Fix #3875 and other details

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -950,7 +950,7 @@ object Scanners {
         setStrVal()
       } else {
         token = op
-        strVal = name.toString
+        strVal = if (name != null) name.toString else null
         litBuf.clear()
       }
     }

library/src/scala/quoted/Liftable.scala

@@ -14,7 +14,9 @@ abstract class Liftable[T] {
  */
 object Liftable {
 
-  final class ConstantExpr[T] private[Liftable](val value: T) extends Expr[T]
+  final class ConstantExpr[T] private[Liftable](val value: T) extends Expr[T] {
+    override def toString: String = s"Expr($value)"
+  }
 
   implicit def BooleanIsLiftable: Liftable[Boolean] = (x: Boolean) => new ConstantExpr(x)
   implicit def ByteLiftable: Liftable[Byte] = (x: Byte) => new ConstantExpr(x)

library/src/scala/quoted/TastyExpr.scala

@@ -4,5 +4,5 @@ import scala.runtime.quoted.Unpickler.Pickled
 
 /** An Expr backed by a pickled TASTY tree */
 final case class TastyExpr[T](tasty: Pickled, args: Seq[Any]) extends Expr[T] with TastyQuoted {
-  override def toString(): String = s"TastyExpr(<TASTY>, $args)"
+  override def toString(): String = s"Expr(<pickled>)"
 }

library/src/scala/quoted/TastyType.scala

@@ -4,5 +4,5 @@ import scala.runtime.quoted.Unpickler.Pickled
 
 /** A Type backed by a pickled TASTY tree */
 final case class TastyType[T](tasty: Pickled, args: Seq[Any]) extends Type[T] with TastyQuoted {
-  override def toString(): String = s"TastyType(<TASTY>, $args)"
+  override def toString(): String = s"Type(<pickled>)"
 }

library/src/scala/quoted/Type.scala

@@ -9,7 +9,9 @@ abstract class Type[T] extends Quoted {
 /** Some basic type tags, currently incomplete */
 object Type {
 
-  class TaggedPrimitive[T] private[Type] (implicit val ct: ClassTag[T]) extends Type[T]
+  final class TaggedPrimitive[T] private[Type] (implicit val ct: ClassTag[T]) extends Type[T] {
+    override def toString: String = s"Type($ct)"
+  }
 
   implicit def UnitTag: Type[Unit] = new TaggedPrimitive[Unit]
   implicit def BooleanTag: Type[Boolean] = new TaggedPrimitive[Boolean]