Add ReusableInstance

Author: liufengyun

compiler/src/dotty/tools/dotc/util/ReusableInstance.scala

@@ -0,0 +1,37 @@
+package dotty.tools.dotc.util
+
+import scala.collection.mutable.ArrayBuffer
+import scala.util.chaining._
+
+/** A wrapper for a list of cached instances of a type `T`.
+  * The wrapper is recursion-reentrant: several instances are kept, so
+  * at each depth of reentrance we are reusing the instance for that.
+  *
+  * An instance is created upon creating this object, and more instances
+  * are allocated dynamically, on demand, when reentrance occurs.
+  *
+  * Not thread safe.
+  *
+  * Ported from scala.reflect.internal.util.ReusableInstance
+  */
+final class ReusableInstance[T <: AnyRef] private (make: => T, enabled: Boolean) {
+  private[this] val cache = if (enabled) new ArrayBuffer[T](ReusableInstance.InitialSize).tap(_.addOne(make)) else null
+  private[this] var taken = 0
+
+  @inline def using[R](action: T => R): R =
+    if (!enabled)
+      action(make)
+    else {
+      if (taken == cache.size)
+        cache += make
+      taken += 1
+      try action(cache(taken-1)) finally taken -= 1
+    }
+}
+
+object ReusableInstance {
+  private final val InitialSize = 4
+
+  def apply[T <: AnyRef](make: => T): ReusableInstance[T]                   = new ReusableInstance[T](make, enabled = true)
+  def apply[T <: AnyRef](make: => T, enabled: Boolean): ReusableInstance[T] = new ReusableInstance[T](make, enabled = enabled)
+}