Documentation of PartialEq has example that violates transitivity

[robamler]

The Documentation of PartialEq states that implementations must be transitive: for all for all a, b and c, a == b and b == c implies a == c. This is followed by a note indicating that the transitivity must also hold if a, b, and c are of different types.

However, the last example implementation in the documentation is not transitive:

let b1 = Book { isbn: 1, format: BookFormat::Paperback };
let b2 = Book { isbn: 2, format: BookFormat::Paperback };

assert!(b1 == BookFormat::Paperback);
assert!(BookFormat::Paperback == b2);

// The following should hold by transitivity but doesn't.
assert!(b1 == b2); // <-- PANICS

I can think of three possible ways to fix this:

• Remove the last example.
• Keep the last example as an explicit counterexample to warn how easy it is to accidentally write implementations that violate the contract.
• Modify the last example in a way that fixes the issue (I'm not sure what that would look like).

I can submit a pull request if I get some guidance which of the above solutions to choose (I personally have a weak preference for the second one).

[Centril]

Keep the last example as an explicit counterexample to warn how easy it is to accidentally write implementations that violate the contract.

This option seems quite helpful.