Scala compiler-plugin, finding an annotation

Question

I would like this plugin to fetch the contents of an annotation (@Typestate(filename)). But at the moment even if I print out the entire tree I can't find the annotation anywhere.

How to get an annotation from source code or alternatively, where to find good documentation on how to do this?

I pulled most of this code from this Scala plugin tutorial.

File with the annotation:

class Typestate(filename:String) extends scala.annotation.StaticAnnotation


@Typestate(filename = "MyProtocol.txt")
class Cat{
  def comeAlive(): Unit = println("The cat is alive")
}

object Main extends App {
  val cat = new Cat()
  cat.comeAlive()
}

Plugin code:

package compilerPlugin

import scala.tools.nsc
import nsc.Global
import nsc.Phase
import nsc.plugins.Plugin
import nsc.plugins.PluginComponent
import scala.collection.mutable.ListBuffer

class GetFileFromAnnotation(val global: Global) extends Plugin {
  import global._

  val name = "GetFileFromAnnotation"
  val description = "gets file from typestate annotation"
  val components: List[PluginComponent] = List[PluginComponent](Component)

  private object Component extends PluginComponent {
    val global: GetFileFromAnnotation.this.global.type = GetFileFromAnnotation.this.global
    val runsAfter: List[String] = List[String]("refchecks")
    val phaseName: String = GetFileFromAnnotation.this.name
    def newPhase(_prev: Phase) = new GetFileFromAnnotationPhase(_prev)

    class GetFileFromAnnotationPhase(prev: Phase) extends StdPhase(prev) {
      override def name: String = GetFileFromAnnotation.this.name

      def apply(unit: CompilationUnit): Unit = {
        printRaw(unit.body)

        for(select @ Select(statements, expr) <- unit.body){
              //global.reporter.error(tree.pos, "file name is here")
        }            
      }
    }
  }
}

Entire tree:

PackageDef(Ident(<empty>), List(ClassDef(Modifiers(), Typestate, List(), Template(List(TypeTree(), TypeTree().setOriginal(Select(Select(Ident(scala), scala.annotation), scala.annotatio
n.StaticAnnotation))), noSelfType, List(ValDef(Modifiers(PRIVATE | LOCAL | PARAMACCESSOR), TermName("filename"), TypeTree().setOriginal(Select(Select(Ident(scala), scala.Predef), TypeN
ame("String"))), EmptyTree), DefDef(Modifiers(), termNames.CONSTRUCTOR, List(), List(List(ValDef(Modifiers(PARAM | PARAMACCESSOR), TermName("filename"), TypeTree().setOriginal(Select(S
elect(Ident(scala), scala.Predef), TypeName("String"))), EmptyTree))), TypeTree(), Block(List(Apply(Select(Super(This(TypeName("Typestate")), typeNames.EMPTY), termNames.CONSTRUCTOR),
List())), Literal(Constant(()))))))), ClassDef(Modifiers(), Cat, List(), Template(List(Select(Ident(scala), TypeName("AnyRef"))), noSelfType, List(DefDef(Modifiers(), termNames.CONSTRU
CTOR, List(), List(List()), TypeTree(), Block(List(Apply(Select(Super(This(TypeName("Cat")), typeNames.EMPTY), termNames.CONSTRUCTOR), List())), Literal(Constant(())))), DefDef(Modifie
rs(), TermName("comeAlive"), List(), List(List()), TypeTree().setOriginal(Select(Ident(scala), scala.Unit)), Apply(Select(Select(Ident(scala), scala.Predef), TermName("println")), List
(Literal(Constant("The cat is alive")))))))), ModuleDef(Modifiers(), Main, Template(List(TypeTree(), TypeTree().setOriginal(Select(Ident(scala), scala.App))), noSelfType, List(DefDef(M
odifiers(), termNames.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(Apply(Select(Super(This(TypeName("Main")), typeNames.EMPTY), termNames.CONSTRUCTOR), List())), Literal(C
onstant(())))), ValDef(Modifiers(PRIVATE | LOCAL), TermName("cat "), TypeTree(), Apply(Select(New(Ident(Cat)), termNames.CONSTRUCTOR), List())), DefDef(Modifiers(METHOD | STABLE | ACCE
SSOR), TermName("cat"), List(), List(), TypeTree(), Select(This(TypeName("Main")), TermName("cat "))), Apply(Select(Select(This(TypeName("Main")), TermName("cat")), TermName("comeAlive
")), List()))))))

Dmytro Mitin · Accepted Answer · 2020-07-10 22:29:53Z

Phase seems to be wrong. If I change refchecks to parser then the plugin with the following component

private object Component extends PluginComponent {
  val global: GetFileFromAnnotation.this.global.type = GetFileFromAnnotation.this.global
  val runsAfter: List[String] = List[String]("parser")
  val phaseName: String = GetFileFromAnnotation.this.name
  def newPhase(_prev: Phase) = new GetFileFromAnnotationPhase(_prev)

  class GetFileFromAnnotationPhase(prev: Phase) extends StdPhase(prev) {
    override def name: String = GetFileFromAnnotation.this.name

    def apply(unit: CompilationUnit): Unit = {
      for(tree@q"$mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents { $self => ..$stats }" <- unit.body){
        global.reporter.echo(tree.pos, s"tree=$tree, showRaw(tree)=${showRaw(tree)}, mods.annotations=${mods.annotations}")
      }
    }
  }
}

produces output

[info] .../core/src/main/scala/Main.scala:5:7: tree=@new Typestate(filename = "MyProtocol.txt") class Cat extends scala.AnyRef {
[info]   def <init>() = {
[info]     super.<init>();
[info]     ()
[info]   };
[info]   def comeAlive(): Unit = println("The cat is alive")
[info] }, showRaw(tree)=ClassDef(Modifiers(NoFlags, typeNames.EMPTY, List(Apply(Select(New(Ident(TypeName("Typestate"))), termNames.CONSTRUCTOR), List(NamedArg(Ident(TermName("filename")), Literal(Constant("MyProtocol.txt"))))))), TypeName("Cat"), List(), Template(List(Select(Ident(scala), TypeName("AnyRef"))), noSelfType, List(DefDef(Modifiers(), termNames.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(pendingSuperCall), Literal(Constant(())))), DefDef(Modifiers(), TermName("comeAlive"), List(), List(List()), Ident(TypeName("Unit")), Apply(Ident(TermName("println")), List(Literal(Constant("The cat is alive")))))))), mods.annotations=List(new Typestate(filename = "MyProtocol.txt"))
[info] class Cat{
[info]       ^

with correct mods.annotations=List(new Typestate(filename = "MyProtocol.txt")).

But if I change phase even to namer (the next phase after parser) then the plugin produces

[info] .../core/src/main/scala/Main.scala:5:7: tree=@Typestate("MyProtocol.txt") class Cat extends scala.AnyRef {
[info]   def <init>(): Cat = {
[info]     Cat.super.<init>();
[info]     ()
[info]   };
[info]   def comeAlive(): Unit = scala.Predef.println("The cat is alive")
[info] }, showRaw(tree)=ClassDef(Modifiers(), Cat, List(), Template(List(Select(Ident(scala), TypeName("AnyRef"))), noSelfType, List(DefDef(Modifiers(), termNames.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(Apply(Select(Super(This(TypeName("Cat")), typeNames.EMPTY), termNames.CONSTRUCTOR), List())), Literal(Constant(())))), DefDef(Modifiers(), TermName("comeAlive"), List(), List(List()), TypeTree().setOriginal(Select(Ident(scala), scala.Unit)), Apply(Select(Select(Ident(scala), scala.Predef), TermName("println")), List(Literal(Constant("The cat is alive")))))))), mods.annotations=List()
[info] class Cat{
[info]       ^

with already empty mods.annotations=List().

After namer phase you should look for annotations not in a tree but in its symbol.

Indeed,

global.reporter.echo(tree.pos, tree.symbol.annotations.mkString(", "))

produces

[info] .../core/src/main/scala/Main.scala:5:7: Typestate("MyProtocol.txt")
[info] class Cat{
[info]       ^

Learning resource:

Seth Tisue - Scala Compiler Plugins 101

video https://www.youtube.com/watch?v=h5NZjuxS5Qo

slides https://docs.google.com/presentation/d/1KtJMd27yGWmr7E2yxKC_ipMeaP_vr1-6wVJ-QcqS_Vc/edit?usp=sharing

code https://github.com/SethTisue/cloc-plugin

Changes in Dotty: http://dotty.epfl.ch/docs/reference/changed-features/compiler-plugins.html

Thanks! That was really useful! The "annotations" in mods.annotations came up in red in Intellij (cannot resolve symbol error) but it worked fine anyway.
@aurorarized IntelliJ highlighting is irrelevant. You can try quasiquote q"${mods: Modifiers} class...

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