Add loops test with separate exit and continue boundaries

Note that this does not rewrite to labeled returns yet because of some
inlining limitations.

Author: odersky

tests/run/loops-alt.scala

@@ -0,0 +1,46 @@
+import scala.util.{boundary, break}
+import boundary.Label
+import java.util.concurrent.TimeUnit
+
+object loop:
+  opaque type ExitLabel = Label[Unit]
+  opaque type ContinueLabel = Label[Unit]
+
+  inline def apply(inline op: (ExitLabel, ContinueLabel) ?=> Unit): Unit =
+    boundary { exitLabel ?=>
+      while true do
+        boundary { continueLabel ?=>
+          op(using exitLabel, continueLabel)
+        }
+    }
+  end apply
+
+  inline def exit()(using ExitLabel): Unit =
+    break()
+
+  inline def continue()(using ContinueLabel): Unit =
+    break()
+end loop
+
+def testLoop(xs: List[Int]) =
+  var current = xs
+  var sum = 0
+  loop:
+    // This should be convertible to labeled returns but isn't, since
+    // the following code is still passed as a closure to `boundary`.
+    // That's probably due to the additional facade operations necessary
+    // for opaque types.
+    if current.isEmpty then loop.exit()
+    val hd = current.head
+    current = current.tail
+    if hd == 0 then loop.exit()
+    if hd < 0 then loop.continue()
+    sum += hd
+  sum
+
+@main def Test =
+  assert(testLoop(List(1, 2, 3, -2, 4, -3, 0, 1, 2, 3)) == 10)
+  assert(testLoop(List()) == 0)
+  assert(testLoop(List(-2, -3, 0, 1)) == 0)
+
+