Compile String to AST inside CompilerPlugin?

I would like to create a templating plugin and as the first step convert an arbitrary string to it's "compiled" AST representation (as the scala interpreter does, I guess). So a compiler plugin could e.g assign someString to "HELLO WORLD":

  @StringAnnotation("""("hello world").toString.toUpperCase""")
  var someString = ""

My current first shot plugin does in short:

runafter parser
create a new representation only compiler and a VirtualFile with the annotation content
compile and print unit.body

see: http://paste.pocoo.org/show/326025/

a) Right now, "object o{val x = 0}" returns an AST, but e.g. "var x = 1+ 2" doesn't because it wouldn't be a valid .scala file. How can I fix this?

b) Is onlyPresentation a good choice? Should I instead overriding computeInternalPhases with the appropriate phases or use -Ystop:phase?

c) Is it possible to bind the environment of the outer compiler to the inner one, so that e.g.

  var x = _
  (...)
  @StringAnnotation("x += 3")

would work?

I found following code[1] using an interpreter and one variable which does something similar:

  Interpreter interpreter = new Interpreter(settings);
  String[] context = { "FOO" };
  interpreter.bind("context", "Array[String]", context);
  interpreter
    .interpret("de.tutorials.scala2.Test.main(context)");
  context[0] = "BAR";
  interpreter
    .interpret("de.tutorials.scala2.Test.main(context)");

[1] http://www.tutorials.de/java/320639-beispiel-zur-einbindung-des-scala-interpreters-kompilierte-scala-anwendungen.html#post1653884

thanks

Complete Code:

class AnnotationsPI(val global: Global) extends Plugin {
  import global._
  val name = "a_plugins::AnnotationsPI" //a_ to run before namer
  val description = "AST Trans PI"
  val components = List[PluginComponent](Component)

  private object Component extends PluginComponent with Transform with TypingTransformers with TreeDSL {
    val global: AnnotationsPI.this.global.type = AnnotationsPI.this.global
    val runsAfter = List[String]("parser");
    val phaseName = AnnotationsPI.this.name

    def newTransformer(unit: CompilationUnit) = {
      new AnnotationsTransformer(unit)
    }

    val SaTpe = "StringAnnotation".toTypeName

    class AnnotationsTransformer(unit: CompilationUnit) extends TypingTransformer(unit) {

      /** When using <code>preTransform</code>, each node is
       *  visited before its children.
       */
      def preTransform(tree: Tree): Tree = tree match {
        case anno@ValDef(Modifiers(_, _, List(Apply(Select(New(Ident(SaTpe)), _), List(Literal(Constant(a))))), _), b, c, d) => //Apply(Select(New(Ident(SaTpe)), /*nme.CONSTRUCTOR*/_), /*List(x)*/x)
          val str = a.toString
          val strArr = str.getBytes("UTF-8")
          import scala.tools.nsc.{ Global, Settings, SubComponent }
          import scala.tools.nsc.reporters.{ ConsoleReporter, Reporter }

          val settings = new Settings()
          val compiler = new Global(settings, new ConsoleReporter(settings)) {
            override def onlyPresentation = true
          }

          val run = new compiler.Run
          val vfName = "Script.scala"
          var vfile = new scala.tools.nsc.io.VirtualFile(vfName)

          val os = vfile.output
          os.write(strArr, 0, str.size) // void  write(byte[] b, int off, int len) 
          os.close
          new scala.tools.nsc.util.BatchSourceFile(vfName, str)
          run.compileFiles(vfile :: Nil)
          for (unit <- run.units) {
            println("Unit: " + unit)
            println("Body:\n" + unit.body)
          }
          tree

        case _ =>
          tree
      }

      override def transform(tree: Tree): Tree = {
        super.transform(preTransform(tree))
      }
    }
  }

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