Add capture-checking annotations to scala.util.boundary

library/src/scala/util/boundary.scala

@@ -1,4 +1,6 @@
 package scala.util
+
+import language.experimental.captureChecking
 import scala.annotation.implicitNotFound
 
 /** A boundary that can be exited by `break` calls.
@@ -27,18 +29,27 @@ import scala.annotation.implicitNotFound
  * ```
  */
 object boundary:
+  import caps.unsafe.unsafeAssumePure
 
   /** User code should call `break.apply` instead of throwing this exception
    *  directly.
    */
-  final class Break[T] private[boundary](val label: Label[T], val value: T)
+  final class Break[T] private[boundary] (private[boundary] val label: Label[T]^{}, val value: T)
   extends RuntimeException(
-    /*message*/ null, /*cause*/ null, /*enableSuppression=*/ false, /*writableStackTrace*/ false)
+    /*message*/ null, /*cause*/ null, /*enableSuppression=*/ false, /*writableStackTrace*/ false):
+
+    /** Compare the given [[Label]] to the one this [[Break]] was constructed with. */
+    inline def isSameLabelAs(other: Label[T]) = label eq other
+
+  object Break:
+    def apply[T](label: Label[T], value: T) =
+      // SAFETY: labels cannot leak from [[Break]], and is only used for equality comparison.
+      new Break(label.unsafeAssumePure, value)
 
   /** Labels are targets indicating which boundary will be exited by a `break`.
    */
   @implicitNotFound("explain=A Label is generated from an enclosing `scala.util.boundary` call.\nMaybe that boundary is missing?")
-  final class Label[-T]
+  final class Label[-T] extends caps.Capability
 
   /** Abort current computation and instead return `value` as the value of
    *  the enclosing `boundary` call that created `label`.
@@ -60,7 +71,7 @@ object boundary:
     val local = Label[T]()
     try body(using local)
     catch case ex: Break[T] @unchecked =>
-      if ex.label eq local then ex.value
+      if ex.isSameLabelAs(local) then ex.value
       else throw ex
 
 end boundary

tests/neg-custom-args/captures/boundary.check

@@ -0,0 +1,27 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/boundary.scala:8:31 --------------------------------------
+8 |      boundary.break(l2)(using l1) // error
+  |                               ^^
+  |                               Found:    (local : scala.util.boundary.Label[scala.util.boundary.Label[Unit]]^)
+  |                               Required: scala.util.boundary.Label[box scala.util.boundary.Label[Unit]^{local²}]^
+  |
+  |                               where:    local  is a value locally defined in object test
+  |                                         local² is a value locally defined in object test
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/boundary.scala:6:32 --------------------------------------
+ 6 |  boundary[boundary.Label[Unit]]: l1 ?=> // error
+   |  ^
+   |  Found:    scala.util.boundary.Break[scala.util.boundary.Label[Unit]] @unchecked
+   |  Required: scala.util.boundary.Break[box scala.util.boundary.Label[Unit]^] @unchecked
+ 7 |    boundary[Unit]: l2 ?=>
+ 8 |      boundary.break(l2)(using l1) // error
+ 9 |    ???
+   |--------------------------------------------------------------------------------------------------------------------
+   |Inline stack trace
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   |This location contains code that was inlined from boundary.scala:73
+73 |    catch case ex: Break[T] @unchecked =>
+   |                   ^
+    --------------------------------------------------------------------------------------------------------------------
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/boundary.scala

@@ -0,0 +1,9 @@
+import language.experimental.captureChecking
+
+import scala.util.boundary
+
+object test:
+  boundary[boundary.Label[Unit]]: l1 ?=> // error
+    boundary[Unit]: l2 ?=>
+      boundary.break(l2)(using l1) // error
+    ???