Add NonEmptyList#toNes (issue #2346) (#2557)

* Add NonEmptyList#toNes (issue #2346)

* Fixed NonEmptyList.scala formating

* Fixed NonEmptyListSuite.scala formating

* Revert "Fixed NonEmptyListSuite.scala formating"

This reverts commit 1a4ff0d.

* Fixed NonEmptyListSuite.scala formating, this time for sure

core/src/main/scala/cats/data/NonEmptyList.scala

@@ -417,6 +417,19 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {
    */
   def toNem[T, U](implicit ev: A <:< (T, U), order: Order[T]): NonEmptyMap[T, U] =
     NonEmptyMap.fromMapUnsafe(SortedMap(toList.map(ev): _*)(order.toOrdering))
+
+  /**
+   * Creates new `NonEmptySet`, similarly to List#toSet from scala standard library.
+   *{{{
+   * scala> import cats.data._
+   * scala> import cats.instances.int._
+   * scala> val nel = NonEmptyList(1, List(2,2,3,4))
+   * scala> nel.toNes
+   * res0: cats.data.NonEmptySet[Int] = TreeSet(1, 2, 3, 4)
+   *}}}
+   */
+  def toNes[B >: A](implicit order: Order[B]): NonEmptySet[B] =
+    NonEmptySet.of(head, tail: _*)
 }
 
 object NonEmptyList extends NonEmptyListInstances {

tests/src/test/scala/cats/tests/NonEmptyListSuite.scala

@@ -3,7 +3,7 @@ package tests
 
 import cats.kernel.laws.discipline.{EqTests, OrderTests, PartialOrderTests, SemigroupTests}
 
-import cats.data.{NonEmptyList, NonEmptyMap, NonEmptyVector}
+import cats.data.{NonEmptyList, NonEmptyMap, NonEmptySet, NonEmptyVector}
 import cats.data.NonEmptyList.ZipNonEmptyList
 import cats.laws.discipline.arbitrary._
 import cats.laws.discipline.{
@@ -15,6 +15,7 @@ import cats.laws.discipline.{
   SerializableTests
 }
 import scala.collection.immutable.SortedMap
+import scala.collection.immutable.SortedSet
 
 class NonEmptyListSuite extends CatsSuite {
   // Lots of collections here.. telling ScalaCheck to calm down a bit
@@ -329,6 +330,12 @@ class NonEmptyListSuite extends CatsSuite {
       nel.toNem should ===(NonEmptyMap.fromMapUnsafe(SortedMap.empty[Int, String] ++ nel.toList.toMap))
     }
   }
+
+  test("NonEmptyList#toNes is consistent with List#toSet and creating NonEmptySet from it") {
+    forAll { nel: NonEmptyList[Int] =>
+      nel.toNes should ===(NonEmptySet.fromSetUnsafe(SortedSet.empty[Int] ++ nel.toList.toSet))
+    }
+  }
 }
 
 @deprecated("to be able to test deprecated methods", since = "1.0.0-RC1")