Comment by tialaramex

I take your point but I do still think it's a violation here, if I see a type which implements Hash and Eq this doesn't feel like a reasonable outcome to me.

Do you have a better way to describe how Maxwell<T> works here? Or, failing that, an idea for a type which you feel can be perverse in a more interesting way here?

By the way (though you should not design things where this happens) you can alway remove things from a HashMap (or HashSet) because such collections provide e.g. retain and extract_if which both take callables, your callable is given a reference to each thing in the collection and asked if you want it, they behave differently and have different intended purposes but either will let you fish out a NaN you mistakenly stored in an ordered collection for example.