🦀HashWith🦀

HashWith is a Rust procedural macro crate that allows you to automatically implement Hash for structs, overriding fields that don’t natively implement Hash (like f64 or structs which use them). It supports custom hash functions per field.

Features

• Derive Hash on structs with fields that normally cannot be hashed.
• Support for custom hash functions per field via the #[hash_with = "..."] attribute.
• Inline closures for per-field hashing.

Usage

Basic Inline Hash Expression

You can use #[hash_with(expr)] to specify a custom hashing expression for a field:

use hash_with::HashWith;

#[derive(HashWith)]
struct Brightness {
    /// Use a custom hash expression for this field.
    #[hash_with(self.inner.to_bits())]
    inner: f64,
}

let b1 = Brightness { inner: 1.1 };
let b2 = Brightness { inner: 2.2 };

assert_ne!(b1.get_hash(), b2.get_hash());

Custom Hash Function

You can also specify a custom function for hashing a field:

use hash_with::HashWith;
use std::hash::Hash;

/// Custom hash function for f64
fn hash_f64_bits<H: std::hash::Hasher>(val: &f64, state: &mut H) {
    val.to_bits().hash(state);
}

#[derive(HashWith)]
struct Config {
    name: String,
    /// Hash using a custom function
    #[hash_with = "hash_f64_bits"]
    brightness: f64,
}

Ignoring Fields

Fields can be excluded from the hash calculation using #[hash_without]:

use hash_with::HashWith;

#[derive(HashWith)]
struct User {
    id: u32,
    /// This field is ignored in the hash
    #[hash_without]
    session_token: String,
}

let user1 = User { id: 1, session_token: "abc".into() };
let user2 = User { id: 1, session_token: "xyz".into() };

// The hash ignores `session_token`
assert_eq!(user1.get_hash(), user2.get_hash());

Why Use hash_with?

• Rust's default Hash implementation does not support some primitive types like f64 directly.
• Simplifies implementing Hash for structs with attributes from external or third-party types that you cannot modify.
• Lets you ignore irrelevant fields without implementing Hash manually.

Installation

Add this to your Cargo.toml:

[dependencies]
hash_with = { git = "https://github.com/some1and2-xc/hash-with" }