Trait Hash

1.6.0 · source

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

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

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§

1.6.0 · source

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

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§

1.6.0 · source

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

Trait HashCopy item path

§Implementing Hash

§Hash and Eq

§Prefix collisions

§Portability

Required Methods§

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

§Examples

Provided Methods§

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

§Examples

Object Safety§

Implementors§

impl Hash for wasmtime_environ::component::dfg::CoreDef

impl Hash for Trampoline

impl Hash for wasmtime_environ::component::CoreDef

impl Hash for FixedEncoding

impl Hash for InterfaceType

impl Hash for StringEncoding

impl Hash for Transcode

impl Hash for ConstOp

impl Hash for EngineOrModuleTypeIndex

impl Hash for EntityIndex

impl Hash for MemoryStyle

impl Hash for TableStyle

impl Hash for Trap

impl Hash for WasmCompositeType

impl Hash for WasmHeapType

impl Hash for WasmStorageType

impl Hash for WasmValType

impl Hash for LibCall

impl Hash for wasmtime_environ::wasmparser::AbstractHeapType

impl Hash for ComdatSymbolKind

impl Hash for CompositeType

impl Hash for wasmtime_environ::wasmparser::HeapType

impl Hash for wasmtime_environ::wasmparser::Ordering

impl Hash for RelocAddendKind

impl Hash for RelocationType

impl Hash for wasmtime_environ::wasmparser::StorageType

impl Hash for UnpackedIndex

impl Hash for wasmtime_environ::wasmparser::ValType

impl Hash for ComponentNameKind<'_>

impl Hash for AnyTypeId

impl Hash for ComponentAnyTypeId

impl Hash for ComponentCoreTypeId

impl Hash for AsciiChar

impl Hash for wasmtime_environ::__core::cmp::Ordering

impl Hash for Infallible

impl Hash for IpAddr

impl Hash for Ipv6MulticastScope

impl Hash for SocketAddr

impl Hash for wasmtime_environ::__core::sync::atomic::Ordering

impl Hash for ErrorKind

impl Hash for DemangleNodeType

impl Hash for cpp_demangle::error::Error

impl Hash for Format

impl Hash for gimli::common::SectionId

impl Hash for RunTimeEndian

impl Hash for CallFrameInstruction

impl Hash for Address

impl Hash for Reference

impl Hash for LineString

impl Hash for gimli::write::loc::Location

impl Hash for gimli::write::range::Range

impl Hash for Level

impl Hash for LevelFilter

impl Hash for AddressSize

impl Hash for object::common::Architecture

impl Hash for object::common::BinaryFormat

impl Hash for ComdatKind

impl Hash for FileFlags

impl Hash for RelocationEncoding

impl Hash for RelocationFlags

impl Hash for RelocationKind

impl Hash for SectionFlags

impl Hash for SectionKind

impl Hash for object::common::SegmentFlags

impl Hash for SubArchitecture

impl Hash for SymbolKind

impl Hash for SymbolScope

Trait Hash

§Implementing `Hash`

§`Hash` and `Eq`

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

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