allow usage of rules compiled against 2.11 & 2.13

Benefits
1. Rules using the presentation compiler can now work on 2.11 and 2.13
   input as long as they are cross-compiled
2. Advanced users with local rules targeting 2.11 or 2.13 code will be
   able to simplify their build

2.12 remains the default until 1.0 to be backward compatible with
external rules (i.e loaded through `scalafixDependencies` or
`dependency:`) that are not (yet) cross-compiled

src/main/scala/scalafix/internal/sbt/ScalafixCoursier.scala

@@ -11,28 +11,30 @@ import scalafix.sbt.BuildInfo
 import scala.collection.JavaConverters._
 
 object ScalafixCoursier {
-  private def scalafixCliModule: Module =
+  private def scalafixCliModule(scalaVersion: String): Module =
     Module.of(
       "ch.epfl.scala",
-      s"scalafix-cli_${BuildInfo.scala212}"
+      s"scalafix-cli_$scalaVersion"
     )
-  private def scalafixCli: Dependency =
+  private def scalafixCli(scalaVersion: String): Dependency =
     Dependency.of(
-      scalafixCliModule,
+      scalafixCliModule(scalaVersion),
       BuildInfo.scalafixVersion
     )
 
   def scalafixCliJars(
+      scalaVersion: String,
       repositories: Seq[cs.Repository]
   ): List[Path] = {
     runFetch(
-      newFetch()
-        .addDependencies(scalafixCli)
+      newFetch(scalaVersion)
+        .addDependencies(scalafixCli(scalaVersion))
         .addRepositories(toCoursierRepositories(repositories): _*)
     )
   }
 
   def scalafixToolClasspath(
+      scalaVersion: String,
       deps: Seq[ModuleID],
       customResolvers: Seq[cs.Repository]
   ): Seq[URL] = {
@@ -41,7 +43,7 @@ object ScalafixCoursier {
     } else {
       val jars = dependencyCache.computeIfAbsent(
         deps,
-        fetchScalafixDependencies(customResolvers)
+        fetchScalafixDependencies(scalaVersion, customResolvers)
       )
       jars.map(_.toUri.toURL)
     }
@@ -51,13 +53,16 @@ object ScalafixCoursier {
     jutil.Collections.synchronizedMap(new jutil.HashMap())
   }
   private[scalafix] def fetchScalafixDependencies(
+      scalaVersion: String,
       customResolvers: Seq[cs.Repository]
   ): function.Function[Seq[ModuleID], List[Path]] =
     new function.Function[Seq[ModuleID], List[Path]] {
       override def apply(t: Seq[ModuleID]): List[Path] = {
+        val binaryScalaVersion = CrossVersion.binaryScalaVersion(scalaVersion)
         val dependencies = t.map { module =>
           val binarySuffix =
-            if (module.crossVersion.isInstanceOf[CrossVersion.Binary]) "_2.12"
+            if (module.crossVersion.isInstanceOf[CrossVersion.Binary])
+              s"_$binaryScalaVersion"
             else ""
           Dependency.of(
             module.organization,
@@ -66,9 +71,9 @@ object ScalafixCoursier {
           )
         }
         runFetch(
-          newFetch()
+          newFetch(scalaVersion)
             .addRepositories(toCoursierRepositories(customResolvers): _*)
-            .addDependencies(scalafixCli)
+            .addDependencies(scalafixCli(scalaVersion))
             .addDependencies(dependencies.toArray: _*)
         )
       }
@@ -84,7 +89,7 @@ object ScalafixCoursier {
     }.toArray
   def runFetch(fetch: Fetch): List[Path] =
     fetch.fetch().asScala.iterator.map(_.toPath()).toList
-  def newFetch(): Fetch =
+  def newFetch(scalaVersion: String): Fetch =
     Fetch
       .create()
       .withRepositories()
@@ -93,7 +98,7 @@ object ScalafixCoursier {
           .create()
           .withForceVersions(
             Map(
-              scalafixCliModule -> scalafixCli.getVersion()
+              scalafixCliModule(scalaVersion) -> BuildInfo.scalafixVersion
             ).asJava
           )
       )

src/main/scala/scalafix/internal/sbt/ScalafixInterface.scala

@@ -74,6 +74,7 @@ object Arg {
 }
 
 class ScalafixInterface private (
+    scalaVersion: String,
     scalafixArguments: ScalafixArguments, // hide it to force usage of withArgs so we can intercept arguments
     val args: Seq[Arg]
 ) {
@@ -88,11 +89,13 @@ class ScalafixInterface private (
     .classLoader
 
   private def this(
+      scalaVersion: String,
       api: ScalafixAPI,
       toolClasspath: URLClassLoader,
       mainCallback: ScalafixMainCallback
   ) =
     this(
+      scalaVersion,
       api
         .newArguments()
         .withMainCallback(mainCallback)
@@ -125,6 +128,7 @@ class ScalafixInterface private (
   ): ScalafixInterface = {
     val extraURLs = ScalafixCoursier
       .scalafixToolClasspath(
+        scalaVersion,
         extraExternalDeps,
         customResolvers
       ) ++ extraInternalDeps.map(_.toURI.toURL)
@@ -141,7 +145,7 @@ class ScalafixInterface private (
       try arg(acc)
       catch { case NonFatal(e) => throw new InvalidArgument(e.getMessage) }
     }
-    new ScalafixInterface(newScalafixArguments, this.args ++ args)
+    new ScalafixInterface(scalaVersion, newScalafixArguments, this.args ++ args)
   }
 
   def run(): Seq[ScalafixError] =
@@ -165,19 +169,21 @@ object ScalafixInterface {
     override def apply(): T = _value.get
   }
   def fromToolClasspath(
+      scalafixScalaVersion: String,
       scalafixDependencies: Seq[ModuleID],
       scalafixCustomResolvers: Seq[Repository],
       logger: Logger = Compat.ConsoleLogger(System.out)
   ): () => ScalafixInterface =
     new LazyValue({ () =>
-      val jars = ScalafixCoursier.scalafixCliJars(scalafixCustomResolvers)
+      val jars = ScalafixCoursier
+        .scalafixCliJars(scalafixScalaVersion, scalafixCustomResolvers)
       val urls = jars.map(_.toUri.toURL).toArray
       val interfacesParent =
         new ScalafixInterfacesClassloader(this.getClass.getClassLoader)
       val classloader = new URLClassLoader(urls, interfacesParent)
       val api = ScalafixAPI.classloadInstance(classloader)
       val callback = new ScalafixLogger(logger)
-      new ScalafixInterface(api, classloader, callback)
+      new ScalafixInterface(scalafixScalaVersion, api, classloader, callback)
         .addToolClasspath(
           scalafixDependencies,
           scalafixCustomResolvers,

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -45,6 +45,12 @@ object ScalafixPlugin extends AutoPlugin {
         "Optional list of custom rules to install from Maven Central. " +
           "This setting is read from the global scope so it only needs to be defined once in the build."
       )
+    val scalafixScalaVersion: SettingKey[String] =
+      settingKey[String](
+        "The full scala version used for the scalafix classpath. Rules must be compiled against that binary version." +
+          s"Defaults to the latest supported 2.12 version (${BuildInfo.scala212}). " +
+          "This setting is read from the global scope so it only needs to be defined once in the build."
+      )
     val scalafixConfig: SettingKey[Option[File]] =
       settingKey[Option[File]](
         "Optional location to .scalafix.conf file to specify which scalafix rules should run. " +
@@ -111,6 +117,7 @@ object ScalafixPlugin extends AutoPlugin {
       //   https://github.com/sbt/sbt/issues/3572#issuecomment-417582703
       workingDirectory = baseDirectory.in(ThisBuild).value.toPath
       scalafixInterface = ScalafixInterface.fromToolClasspath(
+        scalafixScalaVersion = scalafixScalaVersion.in(ThisBuild).value,
         scalafixDependencies = scalafixDependencies.in(ThisBuild).value,
         scalafixCustomResolvers = scalafixResolvers.in(ThisBuild).value
       )
@@ -119,6 +126,7 @@ object ScalafixPlugin extends AutoPlugin {
     scalafixCaching := false,
     scalafixResolvers := ScalafixCoursier.defaultResolvers,
     scalafixDependencies := Nil,
+    scalafixScalaVersion := BuildInfo.scala212, // will follow scalafixVersion once community rules are cross-built
     commands += ScalafixEnable.command
   )
 

src/sbt-test/sbt-scalafix/local-rules/build.sbt

@@ -6,27 +6,29 @@ inThisBuild(
       // Custom rule published to Maven Central https://github.com/olafurpg/example-scalafix-rule
       "com.geirsson" %% "example-scalafix-rule" % "1.3.0"
     )
+//    scalaVersion := Versions.scala213,
+//    scalafixScalaVersion := scalaVersion.version
   )
 )
 
 val rules = project
   .disablePlugins(ScalafixPlugin)
   .settings(
-    scalaVersion := Versions.scala212,
-    crossPaths := false,
+    scalaVersion := Versions.scala212, // TODO: remove
+    crossPaths := false, // TODO: remove
     libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion,
     libraryDependencies += "joda-time" % "joda-time" % "2.10.6"
   )
 
 val service = project
   .dependsOn(rules % ScalafixConfig)
   .settings(
-    scalaVersion := Versions.scala213,
+    scalaVersion := Versions.scala213, // TODO: remove
     libraryDependencies += "com.nequissimus" % "sort-imports_2.12" % "0.5.0" % ScalafixConfig
   )
 
 val sameproject = project
   .settings(
-    scalaVersion := Versions.scala212, // the project scala version MUST match the one used by Scalafix
+    scalaVersion := Versions.scala212, // TODO: remove
     libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion % ScalafixConfig
   )

src/sbt-test/sbt-scalafix/scalafixScalaVersion/build.sbt

@@ -0,0 +1,10 @@
+import _root_.scalafix.sbt.{BuildInfo => V}
+
+inThisBuild(
+  List(
+    addCompilerPlugin(scalafixSemanticdb),
+    scalacOptions += "-Yrangepos",
+    scalaVersion := V.scala211,
+    scalafixScalaVersion := scalaVersion.value // this should be the default in 1.x
+  )
+)

src/sbt-test/sbt-scalafix/scalafixScalaVersion/project/plugins.sbt

@@ -0,0 +1,2 @@
+resolvers += Resolver.sonatypeRepo("public")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % sys.props("plugin.version"))

src/sbt-test/sbt-scalafix/scalafixScalaVersion/src/main/scala/NeedExplicitResultTypes.scala

@@ -0,0 +1,6 @@
+object NeedExplicitResultTypes {
+  def a = {
+    println("foobar")
+    123
+  }
+}

src/sbt-test/sbt-scalafix/scalafixScalaVersion/test

@@ -0,0 +1,5 @@
+# ExplicitResultTypes uses the presentation compiler so the rule must run with the same scala binary version as the target
+-> scalafix --check ExplicitResultTypes
+> scalafix ExplicitResultTypes
+> scalafix --check ExplicitResultTypes
+

src/test/scala/scalafix/internal/sbt/SbtCompletionsSuite.scala

@@ -4,6 +4,7 @@ import org.eclipse.jgit.lib.AbbreviatedObjectId
 import org.scalatest.Tag
 import sbt.complete.Parser
 import org.scalatest.funsuite.AnyFunSuite
+import scalafix.sbt.BuildInfo
 
 class SbtCompletionsSuite extends AnyFunSuite {
   val fs = new Fs()
@@ -29,6 +30,7 @@ class SbtCompletionsSuite extends AnyFunSuite {
   val mainArgs =
     ScalafixInterface
       .fromToolClasspath(
+        BuildInfo.scala212,
         Seq(exampleDependency),
         ScalafixCoursier.defaultResolvers
       )()

src/test/scala/scalafix/internal/sbt/ScalafixAPISuite.scala

@@ -7,8 +7,9 @@ import org.scalactic.source.Position
 import sbt._
 import sbt.internal.sbtscalafix.Compat
 import scalafix.interfaces.ScalafixError
-
 import org.scalatest.funsuite.AnyFunSuite
+import scalafix.sbt.BuildInfo
+
 import scala.util.Properties
 
 class ScalafixAPISuite extends AnyFunSuite {
@@ -28,6 +29,7 @@ class ScalafixAPISuite extends AnyFunSuite {
     val logger = Compat.ConsoleLogger(new PrintStream(baos))
     val interface = ScalafixInterface
       .fromToolClasspath(
+        BuildInfo.scala212,
         List("com.geirsson" %% "example-scalafix-rule" % "1.3.0"),
         ScalafixCoursier.defaultResolvers,
         logger