[build] Use up-to-date sources of Scala 2.13 stdlib when patching Scala 3 bytecode (#24855)

- Update the version of mtags_shared and Scala 2 stdlib to use 2.13.18
artifact instead of 2.13.16
- Removed no longer used `Build.stdlibBootstrappedVersion`

Author: WojciechMazur

project/Build.scala

@@ -151,15 +151,6 @@ object Build {
     }
   }
 
-  /** Version of the scala-library for which we will generate TASTy.
-   *
-   *  We should never use a nightly version here to release.
-   *
-   *  We can use nightly versions to tests the future compatibility in development.
-   *  Nightly versions: https://scala-ci.typesafe.com/ui/native/scala-integration/org/scala-lang
-   */
-  val stdlibBootstrappedVersion = "2.13.16"
-
   val homepageUrl = "https://scala-lang.org/"
   val dottyOrganization = "org.scala-lang"
   val dottyGithubUrl = "https://github.com/scala/scala3"
@@ -932,7 +923,7 @@ object Build {
       scriptedBatchExecution := true,
       scriptedLaunchOpts ++= Seq(
         s"-Dplugin.scalaVersion=${dottyVersion}",
-        s"-Dplugin.scala2Version=2.13.16",
+        s"-Dplugin.scala2Version=${ScalaLibraryPlugin.scala2Version}",
         s"-Dplugin.scalaJSVersion=${scalaJSVersion}",
       ),
       scriptedBufferLog := true,
@@ -2038,7 +2029,7 @@ object Build {
         "org.scalameta" % "mtags-interfaces" % mtagsVersion,
         "com.google.guava" % "guava" % "33.2.1-jre",
       ),
-      libraryDependencies += ("org.scalameta" % "mtags-shared_2.13.16" % mtagsVersion % SourceDeps),
+      libraryDependencies += ("org.scalameta" % s"mtags-shared_${ScalaLibraryPlugin.scala2Version}" % mtagsVersion % SourceDeps),
       ivyConfigurations += SourceDeps.hide,
       transitiveClassifiers := Seq("sources"),
       publishLocal := publishLocal.dependsOn( // It is best to publish all together. It is not rare to make changes in both compiler / presentation compiler and it can get misaligned

project/MiMaFilters.scala

@@ -20,7 +20,10 @@ object MiMaFilters {
       // Only exceptional cases should be added here.
 
       // Breaking changes since last reference version
-      Build.mimaPreviousDottyVersion -> Seq.empty, // We should never break backwards compatibility
+      Build.mimaPreviousDottyVersion -> Seq(
+        // scala/scala3#24855 - copied from Scala 2.13.16 by ScalaLibraryPlugin, to be removed when Scala 3.8.0 is released
+        ProblemFilters.exclude[MissingClassProblem]("scala.collection.immutable.LazyList$State*"),
+      )
     )
   }
 

project/ScalaLibraryPlugin.scala

@@ -12,7 +12,12 @@ object ScalaLibraryPlugin extends AutoPlugin {
 
   override def trigger = noTrigger
 
-  private val scala2Version = "2.13.16"
+  /** Version of the compatible Scala 2.13 scala-library
+   *  Should be updated when we synchronize with the sources of Scala 2.
+   *
+   *  This version would be used to fetch sources of Scala 2.13 standard library to be used for patching the Scala 3 standard library.
+   */
+  val scala2Version = "2.13.18"
 
   object autoImport {
     val keepSJSIR = settingKey[Boolean]("Should we patch .sjsir too?")