With #51760 landed, the PartialEq documentation now at the same time states

Formally, the equality must be (for all a, b and c):

• symmetric: a == b implies b == a; and
• [...]

And gives an example that is not symmetric: b1 == BookFormat::Paperback typechecks, but BookFormat::Paperback == b1 does not.

Cc @gnzlbg @GuillaumeGomez @steveklabnik