Stick TASTy reading behind a -Ytasty-reader flag

doc/internal/tastyreader.md

@@ -8,6 +8,8 @@ TASTy is an intermediate representation of a Scala program after type checking a
 
 ### Compiler flags
 
+- `-Ytasty-reader` enables the support for reading Scala 3's TASTy files.
+
 - `-Ydebug-tasty` enables rich output when traversing tasty files, important for tracing the history of events when diagnosing errors.
 
 - `-Ytasty-no-annotations` ignores all annotations on tasty symbols, may be useful for ignoring complex annotations that are unsupported, but will prevent safety checks for pattern matching.

src/compiler/scala/tools/nsc/settings/ScalaSettings.scala

@@ -261,6 +261,7 @@ trait ScalaSettings extends StandardScalaSettings with Warnings { _: MutableSett
   val YmacroFresh     = BooleanSetting    ("-Ymacro-global-fresh-names", "Should fresh names in macros be unique across all compilation units")
   val YmacroAnnotations = BooleanSetting  ("-Ymacro-annotations", "Enable support for macro annotations, formerly in macro paradise.")
   val YtastyNoAnnotations = BooleanSetting("-Ytasty-no-annotations", "Disable support for reading annotations from TASTy, this will prevent safety features such as pattern match exhaustivity and reachability analysis.")
+  val YtastyReader        = BooleanSetting("-Ytasty-reader", "Enable support for reading Scala 3's TASTy files, allowing consumption of libraries compiled with Scala 3 (provided they don't use any Scala 3 only features).")
   val Yreplclassbased = BooleanSetting    ("-Yrepl-class-based", "Use classes to wrap REPL snippets instead of objects", default = true)
   val YreplMagicImport = BooleanSetting   ("-Yrepl-use-magic-imports", "In the code that wraps REPL snippets, use magic imports rather than nesting wrapper object/classes", default = true)
   val Yreploutdir     = StringSetting     ("-Yrepl-outdir", "path", "Write repl-generated classfiles to given output directory (use \"\" to generate a temporary dir)" , "")

src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala

@@ -81,6 +81,7 @@ abstract class ClassfileParser(reader: ReusableInstance[ReusableDataReader]) {
   protected def moduleClass: Symbol = staticModule.moduleClass
   protected val TASTYUUIDLength: Int = 16
   private var sawPrivateConstructor = false
+  private var YtastyReader          = false
 
   private def ownerForFlags(jflags: JavaAccFlags) = if (jflags.isStatic) moduleClass else clazz
 
@@ -160,6 +161,7 @@ abstract class ClassfileParser(reader: ReusableInstance[ReusableDataReader]) {
         this.clazz        = clazz
         this.staticModule = module
         this.isScala      = false
+        this.YtastyReader = settings.YtastyReader
 
         val magic = in.getInt(in.bp)
         if (magic != JAVA_MAGIC && file.name.endsWith(".sig")) {
@@ -1114,6 +1116,8 @@ abstract class ClassfileParser(reader: ReusableInstance[ReusableDataReader]) {
         case tpnme.ScalaATTR =>
           isScalaRaw = true
           i = numAttrs
+        case tpnme.TASTYATTR if !YtastyReader =>
+          MissingRequirementError.signal(s"Add -Ytasty-reader to scalac options to parse the TASTy in $file")
         case tpnme.TASTYATTR =>
           isTASTY = true
           TASTYAttrLen = attrLen

src/tastytest/scala/tools/tastytest/TastyTest.scala

@@ -60,7 +60,7 @@ object TastyTest {
 
   private def scalacPos(out: String, sourceRoot: String, additionalSettings: Seq[String], sources: String*): Try[Unit] = {
     println(s"compiling sources in ${yellow(sourceRoot)} with scalac.")
-    successWhen(Scalac.scalac(out, additionalSettings, sources:_*))("scalac failed to compile sources.")
+    successWhen(Scalac.scalac(out, "-Ytasty-reader" +: additionalSettings, sources:_*))("scalac failed to compile sources.")
   }
 
   private def scalacNeg(out: String, additionalSettings: Seq[String], files: String*): Try[Unit] = {
@@ -89,7 +89,7 @@ object TastyTest {
           }
           val compiled = Console.withErr(byteArrayStream) {
             Console.withOut(byteArrayStream) {
-              Scalac.scalac(out, additionalSettings, source)
+              Scalac.scalac(out, "-Ytasty-reader" +: additionalSettings, source)
             }
           }
           byteArrayStream.flush()