Update reference, MiMa previous version and sync TASTy version (scala#22187)

* Update reference version to 3.6.3-RC1 (from 3.6.0)
* Update mima previous binary verison to 3.6.2 (instead of unofficial
3.6.1)
* Set TASTy version to `28.7-experimental-1` - it should have been set
when branching of 3.6.3.
* We now document better how and when tasty version should be set 
* Add additional runtime test to ensure we don't emit invalid TASTy
version during Release / NIGHTLY releases and the expected version set
in build matches version defined in TastyFormat

Author: WojciechMazur

Diff for: project/Build.scala

@@ -93,18 +93,21 @@ object Build {
 
   /** Version of the Scala compiler used to build the artifacts.
    *  Reference version should track the latest version pushed to Maven:
-   *  - In main branch it should be the last RC version (using experimental TASTy required for non-bootstrapped tests)
+   *  - In main branch it should be the last RC version
    *  - In release branch it should be the last stable release
-   *  3.6.0-RC1 was released as 3.6.0 - it's having and experimental TASTy version
+   * 
+   *  Warning: Change of this variable needs to be consulted with `expectedTastyVersion` 
    */
-  val referenceVersion = "3.6.0"
+  val referenceVersion = "3.6.3-RC1"
 
   /** Version of the Scala compiler targeted in the current release cycle
    *  Contains a version without RC/SNAPSHOT/NIGHTLY specific suffixes
    *  Should be updated ONLY after release or cutoff for previous release cycle.
    *
    *  Should only be referred from `dottyVersion` or settings/tasks requiring simplified version string,
    *  eg. `compatMode` or Windows native distribution version.
+   * 
+   *  Warning: Change of this variable might require updating `expectedTastyVersion` 
    */
   val developedVersion = "3.6.4"
 
@@ -116,6 +119,25 @@ object Build {
    *  During final, stable release is set exactly to `developedVersion`.
   */
   val baseVersion = s"$developedVersion-RC1"
+  
+  /** The version of TASTY that should be emitted, checked in runtime test 
+   *  For defails on how TASTY version should be set see related discussions: 
+   *    - https://github.com/scala/scala3/issues/13447#issuecomment-912447107
+   *    - https://github.com/scala/scala3/issues/14306#issuecomment-1069333516
+   *    - https://github.com/scala/scala3/pull/19321
+   *   
+   *  Simplified rules, given 3.$minor.$patch = $developedVersion
+   *    - Major version is always 28
+   *    - TASTY minor version:
+   *      - in main (NIGHTLY): {if $patch == 0 then $minor else ${minor + 1}}
+   *      - in release branch is always equal to $minor
+   *    - TASTY experimental version:
+   *      - in main (NIGHTLY) is always experimental
+   *      - in release candidate branch is experimental if {patch == 0}
+   *      - in stable release is always non-experimetnal
+   */
+  val expectedTastyVersion = "28.7-experimental-1"
+  checkReleasedTastyVersion()
 
   /** Final version of Scala compiler, controlled by environment variables. */
   val dottyVersion = {
@@ -149,9 +171,9 @@ object Build {
    *  For a developedVersion `3.M.P` the mimaPreviousDottyVersion should be set to:
    *   - `3.M.0`     if `P > 0`
    *   - `3.(M-1).0` if `P = 0`
-   *  3.6.1 is an exception from this rule - 3.6.0 was a broken release
+   *  3.6.2 is an exception from this rule - 3.6.0 was a broken release, 3.6.1 was hotfix (unstable) release
    */
-  val mimaPreviousDottyVersion = "3.6.1"
+  val mimaPreviousDottyVersion = "3.6.2"
 
   /** LTS version against which we check binary compatibility.
    *
@@ -2424,6 +2446,9 @@ object Build {
       settings(disableDocSetting).
       settings(
         versionScheme := Some("semver-spec"),
+        Test / envVars ++= Map(
+          "EXPECTED_TASTY_VERSION" -> expectedTastyVersion,
+        ),
         if (mode == Bootstrapped) Def.settings(
           commonMiMaSettings,
           mimaForwardIssueFilters := MiMaFilters.TastyCore.ForwardsBreakingChanges,
@@ -2473,6 +2498,34 @@ object Build {
       case Bootstrapped => commonBootstrappedSettings
     })
   }
+  
+  /* Tests TASTy version invariants during NIGHLY, RC or Stable releases */
+  def checkReleasedTastyVersion(): Unit = {
+    lazy val (scalaMinor, scalaPatch, scalaIsRC) = baseVersion.split("\\.|-").take(4) match {
+      case Array("3", minor, patch)    => (minor.toInt, patch.toInt, false)
+      case Array("3", minor, patch, _) => (minor.toInt, patch.toInt, true)
+      case other => sys.error(s"Invalid Scala base version string: $baseVersion")
+    }
+    lazy val (tastyMinor, tastyIsExperimental) = expectedTastyVersion.split("\\.|-").take(4) match {
+      case Array("28", minor)                    => (minor.toInt, false)
+      case Array("28", minor, "experimental", _) => (minor.toInt, true)
+      case other => sys.error(s"Invalid TASTy version string: $expectedTastyVersion")
+    }
+     
+    if(isNightly) {
+      assert(tastyIsExperimental, "TASTY needs to be experimental in nightly builds")
+      val expectedTastyMinor = if(scalaPatch == 0) scalaMinor else scalaMinor + 1
+      assert(tastyMinor == expectedTastyMinor, "Invalid TASTy minor version")
+    }
+    
+    if(isRelease) {
+      assert(scalaMinor == tastyMinor, "Minor versions of TASTY vesion and Scala version should match in release builds")
+      if (scalaIsRC && scalaPatch == 0)
+        assert(tastyIsExperimental, "TASTy should be experimental when releasing a new minor version RC")
+      else
+        assert(!tastyIsExperimental, "Stable version cannot use experimental TASTY") 
+    }
+  }
 }
 
 object ScaladocConfigs {

Diff for: tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -324,7 +324,7 @@ object TastyFormat {
    *  compatibility, but remains backwards compatible, with all
    *  preceding `MinorVersion`.
    */
-  final val MinorVersion: Int = 6
+  final val MinorVersion: Int = 7
 
   /** Natural Number. The `ExperimentalVersion` allows for
    *  experimentation with changes to TASTy without committing

Diff for: tasty/test/dotty/tools/tasty/BuildTastyVersionTest.scala

@@ -0,0 +1,26 @@
+package dotty.tools.tasty
+
+import org.junit.Assert._
+import org.junit.Test
+
+import TastyBuffer._
+
+// Tests ensuring TASTY version emitted by compiler is matching expected TASTY version
+class BuildTastyVersionTest {
+  
+  val CurrentTastyVersion = TastyVersion(TastyFormat.MajorVersion, TastyFormat.MinorVersion, TastyFormat.ExperimentalVersion)
+  
+  // Needs to be defined in build Test/envVars
+  val ExpectedTastyVersionEnvVar = "EXPECTED_TASTY_VERSION"
+  
+  @Test def testBuildTastyVersion(): Unit = {
+    val expectedVersion = sys.env.get(ExpectedTastyVersionEnvVar)
+      .getOrElse(fail(s"Env variable $ExpectedTastyVersionEnvVar not defined"))
+      .match {
+        case s"$major.$minor-experimental-$experimental" => TastyVersion(major.toInt, minor.toInt, experimental.toInt)
+        case s"$major.$minor" if minor.forall(_.isDigit) => TastyVersion(major.toInt, minor.toInt, 0)
+        case other => fail(s"Invalid TASTY version string: $other")
+      }
+    assertEquals(CurrentTastyVersion, expectedVersion)
+  }
+}