wasmer_types::lib::std::hash

Trait Hash

1.0.0 · 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 { ... }
}

Available on crate feature std only.

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".

Implementors§

1.29.0 · Source§

impl Hash for !

1.0.0 · Source§

Trait Hash Copy 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 !

impl Hash for ()

impl Hash for ATerm

impl Hash for Aarch64Architecture

impl Hash for Alignment

impl Hash for Architecture

impl Hash for ArchivedCString

impl Hash for ArchivedDuration

impl Hash for ArchivedIpAddr

impl Hash for ArchivedIpv4Addr

impl Hash for ArchivedIpv6Addr

impl Hash for ArchivedOptionNonZeroI8

impl Hash for ArchivedOptionNonZeroI16

impl Hash for ArchivedOptionNonZeroI32

impl Hash for ArchivedOptionNonZeroI64

impl Hash for ArchivedOptionNonZeroI128

impl Hash for ArchivedOptionNonZeroU8

impl Hash for ArchivedOptionNonZeroU16

impl Hash for ArchivedOptionNonZeroU32

impl Hash for ArchivedOptionNonZeroU64

impl Hash for ArchivedOptionNonZeroU128

impl Hash for ArchivedRangeFull

impl Hash for ArchivedSocketAddr

impl Hash for ArchivedSocketAddrV4

impl Hash for ArchivedSocketAddrV6

impl Hash for ArchivedString

impl Hash for ArmArchitecture

impl Hash for AsciiChar

impl Hash for AtomicsError

impl Hash for B0

impl Hash for B1

impl Hash for BinaryFormat

impl Hash for ByteStr

impl Hash for ByteString

impl Hash for bytes::bytes::Bytes

impl Hash for wasmer_types::Bytes

impl Hash for BytesMut

impl Hash for CDataModel

impl Hash for CStr

impl Hash for CString

impl Hash for CallingConvention

impl Hash for CharCase

impl Hash for CleverArchitecture

impl Hash for CpuFeature

impl Hash for CustomSectionIndex

impl Hash for CustomVendor

impl Hash for DataIndex

impl Hash for DefaultToHost

impl Hash for DefaultToUnknown

impl Hash for DeploymentTarget

impl Hash for Duration

impl Hash for ElemIndex

impl Hash for Endianness

impl Hash for Environment

impl Hash for Equal

impl Hash for Error

impl Hash for ErrorKind

impl Hash for ExportIndex

impl Hash for ExternType

impl Hash for Failure

impl Hash for Features

impl Hash for FieldId

impl Hash for FileType

impl Hash for FunctionIndex

impl Hash for FunctionType

impl Hash for GlobalIndex

impl Hash for GlobalType

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,