its implementation is also explicitly left unspecified. It isn’t guaranteed to be equivalent to repeated calls of hash and implementations of Hash should keep that in mind and call hash themselves if the slice isn’t treated as a whole unit in the PartialEq implementation.

For example, a VecDeque implementation might naïvely call as_slices and then hash_slice on each slice, but this is wrong since the two slices can change with a call to make_contiguous without affecting the PartialEq result. Since these slices aren’t treated as singular units, and instead part of a larger deque, this method cannot be used.

Examples

use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
let numbers = [6, 28, 496, 8128];
Hash::hash_slice(&numbers, &mut hasher);
println!("Hash is {:x}!", hasher.finish());

Object Safety

This trait is not object safe.

Implementors

impl Hash for AsciiChar

impl Hash for core::cmp::Ordering

impl Hash for Infallible

impl Hash for IpAddr

impl Hash for Ipv6MulticastScope

impl Hash for SocketAddr

impl Hash for core::sync::atomic::Ordering

impl Hash for bool

impl Hash for char

impl Hash for i8

impl Hash for i16

impl Hash for i32

impl Hash for i64

impl Hash for i128

impl Hash for isize

impl Hash for !

impl Hash for str

impl Hash for u8

impl Hash for u16

impl Hash for u32

impl Hash for u64

impl Hash for u128

impl Hash for ()

impl Hash for usize

impl Hash for Layout

impl Hash for TypeId

impl Hash for CStr

impl Hash for Error

impl Hash for PhantomPinned

impl Hash for Ipv4Addr

impl Hash for Ipv6Addr

impl Hash for SocketAddrV4

impl Hash for SocketAddrV6

impl Hash for RangeFull

impl Hash for Alignment

impl Hash for Duration

impl<'a> Hash for Location<'a>

impl<B: Hash, C: Hash> Hash for ControlFlow<B, C>

impl<Dyn: ?Sized> Hash for DynMetadata<Dyn>

impl<F: FnPtr> Hash for F

impl<Idx: Hash> Hash for core::ops::Range<Idx>

impl<Idx: Hash> Hash for core::ops::RangeFrom<Idx>

impl<Idx: Hash> Hash for core::ops::RangeInclusive<Idx>

impl<Idx: Hash> Hash for RangeTo<Idx>

impl<Idx: Hash> Hash for RangeToInclusive<Idx>

impl<Idx: Hash> Hash for core::range::Range<Idx>

impl<Idx: Hash> Hash for core::range::RangeFrom<Idx>

impl<Idx: Hash> Hash for core::range::RangeInclusive<Idx>

impl<Ptr: Deref<Target: Hash>> Hash for Pin<Ptr>

impl<T> Hash for (T₁, T₂, …, Tₙ)

where T: ?Sized + Hash,

This trait is implemented for tuples up to twelve items long.

impl<T> Hash for Discriminant<T>

impl<T> Hash for NonZero<T>

where T: ZeroablePrimitive + Hash,

impl<T, const N: usize> Hash for Simd<T, N>

where LaneCount<N>: SupportedLaneCount, T: SimdElement + Hash,

impl<T: Hash + ?Sized> Hash for ManuallyDrop<T>

impl<T: Hash> Hash for Bound<T>

impl<T: Hash> Hash for Option<T>

impl<T: Hash> Hash for Poll<T>

impl<T: Hash> Hash for [T]

impl<T: Hash> Hash for Reverse<T>

impl<T: Hash> Hash for Saturating<T>

impl<T: Hash> Hash for Wrapping<T>

impl<T: Hash, E: Hash> Hash for Result<T, E>

impl<T: Hash, const N: usize> Hash for [T; N]

The hash of an array is the same as that of the corresponding slice, as required by the Borrow implementation.

use std::hash::BuildHasher;

let b = std::hash::RandomState::new();
let a: [u8; 3] = [0xa8, 0x3c, 0x09];
let s: &[u8] = &[0xa8, 0x3c, 0x09];
assert_eq!(b.hash_one(a), b.hash_one(s));

impl<T: ?Sized + Hash> Hash for &T

impl<T: ?Sized + Hash> Hash for &mut T

impl<T: ?Sized> Hash for *const T

impl<T: ?Sized> Hash for *mut T

impl<T: ?Sized> Hash for PhantomData<T>

impl<T: ?Sized> Hash for NonNull<T>

impl<Y: Hash, R: Hash> Hash for CoroutineState<Y, R>