Handle capture checking language imports correctly for -Ytest-pickler

The pickler test did not track whether capture checking was on. This meant that files with

  import language.experimental.captureChecking

has to be compiled additionally with the capture checking setting

  -language.experimental.captureChecking

to pass pickler tests. This made it impossible to test mixed code bases where
some files require capture checking and others don't.

We now use the previous setting of `CompilationUnit#needsCaptureChecking` when unpickling.

Author: odersky

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -48,7 +48,7 @@ class Pickler extends Phase {
 
   // Maps that keep a record if -Ytest-pickler is set.
   private val beforePickling = new mutable.HashMap[ClassSymbol, String]
-  private val pickledBytes = new mutable.HashMap[ClassSymbol, Array[Byte]]
+  private val pickledBytes = new mutable.HashMap[ClassSymbol, (CompilationUnit, Array[Byte])]
 
   /** Drop any elements of this list that are linked module classes of other elements in the list */
   private def dropCompanionModuleClasses(clss: List[ClassSymbol])(using Context): List[ClassSymbol] = {
@@ -137,7 +137,7 @@ class Pickler extends Phase {
         else
           val pickled = computePickled()
           reportPositionWarnings()
-          if ctx.settings.YtestPickler.value then pickledBytes(cls) = pickled
+          if ctx.settings.YtestPickler.value then pickledBytes(cls) = (unit, pickled)
           () => pickled
 
       unit.pickled += (cls -> demandPickled)
@@ -163,21 +163,21 @@ class Pickler extends Phase {
     result
   }
 
-  private def testUnpickler(using Context): Unit = {
+  private def testUnpickler(using Context): Unit =
     pickling.println(i"testing unpickler at run ${ctx.runId}")
     ctx.initialize()
     val unpicklers =
-      for ((cls, bytes) <- pickledBytes) yield {
+      for ((cls, (unit, bytes)) <- pickledBytes) yield {
         val unpickler = new DottyUnpickler(bytes)
         unpickler.enter(roots = Set.empty)
-        cls -> unpickler
+        cls -> (unit, unpickler)
       }
     pickling.println("************* entered toplevel ***********")
-    for ((cls, unpickler) <- unpicklers) {
+    val rootCtx = ctx
+    for ((cls, (unit, unpickler)) <- unpicklers) do
+      ctx.compilationUnit.needsCaptureChecking = unit.needsCaptureChecking
       val unpickled = unpickler.rootTrees
       testSame(i"$unpickled%\n%", beforePickling(cls), cls)
-    }
-  }
 
   private def testSame(unpickled: String, previous: String, cls: ClassSymbol)(using Context) =
     import java.nio.charset.StandardCharsets.UTF_8

tests/pos/cc-backwards-compat/A.scala

@@ -0,0 +1,4 @@
+package p
+class A:
+  def map(other: Iter): Iter = other
+  def pair[T](x: T): (T, T) = (x, x)

tests/pos/cc-backwards-compat/Iter.scala

@@ -0,0 +1,10 @@
+package p
+import language.experimental.captureChecking
+
+class Iter:
+  self: Iter^ =>
+
+def test(it: Iter^) =
+  val a = A()
+  //val b = a.map(it)   // does not work yet
+  val c = a.pair(it)