Consider source code as Scala 2 under -Ycompile-scala2-library

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -861,7 +861,7 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
       assert(isModifierTag(tag))
       writeByte(tag)
     }
-    assert(!flags.is(Scala2x))
+    if flags.is(Scala2x) then assert(attributes.scala2StandardLibrary)
     if (flags.is(Private)) writeModTag(PRIVATE)
     if (flags.is(Protected)) writeModTag(PROTECTED)
     if (flags.is(Final, butNot = Module)) writeModTag(FINAL)

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

@@ -256,7 +256,10 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
               for publicInBinaryAnnot <- publicInBinaryAnnotOpt do sym.addAnnotation(publicInBinaryAnnot)
             else
               sym.keepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot, defn.FieldMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
-          if sym.isScala2Macro && !ctx.settings.XignoreScala2Macros.value then
+          if sym.isScala2Macro && !ctx.settings.XignoreScala2Macros.value &&
+             sym != defn.StringContext_raw &&
+             sym != defn.StringContext_f &&
+             sym != defn.StringContext_s then
             if !sym.owner.unforcedDecls.exists(p => !p.isScala2Macro && p.name == sym.name && p.signature == sym.signature)
                // Allow scala.reflect.materializeClassTag to be able to compile scala/reflect/package.scala
                // This should be removed on Scala 3.x

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -246,7 +246,9 @@ class Namer { typer: Typer =>
 
     tree match {
       case tree: TypeDef if tree.isClassDef =>
-        val flags = checkFlags(tree.mods.flags)
+        var flags = checkFlags(tree.mods.flags)
+        if ctx.settings.YcompileScala2Library.value then
+          flags |= Scala2x
         val name = checkNoConflict(tree.name, flags.is(Private), tree.span).asTypeName
         val cls =
           createOrRefine[ClassSymbol](tree, name, flags, ctx.owner,

project/Scala2LibraryBootstrappedMiMaFilters.scala

@@ -66,17 +66,8 @@ object Scala2LibraryBootstrappedMiMaFilters {
         ProblemFilters.exclude[FinalMethodProblem]("scala.io.Source.NoPositioner"),
         ProblemFilters.exclude[FinalMethodProblem]("scala.io.Source.RelaxedPosition"),
         ProblemFilters.exclude[FinalMethodProblem]("scala.io.Source.RelaxedPositioner"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.collection.immutable.SortedMapOps.coll"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.collection.immutable.TreeMap.empty"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.collection.immutable.TreeMap.fromSpecific"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.collection.mutable.ArrayBuilder#ofUnit.addAll"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.collection.mutable.TreeMap.empty"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.collection.mutable.TreeMap.fromSpecific"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.reflect.ManifestFactory#NothingManifest.newArray"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.reflect.ManifestFactory#NullManifest.newArray"),
         ProblemFilters.exclude[MissingFieldProblem]("scala.collection.ArrayOps#ReverseIterator.xs"),
         ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.NonLocalReturnControl.value"),
-        ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.collection.immutable.SortedMapOps.coll"),
       ) ++
       Seq( // DirectMissingMethodProblem
         "scala.collection.LinearSeqIterator#LazyCell.this",

scala2-library-cc/src/scala/collection/concurrent/TrieMap.scala

@@ -1071,6 +1071,7 @@ object TrieMap extends MapFactory[TrieMap] {
 
 // non-final as an extension point for parallel collections
 private[collection] class TrieMapIterator[K, V](var level: Int, private var ct: TrieMap[K, V], mustInit: Boolean = true) extends AbstractIterator[(K, V)] {
+  this:TrieMapIterator[K, V]^ =>
   private val stack = new Array[Array[BasicNode]](7)
   private val stackpos = new Array[Int](7)
   private var depth = -1
@@ -1161,7 +1162,7 @@ private[collection] class TrieMapIterator[K, V](var level: Int, private var ct:
   /** Returns a sequence of iterators over subsets of this iterator.
     *  It's used to ease the implementation of splitters for a parallel version of the TrieMap.
     */
-  protected def subdivide(): Seq[Iterator[(K, V)]] = if (subiter ne null) {
+  protected def subdivide(): Seq[Iterator[(K, V)]^{this}] = if (subiter ne null) {
     // the case where an LNode is being iterated
     val it = newIterator(level + 1, ct, _mustInit = false)
     it.depth = -1

scala2-library-cc/src/scala/collection/immutable/Range.scala

@@ -643,6 +643,7 @@ private class RangeIterator(
   lastElement: Int,
   initiallyEmpty: Boolean
 ) extends AbstractIterator[Int] with Serializable {
+  this: RangeIterator^ =>
   private[this] var _hasNext: Boolean = !initiallyEmpty
   private[this] var _next: Int = start
   override def knownSize: Int = if (_hasNext) (lastElement - _next) / step + 1 else 0
@@ -656,7 +657,7 @@ private class RangeIterator(
     value
   }
 
-  override def drop(n: Int): Iterator[Int] = {
+  override def drop(n: Int): Iterator[Int]^{this} = {
     if (n > 0) {
       val longPos = _next.toLong + step * n
       if (step > 0) {

scala2-library-cc/src/scala/collection/mutable/UnrolledBuffer.scala

@@ -18,6 +18,7 @@ import scala.collection.generic.DefaultSerializable
 import scala.reflect.ClassTag
 import scala.collection.immutable.Nil
 import language.experimental.captureChecking
+import caps.unsafe.unsafeAssumePure
 
 /** A buffer that stores elements in an unrolled linked list.
   *
@@ -259,13 +260,14 @@ object UnrolledBuffer extends StrictOptimizedClassTagSeqFactory[UnrolledBuffer]
   /** Unrolled buffer node.
     */
   class Unrolled[T: ClassTag] private[collection] (var size: Int, var array: Array[T], var next: Unrolled[T], val buff: UnrolledBuffer[T] = null) {
+    //this: Unrolled[T]^ =>
     private[collection] def this() = this(0, new Array[T](unrolledlength), null, null)
     private[collection] def this(b: UnrolledBuffer[T]) = this(0, new Array[T](unrolledlength), null, b)
 
     private def nextlength = if (buff eq null) unrolledlength else buff.calcNextLength(array.length)
 
     // adds and returns itself or the new unrolled if full
-    @tailrec final def append(elem: T): Unrolled[T] = if (size < array.length) {
+    @tailrec final def append(elem: T): Unrolled[T]^{this} = if (size < array.length) {
       array(size) = elem
       size += 1
       this
@@ -307,21 +309,21 @@ object UnrolledBuffer extends StrictOptimizedClassTagSeqFactory[UnrolledBuffer]
       if (idx < size) array(idx) else next.apply(idx - size)
     @tailrec final def update(idx: Int, newelem: T): Unit =
       if (idx < size) array(idx) = newelem else next.update(idx - size, newelem)
-    @tailrec final def locate(idx: Int): Unrolled[T] =
+    @tailrec final def locate(idx: Int): Unrolled[T]^{this} =
       if (idx < size) this else next.locate(idx - size)
-    def prepend(elem: T) = if (size < array.length) {
+    def prepend(elem: T): Unrolled[T] = if (size < array.length) {
       // shift the elements of the array right
       // then insert the element
       shiftright()
       array(0) = elem
       size += 1
-      this
+      this.unsafeAssumePure
     } else {
       // allocate a new node and store element
       // then make it point to this
       val newhead = new Unrolled[T](buff)
       newhead append elem
-      newhead.next = this
+      newhead.next = this.unsafeAssumePure
       newhead
     }
     // shifts right assuming enough space
@@ -340,7 +342,7 @@ object UnrolledBuffer extends StrictOptimizedClassTagSeqFactory[UnrolledBuffer]
         val r = array(idx)
         shiftleft(idx)
         size -= 1
-        if (tryMergeWithNext()) buffer.lastPtr = this
+        if (tryMergeWithNext()) buffer.lastPtr = this.unsafeAssumePure
         r
       } else next.remove(idx - size, buffer)
 
@@ -397,7 +399,7 @@ object UnrolledBuffer extends StrictOptimizedClassTagSeqFactory[UnrolledBuffer]
         curr.next = newnextnode
 
         // try to merge the last node of this with the newnextnode and fix tail pointer if needed
-        if (curr.tryMergeWithNext()) buffer.lastPtr = curr
+        if (curr.tryMergeWithNext()) buffer.lastPtr = curr.unsafeAssumePure
         else if (newnextnode.next eq null) buffer.lastPtr = newnextnode
         appended
       }