More Scala 3 compatibility backports (#3724)

Turned on `-Xsource:3` and fixed the most obvious syntax issues,
silenced the other less-trivial warnings

build.mill

@@ -411,7 +411,12 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
       "-P:acyclic:force",
       "-feature",
       "-Xlint:unused",
-      "-Xlint:adapted-args"
+      "-Xlint:adapted-args",
+      "-Xsource:3",
+      "-Wconf:msg=inferred type changes:silent",
+      "-Wconf:msg=case companions no longer extend FunctionN:silent",
+      "-Wconf:msg=access modifiers for:silent",
+      "-Wconf:msg=found in a package prefix of the required type:silent"
     )
 
   def scalacPluginIvyDeps =

integration/feature/output-directory/src/OutputDirectoryLockTests.scala

@@ -11,7 +11,7 @@ import scala.concurrent.{Await, ExecutionContext, Future}
 object OutputDirectoryLockTests extends UtestIntegrationTestSuite {
 
   private val pool = Executors.newCachedThreadPool()
-  private implicit val ec = ExecutionContext.fromExecutorService(pool)
+  private implicit val ec: ExecutionContext = ExecutionContext.fromExecutorService(pool)
 
   override def utestAfterAll(): Unit = {
     pool.shutdown()

main/eval/test/src/mill/eval/ModuleTests.scala

@@ -9,7 +9,7 @@ import mill.define.Discover
 import utest._
 
 object TestExternalModule extends mill.define.ExternalModule with mill.define.TaskModule {
-  def defaultCommandName = "x"
+  def defaultCommandName() = "x"
   def x = Task { 13 }
   object inner extends mill.Module {
     def y = Task { 17 }

runner/src/mill/runner/MillBuildRootModule.scala

@@ -226,7 +226,7 @@ abstract class MillBuildRootModule()(implicit
     )
   }
 
-  override def bindDependency: Task[Dep => BoundDep] = Task.Anon { dep: Dep =>
+  override def bindDependency: Task[Dep => BoundDep] = Task.Anon { (dep: Dep) =>
     super.bindDependency().apply(dep).exclude(resolveDepsExclusions(): _*)
   }
 

scalalib/src/mill/scalalib/CoursierModule.scala

@@ -23,7 +23,7 @@ trait CoursierModule extends mill.Module {
    * Bind a dependency ([[Dep]]) to the actual module contetxt (e.g. the scala version and the platform suffix)
    * @return The [[BoundDep]]
    */
-  def bindDependency: Task[Dep => BoundDep] = Task.Anon { dep: Dep =>
+  def bindDependency: Task[Dep => BoundDep] = Task.Anon { (dep: Dep) =>
     BoundDep((resolveCoursierDependency(): @nowarn).apply(dep), dep.force)
   }
 
@@ -80,7 +80,7 @@ trait CoursierModule extends mill.Module {
    * Map dependencies before resolving them.
    * Override this to customize the set of dependencies.
    */
-  def mapDependencies: Task[Dependency => Dependency] = Task.Anon { d: Dependency => d }
+  def mapDependencies: Task[Dependency => Dependency] = Task.Anon { (d: Dependency) => d }
 
   /**
    * The repositories used to resolved dependencies with [[resolveDeps()]].

scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala

@@ -144,7 +144,7 @@ class ZincWorkerImpl(
       compilerClasspath,
       scalacPluginClasspath,
       Seq()
-    ) { compilers: Compilers =>
+    ) { (compilers: Compilers) =>
       if (ZincWorkerUtil.isDotty(scalaVersion) || ZincWorkerUtil.isScala3Milestone(scalaVersion)) {
         // dotty 0.x and scala 3 milestones use the dotty-doc tool
         val dottydocClass =
@@ -388,7 +388,7 @@ class ZincWorkerImpl(
       compilerClasspath = compilerClasspath,
       scalacPluginClasspath = scalacPluginClasspath,
       javacOptions = javacOptions
-    ) { compilers: Compilers =>
+    ) { (compilers: Compilers) =>
       compileInternal(
         upstreamCompileOutput = upstreamCompileOutput,
         sources = sources,