Tracking Issue for future-incompatibility lint `internal_eq_trait_method_impls`

[cyrgani]

This is the tracking issue for the internal_eq_trait_method_impls future-compatibility warning and other related errors. The goal of this page is to describe why this change was made and how you can fix code that is affected by it. It also provides a place to ask questions or register a complaint if you feel the change should not be made. For more information on the policy around future-compatibility warnings, see our breaking change policy guidelines.

What is the warning for?

The Eq trait has the internal, undocumented helper method assert_receiver_is_total_eq. It was never meant to be called by anyone except for the expansion of #[derive(Eq)]. The internal_eq_trait_method_impls lint will issue a warning for any other implementations of this method.

Why was this change made?

The method dates back to the 1.0 release of Rust. Since then, it has gotten new syntax like const _ that will allow the derive macro to no longer need this method. There have also been cases where IDEs detected the presence of the method and automatically added a useless default implementation of the method. To prevent further confusion from this, this future-compatibility warning was added.
The lang and libs-api teams approved this lint in #149978 (comment).

Example

#[derive(PartialEq)]
pub struct Foo;

impl Eq for Foo {
    fn assert_receiver_is_total_eq(&self) {}
}

Recommendations

Remove any manual implementations of the method and just write impl Eq for Foo {}.

When will this warning become a hard error?

Unknown

Steps

• Implement the lint (deprecate Eq::assert_receiver_is_total_eq and emit FCW on manual impls #149978)
• Raise lint level to deny
• Change the lint to report in dependencies
• Switch to a hard error

Implementation history

• deprecate Eq::assert_receiver_is_total_eq and emit FCW on manual impls #149978