ensure/document support of Scala 3 in ScalafixTestkitPlugin

* setup cross-building like https://github.com/scalacenter/scalafix.g8
* rely on scalafix-testkit to add semanticdbTargetRoot to classpath
  based on scalacOptions

project/Dependencies.scala

@@ -2,7 +2,7 @@ import sbt._
 
 object Dependencies {
   val x = List(1) // scalafix:ok
-  def scalafixVersion: String = "0.9.27+52-6c9eeec9-SNAPSHOT"
+  def scalafixVersion: String = "0.9.27+78-f41dec8f-SNAPSHOT"
 
   val all = List(
     "org.eclipse.jgit" % "org.eclipse.jgit" % "5.11.1.202105131744-r",

src/main/scala/scalafix/sbt/ScalafixTestkitPlugin.scala

@@ -38,16 +38,11 @@ object ScalafixTestkitPlugin extends AutoPlugin {
       scalafixTestkitInputScalaVersion := scalaVersion.value,
       resourceGenerators.in(Test) += Def.task {
         val props = new java.util.Properties()
-        val targetrootClasspath = semanticdbOption(
-          scalafixTestkitInputScalacOptions.value,
-          "targetroot"
-        ).map(file).toSeq
         val values = Map[String, Seq[File]](
           "sourceroot" ->
             List(baseDirectory.in(ThisBuild).value),
           "inputClasspath" ->
-            (scalafixTestkitInputClasspath.value
-              .map(_.data) ++ targetrootClasspath),
+            scalafixTestkitInputClasspath.value.map(_.data),
           "inputSourceDirectories" ->
             scalafixTestkitInputSourceDirectories.value,
           "outputSourceDirectories" ->
@@ -71,15 +66,4 @@ object ScalafixTestkitPlugin extends AutoPlugin {
         List(out)
       }
     )
-
-  private def semanticdbOption(
-      scalacOptions: Seq[String],
-      name: String
-  ): Option[String] = {
-    val flag = s"-P:semanticdb:$name:"
-    scalacOptions
-      .filter(_.startsWith(flag))
-      .lastOption
-      .map(_.stripPrefix(flag))
-  }
 }

src/sbt-test/sbt-1.3/testkit/build.sbt → src/sbt-test/sbt-1.5/testkit/build.sbt

@@ -1,22 +1,43 @@
 import _root_.scalafix.sbt.{BuildInfo => Versions}
 
+lazy val scala3InputOutputVersion = "3.0.0"
+
 inThisBuild(
   List(
     scalaVersion := Versions.scala212,
+    crossScalaVersions := List(
+      Versions.scala213,
+      Versions.scala212,
+      Versions.scala211
+    ),
     semanticdbEnabled := true,
     semanticdbVersion := scalafixSemanticdb.revision
   )
 )
 
+crossScalaVersions := Nil
+
 val rules = project
   .disablePlugins(ScalafixPlugin)
   .settings(
     libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion
   )
 
 lazy val input = project
+  .settings(
+    crossScalaVersions := List(scala3InputOutputVersion)
+  )
 
 lazy val output = project
+  .settings(
+    crossScalaVersions := List(scala3InputOutputVersion)
+  )
+
+lazy val testsScala3 = project
+  .aggregate(tests)
+  .settings(
+    crossScalaVersions := List(scala3InputOutputVersion)
+  )
 
 lazy val tests = project
   .settings(
@@ -25,7 +46,8 @@ lazy val tests = project
     scalafixTestkitOutputSourceDirectories :=
       sourceDirectories.in(output, Compile).value,
     scalafixTestkitInputSourceDirectories :=
-      sourceDirectories.in(input, Compile).value,
+      // https://github.com/sbt/sbt/pull/6511
+      sourceDirectories.in(input, Compile).value.distinct,
     scalafixTestkitInputClasspath :=
       fullClasspath.in(input, Compile).value,
     scalafixTestkitInputScalacOptions :=
@@ -34,5 +56,5 @@ lazy val tests = project
       scalaVersion.in(input, Compile).value,
     mainClass in (Test, run) := Some("fix.IntputOutputSuite")
   )
-  .dependsOn(input, rules)
+  .dependsOn(rules)
   .enablePlugins(ScalafixTestkitPlugin)

src/sbt-test/sbt-1.5/testkit/project/build.properties

@@ -0,0 +1 @@
+sbt.version=1.5.2

src/sbt-test/sbt-1.3/testkit/test → src/sbt-test/sbt-1.5/testkit/test

@@ -1,3 +1,3 @@
 -> tests/test
 > tests/test:run --save-expect
-> tests/test
+> +test