allow tasty version to be inspected

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -247,7 +247,7 @@ class CompilationTests {
           //  "-source", "3.1",  // TODO: re-enable once we allow : @unchecked in pattern definitions. Right now, lots of narrowing pattern definitions fail.
           ))(libGroup)
 
-    val tastyCoreSources = sources(Paths.get("tasty/src"))
+    val tastyCoreSources = sources(Paths.get("tasty/src")) ++ sources(Paths.get("tasty/src-bootstrapped"))
     val tastyCore = compileList("tastyCore", tastyCoreSources, opt)(tastyCoreGroup)
 
     val compilerSources = sources(Paths.get("compiler/src")) ++ sources(Paths.get("compiler/src-bootstrapped"))

tasty/src-bootstrapped/dotty/tools/tasty/TastyHeaderUnpickler.scala

@@ -0,0 +1,46 @@
+package dotty.tools.tasty
+
+import java.util.UUID
+
+import TastyFormat.{MajorVersion, MinorVersion, header}
+
+sealed abstract case class TastyHeader(uuid: UUID, majorVersion: Int, minorVersion: Int)
+
+class TastyHeaderUnpickler(reader: TastyReader) {
+  import reader._
+
+  def this(bytes: Array[Byte]) = this(new TastyReader(bytes))
+
+  /** delegates to `readFullHeader`, extracting the UUID */
+  def readHeader(): UUID =
+    readFullHeader().uuid
+
+  /**Reads the header of a Tasty File, the returned header, `h`,
+   * has the following properties:
+   * - `h.majorVersion == TastyFormat.MajorVersion`
+   * - `0 <= h.minorVersion <= TastyFormat.MinorVersion`
+   */
+  def readFullHeader(): TastyHeader = {
+    for (i <- 0 until header.length)
+      check(readByte() == header(i), "not a TASTy file")
+    val majorVersion = readNat()
+    val minorVersion = readNat()
+    val validVersion = (
+      majorVersion == MajorVersion
+      && minorVersion >= 0 && minorVersion <= MinorVersion
+    )
+    check(validVersion,
+      s"""TASTy signature has wrong version.
+         | expected: $MajorVersion.$MinorVersion
+         | found   : ${majorVersion}.${minorVersion}""".stripMargin
+    )
+    val uuid = new UUID(readUncompressedLong(), readUncompressedLong())
+    new TastyHeader(uuid, majorVersion, minorVersion) {}
+  }
+
+  def isAtEnd: Boolean = reader.isAtEnd
+
+  private def check(cond: Boolean, msg: => String): Unit = {
+    if (!cond) throw new UnpickleException(msg)
+  }
+}