`non_canonical_partial_ord_impl` will not lint reversed ordering

[samueltardieu]

Summary

In addition to the correct Some(self.cmp(other)), the lint will accept the probably incorrect Some(other.cmp(self)) (and probably other forms of Some(_.cmp(_))) as the canonical implementation of PartialOrd::partial_cmp().

Lint Name

non_canonical_partial_ord_impl

Reproducer

I tried this code:

#![deny(clippy::non_canonical_partial_ord_impl)]

fn main() {
    #[derive(Eq, PartialEq)]
    struct S(i32);

    impl Ord for S {
        fn cmp(&self, other: &Self) -> std::cmp::Ordering {
            todo!()
        }
    }
    
    impl PartialOrd for S {
        fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
            Some(other.cmp(self))  // Note inversion of receiver and argument
        }
    }
}

I expected to see this happen: partial_cmp() should warn about a non-canonical implementation

Instead, this happened: nothing

Version

rustc 1.88.0-nightly (d5b4c2e4f 2025-04-02)
binary: rustc
commit-hash: d5b4c2e4f19b6d7037371cdaecc3cc2c701c68df
commit-date: 2025-04-02
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.1

[samueltardieu]

Noticed during the implementation of #14573, but those are different concerns.