Scala collections breakOut

breakout.scala

// a List isn't a Map
scala> val x: Map[Int,Int] = List(1,2,3).map(i => i -> i)
<console>:16: error: type mismatch;
 found   : List[(Int, Int)]
 required: Map[Int,Int]
       val x: Map[Int,Int] = List(1,2,3).map(i => i -> i)
                                            ^
// first create List, then create Map. Intermediate collection overhead
scala> val x: Map[Int,Int] = List(1,2,3).map(i => i -> i).toMap 
x: Map[Int,Int] = Map(1 -> 1, 2 -> 2, 3 -> 3)

// Create Map instantly, no intermediate, type Map[Int,Int] inferred from expected
scala> val x: Map[Int,Int] = List(1,2,3).map(i => i -> i)(scala.collection.breakOut) 
x: Map[Int,Int] = Map(1 -> 1, 2 -> 2, 3 -> 3)