The trait PartialEq represents a partial equivalence — that is, a relation that is symmetric and transitive. However, Rust doesn't currently enforce these properties in any way. This means we get issues like this:

struct A;
struct B;

impl PartialEq<B> for A {
    fn eq(&self, _other: &B) -> bool {
        true
    }
}

fn main() {
    let a = A {};
    let b = B {};
    a == b; // Works
    b == a; // Error (B does not implement PartialEq<A>)
}

This is confusing, but it's usually not so much of an issue in user code, because it's easy to flip the operands. However, when attempting to write generic functions over these traits, you run into problems (for example in #46934).

At the very least there should be a lint warning/error for this. It'd be nice to have a generic solution for properties of traits, though that could come later. It'd be even nicer if the symmetric case for PartialEq, for instance, could be automatically implemented by Rust, though this could require quite a bit more machinery.