WIP on skipping contradicted queries

lib/mongoid/criteria.rb

@@ -281,7 +281,7 @@ def none
     #
     # @return [ true | false ] If the criteria is a none.
     def empty_and_chainable?
-      !!@none
+      !!@none || Queryable::Contradiction.contradicted?(self)
     end
 
     # Overriden to include _type in the fields.

lib/mongoid/criteria/queryable/contradiciton.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module Mongoid
+  class Criteria
+    module Queryable
+      # Checks whether the criteria contains a contradiction which
+      # would cause the query to return no results. This can result from:
+      # - Conflicting equals: { 'name' => 'Bob', 'name' => 'Joe' }
+      # - Empty $in: { '$in' => [] }
+      # - Not empty $nin: { '$not' => { '$nin' => [] } }
+      module Contradiction
+        extend self
+
+        def contradicted?(criteria)
+          traverse_selector(criteria.selector)
+        end
+
+        private
+
+        def traverse_selector(selector, negated = false)
+          # return true if selector.any? { |k, v| empty_condition?(k, v, '$in') }
+
+          # selector.all?
+          selector.each do |k, v|
+            return true if empty_condition?(k, v, '$in')
+
+            case k.to_s
+            when '$and'
+              Array(v).any? { |s| traverse_selector(s, negated) }
+            when '$or'
+              Array(v).all? { |s| traverse_selector(s, negated) }
+            when '$nor'
+              Array(v).all? { |s| traverse_selector(s, !negated) }
+            when '$not'
+              traverse_selector(selector, !negated)
+            end
+          end
+        end
+
+        def empty_condition?(key, value, operator)
+          !key.to_s.start_with?('$') &&
+            value.is_a?(Hash) &&
+            value.size == 1 &&
+            (value[operator] || value[operator.to_sym]) == []
+        end
+      end
+    end
+  end
+end