From fa20dd618b96f5038713dd1fb1fbf49285cd37c5 Mon Sep 17 00:00:00 2001
From: statxc <181730535+statxc@users.noreply.github.com>
Date: Sun, 12 Apr 2026 18:30:49 +0200
Subject: [PATCH] [2.x] feat: Add Best Effort compilation support for Scala
 3.5+

---
 main/src/main/scala/sbt/Defaults.scala        |  7 -----
 .../scala/sbt/internal/PluginDiscovery.scala  |  1 +
 .../scala/sbt/plugins/BestEffortPlugin.scala  | 27 ++++++++++++++++++-
 3 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index 5819c6952..1b567de2c 100644
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -204,7 +204,6 @@ object Defaults extends BuildCommon with DefExtra {
       testForkedParallelism :== None,
       javaOptions :== Nil,
       sbtPlugin :== false,
-      bestEffortEnabled :== internal.SysProp.bestEffort,
       isMetaBuild :== false,
       reresolveSbtArtifacts :== false,
       crossPaths :== true,
@@ -1079,12 +1078,6 @@ 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 =
diff --git a/main/src/main/scala/sbt/internal/PluginDiscovery.scala b/main/src/main/scala/sbt/internal/PluginDiscovery.scala
index 934a0a53d..af28f39d0 100644
--- a/main/src/main/scala/sbt/internal/PluginDiscovery.scala
+++ b/main/src/main/scala/sbt/internal/PluginDiscovery.scala
@@ -51,6 +51,7 @@ object PluginDiscovery:
       "sbt.ScriptedPlugin" -> sbt.ScriptedPlugin,
       "sbt.plugins.SbtPlugin" -> sbt.plugins.SbtPlugin,
       "sbt.plugins.SemanticdbPlugin" -> sbt.plugins.SemanticdbPlugin,
+      "sbt.plugins.BestEffortPlugin" -> sbt.plugins.BestEffortPlugin,
       "sbt.plugins.JUnitXmlReportPlugin" -> sbt.plugins.JUnitXmlReportPlugin,
       "sbt.plugins.Giter8TemplatePlugin" -> sbt.plugins.Giter8TemplatePlugin,
       "sbt.plugins.DependencyTreePlugin" -> sbt.plugins.DependencyTreePlugin,
diff --git a/main/src/main/scala/sbt/plugins/BestEffortPlugin.scala b/main/src/main/scala/sbt/plugins/BestEffortPlugin.scala
index 7f8ac0aaf..3809b2771 100644
--- a/main/src/main/scala/sbt/plugins/BestEffortPlugin.scala
+++ b/main/src/main/scala/sbt/plugins/BestEffortPlugin.scala
@@ -9,6 +9,10 @@
 package sbt
 package plugins
 
+import Keys.*
+import sbt.internal.SysProp
+import sbt.librarymanagement.syntax.*
+import ProjectExtra.inConfig
 import sbt.internal.inc.ScalaInstance
 
 /**
@@ -21,9 +25,30 @@ import sbt.internal.inc.ScalaInstance
  * for improved code intelligence.
  */
 object BestEffortPlugin extends AutoPlugin:
-  override def requires = JvmPlugin
+  override def requires = SemanticdbPlugin
   override def trigger = allRequirements
 
+  private val bestEffortFlags = Seq("-Ybest-effort", "-Ywith-best-effort-tasty")
+
+  override lazy val globalSettings: Seq[Def.Setting[?]] = Seq(
+    bestEffortEnabled := SysProp.bestEffort,
+  )
+
+  override lazy val projectSettings: Seq[Def.Setting[?]] =
+    inConfig(Compile)(configurationSettings) ++
+      inConfig(Test)(configurationSettings)
+
+  lazy val configurationSettings: Seq[Def.Setting[?]] = List(
+    scalacOptions := {
+      val orig = scalacOptions.value
+      val enabled = bestEffortEnabled.value
+      val sv = scalaVersion.value
+      if enabled && isScala35Plus(sv) then
+        (orig.filterNot(bestEffortFlags.contains) ++ bestEffortFlags).distinct
+      else orig.filterNot(bestEffortFlags.contains)
+    },
+  )
+
   private[sbt] def isScala35Plus(scalaVersion: String): Boolean =
     ScalaInstance.isDotty(scalaVersion) && {
       val versionPart = scalaVersion.stripPrefix("3.")