Add a warning and instructions for new users.

Author: nicolasstucki

project/Build.scala

@@ -12,6 +12,8 @@ import sbt.Package.ManifestAttributes
 
 object DottyBuild extends Build {
 
+  projectChecks()
+
   val scalacVersion = "2.11.5" // Do not rename, this is grepped in bin/common.
 
   val dottyOrganization = "ch.epfl.lamp"
@@ -903,4 +905,18 @@ object DottyInjectedPlugin extends AutoPlugin {
       }
     ))
   }
+
+  private def projectChecks(): Unit = {
+    val scalaScala = new File("scala-scala")
+    if (!scalaScala.exists()) {
+      println(
+        s"""[WARNING] Missing `dotty/scala-scala` library
+           |You can clone the library with:
+           |  > git clone https://github.com/DarkDimius/scala.git ${scalaScala.getAbsolutePath}
+           |  > cd ${scalaScala.getAbsolutePath}
+           |  > git checkout dotty-library
+           |  > cd ${new File("").getAbsolutePath}
+        """.stripMargin)
+    }
+  }
 }