Why is this an issue?

Having a manual PartialEq implementation for types with a derived Hash can lead to inconsistencies. The contract k1 == k2 ⇒ hash(k1) == hash(k2) must always hold. Inconsistencies can cause undefined behaviors, especially when these types are used in collections such as HashMap.

Code examples

Noncompliant code example

#[derive(Hash)]
struct Foo;

impl PartialEq for Foo {
    fn eq(&self, other: &Self) -> bool {
        // Some custom equality logic
        true // Noncompliant
    }
}

Compliant solution

#[derive(Hash, PartialEq)]
struct Foo;

Resources

Documentation

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