Skip validation if there are no sources to fix.

Previously, the semanticdb-scalac and scalacOptions validation ran for
empty root projects with no sources, failing the build when running
scalafix from the root project.  Since there are no sources to fix we
should not run any validation.

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

@@ -140,33 +140,47 @@ object ScalafixPlugin extends AutoPlugin {
     )
   }
 
+  private def validateProject(
+      files: Seq[Path],
+      dependencies: Seq[ModuleID],
+      args: ScalafixArguments,
+      ruleNames: Seq[String]
+  ): Seq[String] = {
+    if (files.isEmpty) Nil
+    else {
+      val errors = ListBuffer.empty[String]
+      val isSemanticdb =
+        dependencies.exists(_.name.startsWith("semanticdb-scalac"))
+      if (!isSemanticdb) {
+        val names = ruleNames.mkString(", ")
+        errors +=
+          s"""|The semanticdb-scalac compiler plugin is required to run semantic rules like $names.
+              |To fix this problem for this sbt shell session, run `scalafixEnable` and try again.
+              |To fix this problem permanently for your build, add the following settings to build.sbt:
+              |  addCompilerPlugin(scalafixSemanticdb)
+              |  scalacOptions += "-Yrangepos"
+              |""".stripMargin
+      }
+      val validateError = args.validate()
+      if (validateError.isPresent) {
+        errors += validateError.get().getMessage
+      }
+      errors
+    }
+  }
+
   private def scalafixSemantic(
       ruleNames: Seq[String],
       mainArgs: ScalafixArguments,
       shellArgs: ShellArgs,
       config: Configuration
   ): Def.Initialize[Task[Unit]] = Def.taskDyn {
-    val errors = ListBuffer.empty[String]
-    val isSemanticdb =
-      libraryDependencies.value.exists(_.name.startsWith("semanticdb-scalac"))
-    if (!isSemanticdb) {
-      val names = ruleNames.mkString(", ")
-      errors +=
-        s"""|The semanticdb-scalac compiler plugin is required to run semantic rules like $names.
-            |To fix this problem for this sbt shell session, run `scalafixEnable` and try again.
-            |To fix this problem permanently for your build, add the following settings to build.sbt:
-            |  addCompilerPlugin(scalafixSemanticdb)
-            |  scalacOptions += "-Yrangepos"
-            |""".stripMargin
-    }
+    val dependencies = libraryDependencies.value
+    val files = filesToFix(shellArgs, config).value
     val withScalaArgs = mainArgs
       .withScalaVersion(scalaVersion.value)
       .withScalacOptions(scalacOptions.value.asJava)
-    val validateError = withScalaArgs.validate()
-    if (validateError.isPresent()) {
-      errors += validateError.get().getMessage
-    }
-    val files = filesToFix(shellArgs, config).value
+    val errors = validateProject(files, dependencies, withScalaArgs, ruleNames)
     if (errors.isEmpty) {
       Def.task {
         val args = withScalaArgs.withClasspath(

src/sbt-test/sbt-scalafix/root-validation/build.sbt

@@ -0,0 +1,20 @@
+inThisBuild(
+  List(
+    scalaVersion := "2.12.6",
+  )
+)
+
+lazy val root = project
+  .in(file("."))
+  .aggregate(lib)
+
+lazy val lib = project
+  .in(file("lib"))
+  .settings(
+    addCompilerPlugin(scalafixSemanticdb),
+    scalacOptions ++= List(
+      "-Ywarn-unused",
+      "-Yrangepos"
+    )
+  )
+

src/sbt-test/sbt-scalafix/root-validation/lib/src/main/scala/lib/App.scala

@@ -0,0 +1,6 @@
+package lib
+
+import scala.concurrent.Future
+import scala.util.Success
+
+object App { println(Success(1)) }

src/sbt-test/sbt-scalafix/root-validation/project/plugins.sbt

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

src/sbt-test/sbt-scalafix/root-validation/project/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("io.get-coursier" % "sbt-coursier" % "1.1.0-M6")

src/sbt-test/sbt-scalafix/root-validation/test

@@ -0,0 +1,3 @@
+-> root/scalafix --check RemoveUnused
+> root/scalafix RemoveUnused
+> root/scalafix --check RemoveUnused