Add support for the into modifier.

build.sbt

@@ -80,6 +80,7 @@ lazy val testSources = crossProject(JSPlatform, JVMPlatform)
   .settings(
     publish / skip := true,
     scalacOptions += "-Werror",
+    scalacOptions += "-preview",
     javacOptions += "-parameters",
   )
   .dependsOn(scala2TestSources)

tasty-query/shared/src/main/scala/tastyquery/Flags.scala

@@ -60,6 +60,7 @@ private[tastyquery] object Flags:
   val Infix: Flag = newFlag("Infix")
   val Inline: Flag = newFlag("Inline")
   val InlineProxy: Flag = newFlag("InlineProxy")
+  val Into: Flag = newFlag("Into")
   val JavaDefined: Flag = newFlag("JavaDefined")
   val Lazy: Flag = newFlag("Lazy")
   val Local: Flag = newFlag("Local")

tasty-query/shared/src/main/scala/tastyquery/Symbols.scala

@@ -692,6 +692,9 @@ object Symbols {
 
     final def isOpaqueTypeAlias: Boolean = flags.is(Opaque)
 
+    /** Is this a type with the `into` modifier? */
+    final def isIntoType: Boolean = flags.is(Into)
+
     private[tastyquery] final def topLevelRef: TypeRef =
       require(owner.isPackage, s"Cannot construct a topLevelRef for non-top-level symbol $this")
       TypeRef(owner.asPackage.packageRef, this)

tasty-query/shared/src/main/scala/tastyquery/reader/tasties/TastyFormat.scala

@@ -224,6 +224,7 @@ import scala.annotation.switch
   *                  EXPORTED                                                         -- An export forwarder
   *                  OPEN                                                             -- an open class
   *                  INVISIBLE                                                        -- invisible during typechecking
+  *                  INTO                                                             -- has the `into` modifier
   *                  Annotation
   *
   *  Variance      = STABLE                                                           -- invariant
@@ -489,6 +490,7 @@ private[tasties] object TastyFormat:
   final val INVISIBLE = 44
   final val EMPTYCLAUSE = 45
   final val SPLITCLAUSE = 46
+  final val INTO = 49
 
   // Cat. 2:    tag Nat
 
@@ -614,7 +616,7 @@ private[tasties] object TastyFormat:
 
   /** Useful for debugging */
   def isLegalTag(tag: Int): Boolean =
-    firstSimpleTreeTag <= tag && tag <= SPLITCLAUSE
+    firstSimpleTreeTag <= tag && tag <= INTO
       || firstNatTreeTag <= tag && tag <= RENAMED
       || firstASTTreeTag <= tag && tag <= BOUNDED
       || firstNatASTTreeTag <= tag && tag <= NAMEDARG
@@ -628,7 +630,7 @@ private[tasties] object TastyFormat:
     case PRIVATE | PROTECTED | ABSTRACT | FINAL | SEALED | CASE | IMPLICIT | GIVEN | ERASED | LAZY | OVERRIDE | INLINE |
         INLINEPROXY | MACRO | OPAQUE | STATIC | OBJECT | TRAIT | TRANSPARENT | INFIX | ENUM | LOCAL | SYNTHETIC |
         ARTIFACT | MUTABLE | FIELDaccessor | CASEaccessor | COVARIANT | CONTRAVARIANT | HASDEFAULT | STABLE |
-        EXTENSION | PARAMsetter | PARAMalias | EXPORTED | OPEN | INVISIBLE | ANNOTATION | PRIVATEqualified |
+        EXTENSION | PARAMsetter | PARAMalias | EXPORTED | OPEN | INVISIBLE | INTO | ANNOTATION | PRIVATEqualified |
         PROTECTEDqualified =>
       true
     case _ =>
@@ -687,6 +689,7 @@ private[tasties] object TastyFormat:
     case PARAMalias    => "PARAMalias"
     case EMPTYCLAUSE   => "EMPTYCLAUSE"
     case SPLITCLAUSE   => "SPLITCLAUSE"
+    case INTO          => "INTO"
 
     case SHAREDterm    => "SHAREDterm"
     case SHAREDtype    => "SHAREDtype"

tasty-query/shared/src/main/scala/tastyquery/reader/tasties/TreeUnpickler.scala

@@ -328,6 +328,7 @@ private[tasties] class TreeUnpickler private (
         case INVISIBLE     => ignoreFlag()
         case TRANSPARENT   => addFlag(Transparent)
         case INFIX         => addFlag(Infix)
+        case INTO          => addFlag(Into)
         case PRIVATEqualified =>
           ignoreFlag()
           skipTree()

tasty-query/shared/src/test/scala/tastyquery/ReadTreeSuite.scala

@@ -2411,4 +2411,21 @@ class ReadTreeSuite extends RestrictedUnpicklingSuite {
     }
     assert(containsSubtree(myMapStructure)(clue(myMapDef)))
   }
+
+  testUnpickle("into-modifier", "simple_trees.IntoModifier") { tree =>
+    val intoClass: StructureCheck = {
+      case ClassDef(SimpleTypeName("IntoClass"), _, sym) if sym.isIntoType =>
+    }
+    assert(containsSubtree(intoClass)(clue(tree)))
+
+    val intoTrait: StructureCheck = {
+      case ClassDef(SimpleTypeName("IntoTrait"), _, sym) if sym.isIntoType =>
+    }
+    assert(containsSubtree(intoTrait)(clue(tree)))
+
+    val intoEnum: StructureCheck = {
+      case ClassDef(SimpleTypeName("IntoEnum"), _, sym) if sym.isIntoType =>
+    }
+    assert(containsSubtree(intoEnum)(clue(tree)))
+  }
 }

test-sources/src/main/scala/simple_trees/IntoModifier.scala

@@ -0,0 +1,15 @@
+package simple_trees
+
+import scala.Conversion.into
+
+class IntoModifier {
+  def methodWithIntoString(x: into[String]): Unit = ()
+
+  into class IntoClass
+
+  into trait IntoTrait
+
+  into enum IntoEnum {
+    case Foo
+  }
+}