Trait core::hash::Hash

1.0.0 · source ·
pub trait Hash {
    fn hash<H: Hasher>(&self, state: &mut H);

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

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,
}
Run

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);
    }
}
Run

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)].

Prefix collisions

Implementations of hash should ensure that the data they pass to the Hasher are prefix-free. That is, unequal values 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§

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

Feeds this value into the given Hasher.

Examples
use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};

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

Provided Methods§

1.3.0 · source

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::collections::hash_map::DefaultHasher;
use std::hash::{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());
Run

Implementors§

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impl Hash for core::cmp::Ordering

1.44.0 · source§

impl Hash for Infallible

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impl Hash for Which

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

const: unstable · source§

impl Hash for bool

const: unstable · source§

impl Hash for char

const: unstable · source§

impl Hash for i8

const: unstable · source§

impl Hash for i16

const: unstable · source§

impl Hash for i32

const: unstable · source§

impl Hash for i64

const: unstable · source§

impl Hash for i128

const: unstable · source§

impl Hash for isize

1.29.0 (const: unstable) · source§

impl Hash for !

const: unstable · source§

impl Hash for str

const: unstable · source§

impl Hash for u8

const: unstable · source§

impl Hash for u16

const: unstable · source§

impl Hash for u32

const: unstable · source§

impl Hash for u64

const: unstable · source§

impl Hash for u128

const: unstable · source§

impl Hash for ()

const: unstable · source§

impl Hash for usize

1.28.0 · source§

impl Hash for Layout

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impl Hash for TypeId

1.64.0 · source§

impl Hash for CStr

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impl Hash for Error

1.33.0 · source§

impl Hash for PhantomPinned

1.34.0 · source§

impl Hash for NonZeroI8

1.34.0 · source§

impl Hash for NonZeroI16

1.34.0 · source§

impl Hash for NonZeroI32

1.34.0 · source§

impl Hash for NonZeroI64

1.34.0 · source§

impl Hash for NonZeroI128

1.34.0 · source§

impl Hash for NonZeroIsize

1.28.0 · source§

impl Hash for NonZeroU8

1.28.0 · source§

impl Hash for NonZeroU16

1.28.0 · source§

impl Hash for NonZeroU32

1.28.0 · source§

impl Hash for NonZeroU64

1.28.0 · source§

impl Hash for NonZeroU128

1.28.0 · source§

impl Hash for NonZeroUsize

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impl Hash for RangeFull

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impl Hash for Alignment

1.3.0 · source§

impl Hash for Duration

1.10.0 · source§

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

1.55.0 · source§

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

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

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impl<Idx: Hash> Hash for Range<Idx>

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

1.26.0 · source§

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

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impl<Idx: Hash> Hash for RangeTo<Idx>

1.26.0 · source§

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

1.41.0 · source§

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

1.4.0 · source§

impl<Ret, T> Hash for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

1.4.0 · source§

impl<Ret, T> Hash for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

1.4.0 · source§

impl<Ret, T> Hash for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

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impl<Ret, T> Hash for extern "C-unwind" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

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impl<Ret, T> Hash for extern "C-unwind" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

1.4.0 · source§

impl<Ret, T> Hash for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

1.4.0 · source§

impl<Ret, T> Hash for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

1.4.0 · source§

impl<Ret, T> Hash for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

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impl<Ret, T> Hash for unsafe extern "C-unwind" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

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impl<Ret, T> Hash for unsafe extern "C-unwind" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

const: unstable · source§

impl<T> Hash for (T₁, T₂, …, Tₙ)where
    T: ?Sized + Hash,

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

1.21.0 · source§

impl<T> Hash for Discriminant<T>

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impl<T, const LANES: usize> Hash for Simd<T, LANES>where
    LaneCount<LANES>: SupportedLaneCount,
    T: SimdElement + Hash,

1.20.0 · source§

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

1.17.0 · source§

impl<T: Hash> Hash for Bound<T>

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impl<T: Hash> Hash for Option<T>

1.36.0 · source§

impl<T: Hash> Hash for Poll<T>

const: unstable · source§

impl<T: Hash> Hash for [T]

1.19.0 · source§

impl<T: Hash> Hash for Reverse<T>

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impl<T: Hash> Hash for Saturating<T>

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impl<T: Hash> Hash for Wrapping<T>

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

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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.

#![feature(build_hasher_simple_hash_one)]
use std::hash::BuildHasher;

let b = std::collections::hash_map::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));
Run
const: unstable · source§

impl<T: ?Sized + Hash> Hash for &T

const: unstable · source§

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

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impl<T: ?Sized> Hash for *const T

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impl<T: ?Sized> Hash for *mut T

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

1.25.0 · source§

impl<T: ?Sized> Hash for NonNull<T>

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