Custom attributes

The Hash trait supports the following attributes:

• Container attributes
  • Hash(bound="<where-clause or empty>")
• Field attributes
  • Hash(bound="<where-clause or empty>")
  • Hash(format_with="<path>")
  • Hash="ignore"

Ignoring a field

You can use derivative to ignore fields from a Hash implementation:

#[derive(Derivative)]
#[derivative(Hash)]
struct Foo {
    foo: u8,
    #[derivative(Hash="ignore")]
    bar: i32,
}

#[derive(Hash)]
struct Bar {
    foo: u8,
}

assert_eq!(hash(Foo { foo: 42, bar: -1337 }), hash(Bar { foo: 42 }));

Hash with

You can pass a field to a hash function:

#[derive(Derivative)]
#[derivative(Hash)]
struct Foo {
    foo: u32,
    #[derivative(Hash(hash_with="path::to::my_hash_fn"))]
    bar: SomeTypeThatMightNotBeHash,
}

The field bar will be hashed with path::to::my_hash_fn(&bar, &mut state) where state is the current Hasher.

The function must the following prototype:

fn my_hash_fn<H>(&T, state: &mut H) where H: Hasher;

Limitations

On structure, derivative(Hash) will produce the same hash as derive(Hash). On unions however, it will produces the same hashes only for unitary variants!

Custom bound

As most other traits, Hash supports a custom bound on container and fields. See Debug's documentation for more information.