Optimize encoding of TastyString

Most pickled quotes fit in a single string. We avoid the creation of the list for single string by typing as `String | List[String]`.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/quoted/PickledQuotes.scala

@@ -51,7 +51,7 @@ object PickledQuotes {
   }
 
   /** Unpickle the tree contained in the TastyExpr */
-  def unpickleTerm(pickled: List[String], fillHole: Int => Seq[Any] => Any)(using Context): Tree = {
+  def unpickleTerm(pickled: String | List[String], fillHole: Int => Seq[Any] => Any)(using Context): Tree = {
     val unpickled = withMode(Mode.ReadPositions)(unpickle(pickled, isType = false))
     val Inlined(call, Nil, expnasion) = unpickled
     val inlineCtx = inlineContext(call)
@@ -61,7 +61,7 @@ object PickledQuotes {
   }
 
   /** Unpickle the tree contained in the TastyType */
-  def unpickleTypeTree(pickled: List[String], fillHole: Int => Seq[Any] => Any)(using Context): Tree = {
+  def unpickleTypeTree(pickled: String | List[String], fillHole: Int => Seq[Any] => Any)(using Context): Tree = {
     val unpickled = withMode(Mode.ReadPositions)(unpickle(pickled, isType = true))
     spliceTypes(unpickled, fillHole)
   }
@@ -175,8 +175,11 @@ object PickledQuotes {
   }
 
   /** Unpickle TASTY bytes into it's tree */
-  private def unpickle(pickled: List[String], isType: Boolean)(using Context): Tree = {
-    val bytes = TastyString.unpickle(pickled)
+  private def unpickle(pickled: String | List[String], isType: Boolean)(using Context): Tree = {
+    val bytes = pickled match
+      case pickled: String => TastyString.unpickle(pickled)
+      case pickled: List[String] => TastyString.unpickle(pickled)
+
     quotePickling.println(s"**** unpickling quote from TASTY\n${TastyPrinter.show(bytes)}")
 
     val mode = if (isType) UnpickleMode.TypeTree else UnpickleMode.Term

compiler/src/dotty/tools/dotc/quoted/QuoteContextImpl.scala

@@ -2621,11 +2621,11 @@ class QuoteContextImpl private (ctx: Context) extends QuoteContext, scala.intern
 
   end reflect
 
-  def unpickleExpr[T](pickled: List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Expr[T] =
+  def unpickleExpr[T](pickled: String | List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Expr[T] =
     val tree = PickledQuotes.unpickleTerm(pickled, fillHole)(using reflect.rootContext)
     new scala.quoted.internal.Expr(tree, hash).asInstanceOf[scala.quoted.Expr[T]]
 
-  def unpickleType[T <: AnyKind](pickled: List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Type[T] =
+  def unpickleType[T <: AnyKind](pickled: String | List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Type[T] =
     val tree = PickledQuotes.unpickleTypeTree(pickled, fillHole)(using reflect.rootContext)
     new scala.quoted.internal.Type(tree, hash).asInstanceOf[scala.quoted.Type[T]]
 

compiler/src/dotty/tools/dotc/quoted/TastyString.scala

@@ -22,4 +22,9 @@ object TastyString {
     strings.foreach(string.append)
     Base64.getDecoder().decode(string.result().getBytes(UTF_8))
   }
+
+  /** Decode the Strings into TASTY bytes */
+  def unpickle(string: String): Array[Byte] =
+    Base64.getDecoder().decode(string.getBytes(UTF_8))
+
 }

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

@@ -200,7 +200,10 @@ class PickleQuotes extends MacroTransform {
        *  this closure is always applied directly to the actual context and the BetaReduce phase removes it.
        */
       def pickleAsTasty() = {
-        val pickledQuoteStrings = liftList(PickledQuotes.pickleQuote(body).map(x => Literal(Constant(x))), defn.StringType)
+        val pickleQuote = PickledQuotes.pickleQuote(body)
+        val pickledQuoteStrings = pickleQuote match
+          case x :: Nil => Literal(Constant(x))
+          case xs => liftList(xs.map(x => Literal(Constant(x))), defn.StringType)
         val splicesList = liftList(splices, defn.FunctionType(1).appliedTo(defn.SeqType.appliedTo(defn.AnyType), defn.AnyType))
         val quoteClass = if isType then defn.QuotedTypeClass else defn.QuotedExprClass
         val quotedType = quoteClass.typeRef.appliedTo(originalTp)

library/src/scala/internal/quoted/QuoteContextInternal.scala

@@ -9,12 +9,12 @@ trait QuoteContextInternal { self: QuoteContext =>
   /** Unpickle `repr` which represents a pickled `Expr` tree,
    *  replacing splice nodes with `holes`
    */
-  def unpickleExpr[T](pickled: List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Expr[T]
+  def unpickleExpr[T](pickled: String | List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Expr[T]
 
   /** Unpickle `repr` which represents a pickled `Type` tree,
    *  replacing splice nodes with `holes`
    */
-  def unpickleType[T <: AnyKind](pickled: List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Type[T]
+  def unpickleType[T <: AnyKind](pickled: String | List[String], fillHole: Int => Seq[Any] => Any): scala.quoted.Type[T]
 
   val ExprMatch: ExprMatchModule