Require seeing ScalaSig before scanning for Scala pickling annotations

An alternative fix for scala#15166, which aligns with the behavior of the
Scala 2 classfile parser.

Before this commit, all classfiles, including those produced by the Java
compiler and compilers of other JVM languages, were scanned for Scala
pickling annotations. In certain situations (as in tests/pos/i15166),
this is problematic, as the denotation of an annotation symbol defined as
a Java inner class may be forced before the inner class table is populated
and setClassInfo is called on the class root.

We avoid this situation by only performing the pickling annotation scan for
those classfiles having the `ScalaSig` attribute, i.e. those produced by
the Scala 2 compiler.

We also drop support for pickling TASTy using the classfile annotations
  `scala.annotation.internal.TASTYSignature` and
  `scala.annotation.internal.TASTYLongSignature`.
These were never used by the compiler, there are no plans for future use,
and preserving support would complicate this fix.

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -961,6 +961,8 @@ class Definitions {
   @tu lazy val NativeAnnot: ClassSymbol = requiredClass("scala.native")
   @tu lazy val RepeatedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Repeated")
   @tu lazy val SourceFileAnnot: ClassSymbol = requiredClass("scala.annotation.internal.SourceFile")
+  @tu lazy val ScalaSignatureAnnot: ClassSymbol = requiredClass("scala.reflect.ScalaSignature")
+  @tu lazy val ScalaLongSignatureAnnot: ClassSymbol = requiredClass("scala.reflect.ScalaLongSignature")
   @tu lazy val ScalaStrictFPAnnot: ClassSymbol = requiredClass("scala.annotation.strictfp")
   @tu lazy val ScalaStaticAnnot: ClassSymbol = requiredClass("scala.annotation.static")
   @tu lazy val SerialVersionUIDAnnot: ClassSymbol = requiredClass("scala.SerialVersionUID")

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -50,12 +50,6 @@ object ClassfileParser {
         mapOver(tp)
     }
   }
-
-  private inline def sigOfClassName(n: String) = s"L$n;"
-  val ScalaSignatureAnnot: String = sigOfClassName("scala.reflect.ScalaSignature")
-  val ScalaLongSignatureAnnot: String = sigOfClassName("scala.reflect.ScalaLongSignature")
-  val TASTYSignatureAnnot: String = sigOfClassName("scala.annotation.internal.TASTYSignature")
-  val TASTYLongSignatureAnnot: String = sigOfClassName("scala.annotation.internal.TASTYLongSignature")
 }
 
 class ClassfileParser(
@@ -876,7 +870,7 @@ class ClassfileParser(
 
   /** Parse inner classes. Expects `in.bp` to point to the superclass entry.
    *  Restores the old `bp`.
-   *  @return true iff classfile is from Scala, so no Java info needs to be read.
+   *  @return Some(unpickler) iff classfile is from Scala, so no Java info needs to be read.
    */
   def unpickleOrParseInnerClasses()(using ctx: Context, in: DataReader): Option[Embedded] = {
     val oldbp = in.bp
@@ -1005,25 +999,21 @@ class ClassfileParser(
         // attribute isn't, this classfile is a compilation artifact.
         return Some(NoEmbedded)
 
-      if (scan(tpnme.RuntimeVisibleAnnotationATTR) || scan(tpnme.RuntimeInvisibleAnnotationATTR)) {
+      if (scan(tpnme.ScalaSignatureATTR) && scan(tpnme.RuntimeVisibleAnnotationATTR)) {
         val attrLen = in.nextInt
         val nAnnots = in.nextChar
         var i = 0
         while (i < nAnnots) {
-          val attrSig = pool.getExternalName(in.nextChar).value
+          val attrClass = pool.getType(in.nextChar).typeSymbol
           val nArgs = in.nextChar
           var j = 0
           while (j < nArgs) {
             val argName = pool.getName(in.nextChar)
             if (argName.name == nme.bytes) {
-              if attrSig == ScalaSignatureAnnot then
+              if attrClass == defn.ScalaSignatureAnnot then
                 return unpickleScala(parseScalaSigBytes)
-              else if attrSig == ScalaLongSignatureAnnot then
+              else if attrClass == defn.ScalaLongSignatureAnnot then
                 return unpickleScala(parseScalaLongSigBytes)
-              else if attrSig == TASTYSignatureAnnot then
-                return unpickleTASTY(parseScalaSigBytes)
-              else if attrSig == TASTYLongSignatureAnnot then
-                return unpickleTASTY(parseScalaLongSigBytes)
             }
             parseAnnotArg(skip = true)
             j += 1

library/src/scala/annotation/internal/TASTYLongSignature.java

@@ -7,6 +7,7 @@
 
 @Retention(RetentionPolicy.RUNTIME)
 @Target(ElementType.TYPE)
+@Deprecated
 public @interface TASTYLongSignature {
     public String[] bytes();
 }

library/src/scala/annotation/internal/TASTYSignature.java

@@ -7,6 +7,7 @@
 
 @Retention(RetentionPolicy.RUNTIME)
 @Target(ElementType.TYPE)
+@Deprecated
 public @interface TASTYSignature {
     public String bytes();
 }