Let phases themselves decide whether they have to do anything.

Instead of having `Run.rootContext` hard-code the filtering.

Author: sjrd

compiler/src/dotty/tools/backend/sjs/GenSJSIR.scala

@@ -9,6 +9,7 @@ class GenSJSIR extends Phase {
   def phaseName: String = "genSJSIR"
 
   def run(implicit ctx: Context): Unit = {
-    new JSCodeGen().run()
+    if (ctx.settings.scalajs.value)
+      new JSCodeGen().run()
   }
 }

compiler/src/dotty/tools/dotc/Run.scala

@@ -27,7 +27,6 @@ import collection.mutable
 import dotty.tools.io.VirtualFile
 
 import scala.util.control.NonFatal
-import dotty.tools.backend.sjs
 
 /** A compiler run. Exports various methods to compile source files */
 class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with ConstraintRunInfo {
@@ -42,23 +41,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
    */
   protected[this] def rootContext(implicit ctx: Context): Context = {
     ctx.initialize()(ctx)
-
-    val actualPhases = if (ctx.settings.scalajs.value) {
-      // Remove phases that Scala.js does not want
-      comp.phases.mapConserve(_.filter {
-        case _: transform.FunctionalInterfaces => false
-        case _ => true
-      }).filter(_.nonEmpty)
-    } else {
-      // Remove Scala.js-related phases
-      comp.phases.mapConserve(_.filter {
-        case _: sjs.GenSJSIR => false
-        case _ => true
-      }).filter(_.nonEmpty)
-    }
-
-    ctx.base.setPhasePlan(actualPhases)
-
+    ctx.base.setPhasePlan(comp.phases)
     val rootScope = new MutableScope
     val bootstrap = ctx.fresh
       .setPeriod(Period(comp.nextRunId, FirstPhaseId))

compiler/src/dotty/tools/dotc/transform/FunctionalInterfaces.scala

@@ -33,7 +33,8 @@ class FunctionalInterfaces extends MiniPhase {
     val List(implParamTypes) = implType.paramInfoss
     val implResultType = implType.resultType
 
-    if (defn.isSpecializableFunction(cls, implParamTypes, implResultType)) {
+    if (defn.isSpecializableFunction(cls, implParamTypes, implResultType) &&
+        !ctx.settings.scalajs.value) { // never do anything for Scala.js, but do this test as late as possible not to slow down Scala/JVM
       val names = ctx.atPhase(ctx.erasurePhase) {
         implicit ctx => cls.typeParams.map(_.name)
       }