What should the new rule do?

What new ECMAScript feature does this rule relate to? Note that we only accept new core rules related to new ECMAScript features.

What category of rule is this (place an "X" next to just one item)?

[ ] Warns about a potential problem [ ] Suggests an alternate way of doing something

Please provide some example JavaScript code that this rule will warn about:

<!-- put your code examples here -->

Why should this rule be included in ESLint (instead of a plugin)?