Why is this an issue?

Hashing a unit value doesn’t accomplish anything because the implementation of Hash for () is a no-op. This can lead to confusion or misleading code, as it implies some hashing operation is actually taking place, when in reality it does nothing.

Code examples

Noncompliant code example

enum Foo { Empty, WithValue(u8) }
use Foo::*;

let mut state = DefaultHasher::new();
let my_enum = Foo::Empty;

match my_enum {
    Empty => ().hash(&mut state), // Noncompliant
    WithValue(x) => x.hash(&mut state),
}

Compliant solution

enum Foo { Empty, WithValue(u8) }
use Foo::*;

let mut state = DefaultHasher::new();
let my_enum = Foo::Empty;

match my_enum {
    Empty => 0_u8.hash(&mut state), // Compliant
    WithValue(x) => x.hash(&mut state),
}

Resources

Documentation

• Clippy Lints - https://rust-lang.github.io/rust-clippy/master/index.html#unit_hash