Add library replacement to nonlocal returns

Author: OlivierBlanvillain

library/src/scala/util/control/NonLocalReturns.scala

@@ -0,0 +1,34 @@
+package scala.util.control
+
+/** Library implementation of nonlocal return.
+ *
+ *  Usage:
+ *
+ *     import scala.util.control.NonLocalReturns._
+ *
+ *     returning { ... throwReturn(x) ... }
+ */
+object NonLocalReturns {
+  class ReturnThrowable[T] extends ControlThrowable {
+    private var myResult: T = _
+    def throwReturn(result: T): Nothing = {
+      myResult = result
+      throw this
+    }
+    def result: T = myResult
+  }
+
+  /** Performs a nonlocal return by throwing an exception. */
+  def throwReturn[T](result: T) given (returner: ReturnThrowable[T]): Nothing =
+    returner.throwReturn(result)
+
+  /** Enable nonlocal returns in `op`. */
+  def returning[T](op: given ReturnThrowable[T] => T): T = {
+    val returner = new ReturnThrowable[T]
+    try op given returner
+    catch {
+      case ex: ReturnThrowable[T] =>
+       if (ex.eq(returner)) ex.result else throw ex
+    }
+  }
+}

tests/run/nonlocal-return.scala

@@ -0,0 +1,16 @@
+import scala.util.control.NonLocalReturns._
+
+object Test {
+  def has(xs: List[Int], elem: Int) =
+    returning {
+      for (x <- xs)
+        if (x == elem) throwReturn(true)
+      false
+    }
+
+  def main(arg: Array[String]): Unit = {
+    assert(has(1 :: 2 :: Nil, 1))
+    assert(has(1 :: 2 :: Nil, 2))
+    assert(!has(1 :: 2 :: Nil, 3))
+  }
+}