Derive Eq for std::cmp::Ordering, instead of using manual impl.

[zachs18]

This allows consts of type Ordering to be used in patterns, and with feature(adt_const_params) allows using Ordering as a const generic parameter.

Currently, std::cmp::Ordering implements Eq using a manually written impl Eq for Ordering {}, instead of derive(Eq). This means that it does not implement StructuralEq.

This commit removes the manually written impl, and adds derive(Eq) to Ordering, so that it will implement StructuralEq.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @kennytm (or someone else) soon.

Please see the contribution instructions for more information.

[zachs18]

The manual impl appears to have been added in 2013 with this commit ( a0f8540 ). As far as I can tell, derive(Eq) (deriving(TotalEq)) was possible then, so I don't know if there is a reason it was not derived at the time.

[Dylan-DPC]

r? @Dylan-DPC

[Dylan-DPC]

@bors r+ rollup

[bors]

📌 Commit b13b495 has been approved by Dylan-DPC

[m-ou-se]

This needed a @rust-lang/libs-api FCP, as this was a change to the public stable API.

This change has already shipped stably as part of 1.61. Hopefully everyone agrees this change was uncontroversial.

[BurntSushi]

@m-ou-se What was the public API change here? Is my mental model that derive(Eq) is precisely equivalent to impl Eq for Ordering wrong? (At least, for this specific case.)

[m-ou-se]

This adds an implementation of StructuralEq. (See the top comment.)

[BurntSushi]

Oh wow, okay, that's subtle. Hopefully we can figure out how to make such changes more explicit in the future.

But I think this specific change looks okay at least.

[dtolnay]

fn main() {
    const ORDERING: std::cmp::Ordering = std::cmp::Ordering::Less;
    if let ORDERING = ORDERING {}
}

$ cargo +1.61.0 check
    Checking repro v0.0.0
    Finished dev [unoptimized + debuginfo] target(s) in 0.66s

$ cargo +1.60.0 check
    Checking repro v0.0.0

error: to use a constant of type `std::cmp::Ordering` in a pattern, `std::cmp::Ordering` must be annotated with `#[derive(PartialEq, Eq)]`
 --> src/main.rs:3:12
  |
3 |     if let ORDERING = ORDERING {}
  |            ^^^^^^^^

warning: irrefutable `if let` pattern
 --> src/main.rs:3:8
  |
3 |     if let ORDERING = ORDERING {}
  |        ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(irrefutable_let_patterns)]` on by default
  = note: this pattern will always match, so the `if let` is useless
  = help: consider replacing the `if let` with a `let`

error: could not compile `repro` due to previous error; 1 warning emitted