Add an -Yimplicit-as-given flag to easily test changes in the ecosystem.

It replaces every implicit definition with `given` (including classes,
which I was surprised to see working), and every implicit parameter
clause with `using`, all in the parser phase. Symbols and trees
unpickled from tasty are unaffected by this flag. Trees generated using
the reflection api are unaffected as well.

Author: jchyb

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -458,6 +458,7 @@ private sealed trait YSettings:
 
   val Yinstrument: Setting[Boolean] = BooleanSetting(ForkSetting, "Yinstrument", "Add instrumentation code that counts allocations and closure creations.")
   val YinstrumentDefs: Setting[Boolean] = BooleanSetting(ForkSetting, "Yinstrument-defs", "Add instrumentation code that counts method calls; needs -Yinstrument to be set, too.")
+  val YimplicitAsGiven: Setting[Boolean] = BooleanSetting(ForkSetting, "Yimplicit-as-given", "Interpret the compiled implicit keywords as their scala-3 given counterparts.")
 
   // Deprecated: lifted from -Y to -X
   @deprecated(message = "Lifted to -X, Scheduled for removal.", since = "3.5.0")

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

@@ -3283,6 +3283,7 @@ object Parsers {
     private def modOfToken(tok: Int, name: Name): Mod = tok match {
       case ABSTRACT    => Mod.Abstract()
       case FINAL       => Mod.Final()
+      case IMPLICIT if ctx.settings.YimplicitAsGiven.value => Mod.Given()
       case IMPLICIT    => Mod.Implicit()
       case GIVEN       => Mod.Given()
       case LAZY        => Mod.Lazy()
@@ -3552,7 +3553,7 @@ object Parsers {
 
       def paramMods() =
         if in.token == IMPLICIT then
-          addParamMod(() => Mod.Implicit())
+          addParamMod(() => if (ctx.settings.YimplicitAsGiven.value) Mod.Given() else Mod.Implicit())
         else if isIdent(nme.using) then
           if initialMods.is(Given) then
             syntaxError(em"`using` is already implied here, should not be given explicitly", in.offset)

tests/pos-macros/i22482/Macro_1.scala

@@ -0,0 +1,22 @@
+//> using options -Yimplicit-as-given
+import scala.quoted._
+object Macro:
+  inline def test[I] = ${testImpl[I]}
+  def testImpl[I: Type](using Quotes): Expr[Unit] =
+    import quotes.reflect._
+
+    val implicitClause = '{
+      def clause(implicit num: Int) = ???
+    }
+    val expectedImplicitClause =
+      """|{
+         |  def clause(using num: scala.Int): scala.Nothing = scala.Predef.???
+         |  ()
+         |}""".stripMargin
+
+    assert(implicitClause.show == expectedImplicitClause)
+
+    // test ImplicitClauseInCaseClass
+    assert(TypeRepr.of[I].typeSymbol.primaryConstructor.paramSymss(1)(0).flags.is(Flags.Given))
+
+    '{()}

tests/pos-macros/i22482/Test_2.scala

@@ -0,0 +1,3 @@
+//> using options -Yimplicit-as-given
+case class ImplicitClauseInCaseClass(dummy: Int)(implicit num: Int)
+def Test() = Macro.test[ImplicitClauseInCaseClass]

tests/run/i22482.check

@@ -0,0 +1 @@
+found if given

tests/run/i22482.scala

@@ -0,0 +1,15 @@
+//> using options -Yimplicit-as-given
+
+object Lib:
+  class LibComponent:
+    def msg = "found if given"
+  implicit val libComponent: LibComponent = LibComponent()
+
+  class UserComponent extends LibComponent:
+      override def msg = "found if implicit"
+  implicit val userComponent: UserComponent = UserComponent()
+
+  def printComponent(implicit c: LibComponent) = println(c.msg)
+
+
+@main def Test = Lib.printComponent