[2.x] wip: Save working state before refactoring to plugin approach

Working but architecturally wrong - logic in Defaults.scala instead
of BestEffortPlugin. Saving state before refactoring to proper
plugin pattern (option 1).

main/src/main/scala/sbt/Defaults.scala

@@ -204,6 +204,7 @@ object Defaults extends BuildCommon with DefExtra {
       testForkedParallelism :== None,
       javaOptions :== Nil,
       sbtPlugin :== false,
+      bestEffortEnabled :== internal.SysProp.bestEffort,
       isMetaBuild :== false,
       reresolveSbtArtifacts :== false,
       crossPaths :== true,
@@ -694,7 +695,6 @@ object Defaults extends BuildCommon with DefExtra {
         case vf: VirtualFile => vf
     },
     semanticdbTargetRoot := target.value / (prefix(configuration.value.name) + "meta"),
-    bestEffortTargetRoot := target.value / (prefix(configuration.value.name) + "betasty"),
     compileAnalysisTargetRoot := target.value / (prefix(configuration.value.name) + "zinc"),
     earlyCompileAnalysisTargetRoot := target.value / (prefix(
       configuration.value.name
@@ -1079,6 +1079,12 @@ object Defaults extends BuildCommon with DefExtra {
         then old ++ Seq("-Wconf:cat=unused-nowarn:s", "-Xsource:3")
         else old
       },
+      scalacOptions := {
+        val old = scalacOptions.value
+        if bestEffortEnabled.value && plugins.BestEffortPlugin.isScala35Plus(scalaVersion.value)
+        then old ++ Seq("-Ybest-effort", "-Ywith-best-effort-tasty")
+        else old
+      },
       persistJarClasspath :== true,
       classpathEntryDefinesClassVF := Def.uncached {
         val cache =

main/src/main/scala/sbt/Keys.scala

@@ -258,8 +258,6 @@ object Keys {
   val semanticdbTargetRoot = settingKey[File]("The output directory to produce META-INF/semanticdb/**/*.semanticdb files").withRank(CSetting)
   val semanticdbOptions = settingKey[Seq[String]]("The Scalac options introduced for SemanticDB").withRank(CSetting)
   val bestEffortEnabled = settingKey[Boolean]("Enables Scala 3 Best Effort compilation to produce .betasty files").withRank(CSetting)
-  val bestEffortTargetRoot = settingKey[File]("The output directory for Best Effort TASTy (.betasty) files").withRank(CSetting)
-  val bestEffortOptions = settingKey[Seq[String]]("The Scalac options introduced for Best Effort compilation").withRank(CSetting)
 
   val clean = taskKey[Unit]("Deletes files produced by the build, such as generated sources, compiled classes, and task caches.").withRank(APlusTask)
   val console = taskKey[Unit]("Starts the Scala interpreter with the project classes on the classpath.").withRank(APlusTask)

main/src/main/scala/sbt/plugins/BestEffortPlugin.scala

@@ -9,99 +9,25 @@
 package sbt
 package plugins
 
-import java.io.File
-
-import Keys.*
-import sbt.internal.SysProp
-import sbt.librarymanagement.syntax.*
-import sbt.librarymanagement.Configuration
-import ProjectExtra.inConfig
 import sbt.internal.inc.ScalaInstance
-import sbt.ScopeFilter.Make.*
-import sbt.util.CacheImplicits.given
 
 /**
  * An AutoPlugin that wires Scala 3's Best Effort compilation into sbt.
  *
  * When `bestEffortEnabled` is true and the project uses Scala 3.5+,
- * the plugin appends `-Ybest-effort` and `-Ybest-effort-dir` to scalacOptions
- * so that the compiler produces .betasty files even when compilation fails.
- * These files are consumed by IDEs such as Metals for improved code intelligence.
+ * `-Ybest-effort` and `-Ywith-best-effort-tasty` are appended to scalacOptions
+ * so that the compiler produces .betasty files (to META-INF/best-effort/ inside classes)
+ * even when compilation fails. These files are consumed by IDEs such as Metals
+ * for improved code intelligence.
  */
 object BestEffortPlugin extends AutoPlugin:
   override def requires = JvmPlugin
   override def trigger = allRequirements
 
-  override lazy val globalSettings: Seq[Def.Setting[?]] = Seq(
-    bestEffortEnabled := SysProp.bestEffort,
-    bestEffortOptions := List(),
-  )
-
-  override lazy val projectSettings: Seq[Def.Setting[?]] = Seq(
-    bestEffortOptions ++= {
-      val enabled = bestEffortEnabled.value
-      val sv = scalaVersion.value
-      if enabled && isScala35Plus(sv) then Seq("-Ybest-effort")
-      else Nil
-    },
-  ) ++
-    inConfig(Compile)(configurationSettings) ++
-      inConfig(Test)(configurationSettings)
-
-  lazy val configurationSettings: Seq[Def.Setting[?]] = List(
-    compileIncremental := Def.taskIf {
-      if !bestEffortEnabled.value then compileIncremental.value
-      else compileIncAndCacheBestEffortTargetRootTask.value
-    }.value,
-    bestEffortOptions --= Def.settingDyn {
-      val scalaV = scalaVersion.value
-      val config = configuration.value
-      Def.setting {
-        bestEffortTargetRoot.?.all(ancestorConfigs(config)).value.flatten
-          .flatMap(targetRootOptions(scalaV, _))
-      }
-    }.value,
-    bestEffortOptions ++=
-      targetRootOptions(scalaVersion.value, bestEffortTargetRoot.value),
-    scalacOptions := (Def.taskDyn {
-      val orig = scalacOptions.value
-      val config = configuration.value
-      if bestEffortEnabled.value then
-        Def.task {
-          (orig diff bestEffortOptions.?.all(ancestorConfigs(config)).value.flatten.flatten) ++
-            bestEffortOptions.value
-        }
-      else
-        Def.task {
-          orig
-        }
-    }).value,
-  )
-
   private[sbt] def isScala35Plus(scalaVersion: String): Boolean =
     ScalaInstance.isDotty(scalaVersion) && {
       val versionPart = scalaVersion.stripPrefix("3.")
       val minor = versionPart.takeWhile(_.isDigit)
       minor.nonEmpty && minor.toInt >= 5
     }
-
-  def targetRootOptions(scalaVersion: String, targetRoot: File): Seq[String] =
-    if isScala35Plus(scalaVersion) then
-      Seq("-Ybest-effort-dir", targetRoot.toString)
-    else Nil
-
-  private val compileIncAndCacheBestEffortTargetRootTask = Def.cachedTask {
-    val prev = compileIncremental.value
-    val converter = fileConverter.value
-    val targetRoot = bestEffortTargetRoot.value
-
-    val vfTargetRoot = converter.toVirtualFile(targetRoot.toPath)
-    Def.declareOutputDirectory(vfTargetRoot)
-    prev
-  }
-
-  private def ancestorConfigs(config: Configuration) =
-    def ancestors(configs: Vector[Configuration]): Vector[Configuration] =
-      configs ++ configs.flatMap(conf => ancestors(conf.extendsConfigs))
-    ScopeFilter(configurations = inConfigurations(ancestors(config.extendsConfigs)*))
 end BestEffortPlugin

sbt-app/src/sbt-test/project/best-effort-enabled/test

@@ -1,8 +1,7 @@
-# Step 1: Verify best effort flags appear in scalacOptions for Scala 3.5+
-> show Compile / scalacOptions
+# Step 1: Compile succeeds with best effort enabled (Scala 3.5+)
 > compile
 
-# Step 2: Introduce a type error — compile should fail but produce .betasty
+# Step 2: Introduce a type error — compile should fail
 $ copy-file changes/ErrorFile.scala src/main/scala/A.scala
 -> compile