scala mutable IdentityHashMap implementation

IdentityHashMap.scala

package com.github.gist.jdolson.collection.mutable

import scala.collection.generic.CanBuildFrom
import scala.collection.mutable.{MapLike, MapBuilder, HashMap}

final class IdentityHashMap[A <: AnyRef, B]() extends HashMap[A, B] with MapLike[A, B, IdentityHashMap[A, B]] {
  override protected def elemEquals(key1: A, key2: A): Boolean = key1 eq key2

  override protected def elemHashCode(key: A) = System.identityHashCode(key)

  override def empty: IdentityHashMap[A, B] = IdentityHashMap.empty

  // WTF: resize is private in HashTable!!
  //override def sizeHint(size: Int): Unit = resize(size)
}

object IdentityHashMap {
  type Coll = IdentityHashMap[_, _]

  implicit def canBuildFrom[A <: AnyRef, B] = new CanBuildFrom[Coll, (A, B), IdentityHashMap[A, B]] {
    def apply() = newBuilder[A, B]
    def apply(from: Coll) = {
      val builder = newBuilder[A, B]
      builder.sizeHint(from.size)
      builder
    }
  }

  def empty[A <: AnyRef, B]: IdentityHashMap[A, B] = new IdentityHashMap[A, B]

  def newBuilder[A <: AnyRef, B] = new MapBuilder[A, B, IdentityHashMap[A, B]](empty[A, B]) {
    override def +=(x: (A, B)): this.type = {
      elems += x
      this
    }
    override def sizeHint(size: Int): Unit = elems.sizeHint(size)
  }

  def apply[A <: AnyRef, B](elems: (A, B)*) = (newBuilder[A, B] ++= elems).result()
}