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§

1.0.0 · 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§

1.3.0 · 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());

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so 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

Dyn Compatibility§

Implementors§

impl Hash for Tz

impl Hash for clang_sys::Version

impl Hash for Severity

impl Hash for Format

impl Hash for SectionId

impl Hash for RunTimeEndian

impl Hash for ExtensionType

impl Hash for GeneralCategory

impl Hash for BufferFormat

impl Hash for IpAddrRange

impl Hash for IpNet

impl Hash for IpSubnets

impl Hash for fsconfig_command

impl Hash for membarrier_cmd

impl Hash for membarrier_cmd_flag

impl Hash for InsertError

impl Hash for DataFormat

impl Hash for MZError

impl Hash for MZFlush

impl Hash for MZStatus

impl Hash for TINFLStatus

impl Hash for FlockArg

impl Hash for PosixFadviseAdvice

impl Hash for PrctlMCEKillPolicy

impl Hash for SigHandler

impl Hash for SigevNotify

impl Hash for SigmaskHow

impl Hash for Signal

impl Hash for BaudRate

impl Hash for FlowArg

impl Hash for FlushArg

impl Hash for SetArg

impl Hash for SpecialCharacterIndices

impl Hash for WaitStatus

impl Hash for nom::error::ErrorKind

impl Hash for AddressSize

impl Hash for Architecture

impl Hash for 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 SegmentFlags

impl Hash for SubArchitecture

impl Hash for SymbolKind

impl Hash for SymbolScope

impl Hash for Endianness

impl Hash for ArchiveKind

impl Hash for ImportType

impl Hash for CompressionFormat

impl Hash for FileKind

impl Hash for ObjectKind

impl Hash for RelocationTarget

impl Hash for SymbolSection

impl Hash for InputLocation

impl Hash for LineColLocation

impl Hash for Direction

impl Hash for rustls_pki_types::server_name::IpAddr

impl Hash for QuoteError

impl Hash for unic_ucd_segment::grapheme_cluster_break::GraphemeClusterBreak

impl Hash for unic_ucd_segment::sentence_break::SentenceBreak

impl Hash for unic_ucd_segment::word_break::WordBreak

impl Hash for winnow::error::ErrorKind

impl Hash for zerocopy::byteorder::BigEndian

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,