A rule that disallows comparison to booleans.

For example, these are considered invalid:

foo == true
true != bar
false === baz

This is because these expressions evaluate to true or false, so you could get the same result by using either the variable directly, or negating the variable.

Additionally, with a duck typed language such as Elixir, we should be more interested in whether something is “truthy” or “falsey” than if they are true or false.