lint on manual Hash, *Ord, *Eq impls that access a *Cell

[oli-obk]

even read-only access can be bad, because between two calls to hash, the cell might have changed its value, even if there are only immutable references present.

cc #729

should be fairly straight forward.

1. Find impls of the above traits
2. check if the type has fields that have interior mutability
3. check if the impl-method has an ExprField for one of the interior mutability fields

bonus points for also checking inherent methods that are called in the impl method for interior mutation.

[llogiq]

we should also warn on std::sync::atomic types (as they also can be mutated from a & ref.

[llogiq]

I think that people might want those impls anyway. Especially Ord and Eq might be helpful for sorting or partitioning a collection (or a variety of other things) while having a &mut ref ensures no concurrent modification.