Fix #3823: Unpickle type holes

[nicolasstucki]

No description provided.

[nicolasstucki]

@odersky could you have a look at this one. I did not manage to use the MacroTransformWithImplicits and inferImplicitArg to get the implicit argument. I looked at b466e0a but it might be too outdated.

[nicolasstucki]

This quote will be pickled as

type T = [[1| ]]
{
  val z: T = [[0| ]]
  ()
}

[nicolasstucki]

After ReifyQuotes the tree is

package <empty> {
  import dotty.tools.dotc.quoted.Runners._
  import scala.quoted._
  final lazy module val Test: Test$ = new Test$()
  @scala.annotation.internal.SourceFile("Foo.scala") final module class Test$()
     extends
   Object() { this: Test.type => 
    def main(args: Array[String]): Unit = 
      {
        def f[T >: Nothing <: Any](x: quoted.Expr[T])(implicit t: quoted.Type[T]
          )
        : scala.quoted.Expr[Unit] = 
          scala.runtime.quoted.Unpickler.unpickleExpr[Unit](
            scala.collection.immutable.Nil.::[String](
              
                "\\¡«\037\0203\0200\00\00\00\00\00\00\00\00\00\00\00\00\016\02M0£\01\0204ASTs\01\0206_root_\01\0201\'\01\0203Any\01\0205scala\01\0201z\01\0201T\0200¤\0200¢6\0201\0201\0236\0202u\02036\0204\0214\0226\0214\0211\02\0201\0206\02055\0230ÿ\0201\0200\0203\0211\0206£\0205ÿ\0201\02012\0235\025\025"
              
            )
          , [x,new scala.quoted.Type[T]() : Any])
        println(
          f[Int](
            scala.runtime.quoted.Unpickler.unpickleExpr[Int](
              scala.collection.immutable.Nil.::[String](
                
                  "\\¡«\037\0203\0200\00\00\00\00\00\00\00\00\00\00\00\00\n%fð\0235\01\0204ASTs\01\0206_root_\01\0201\'\01\0203Any\01\0205scala\0200\0216\0200\02146\0201\0201\0210\0202u\02036\0204<\0202\025"
                
              )
            , [ : Any])
          )(quoted.Type.IntTag).show(
            dotty.tools.dotc.quoted.Runners.runner[Unit]
          )
        )
      }
  }
}

Note that the arguments for the holes are x and new scala.quoted.Type[T]() hence when unpickling we get an instance of scala.quoted.Type with an erased T. Instead of new scala.quoted.Type[T]() we should have t.

[nicolasstucki]

If we have

def f[T](x: Expr[T])(implicit t: Type[T]) = '{
   val z: ~t= ~x
}

then the hole receives the t and works as expected.

[odersky]

Otherwise LGTM

[odersky]

Maybe add a dollar to the name? It's unambiguous now, but if we sometimes in the future would add toplevel ValDefs to Scala someone could write val quote = ....

[nicolasstucki]

It should be safe as this will only be contained in expressions Expr[T] and expressions do not have packages (hence not top level vals). Regardless the name with $ is better as it shows that it is a synthetic construct.

[odersky]

Same here, probably safer to write e.g. $typeQuote.