Scala 3 compiler plugin generating expected code but failing at runtime

Question

I'm trying to get started with writing a compiler plugin for scala 3. At this stage, it's primarily based on https://github.com/liufengyun/scala3-plugin-example/blob/master/plugin/src/main/scala/Phases.scala (and the accompanying youtube video explaining how it works).

It's been an interesting process so far, and I'm getting a bit of a feel for some aspects of the compiler.

As a first step, I'm simply trying to wrap a method body into a block, print whatever the returned object was going to be, and then return the object.

This differs from the original plugin mainly in that there was a single side-effecting method call added to each method - this is also assigning a local variable, (which I think is probably the cause of the problem), and moving the method body into a block.

I've produced as minimal of a working example as I could in a fork here: https://github.com/robmwalsh/scala3-plugin-example

The plugin compiles fine, seems to run as part of compilation as expected, and then blows up at runtime. I'm not entirely sure if this is me doing something wrong (not unlikely) or a bug in the compiler (less likely, but a distinct possibility!).

Can anybody please shed some light on why this isn't working? I don't know what flags should be set when creating a new Symbol, so that's one possibility, but there's heaps of stuff that sorta seemed to work so I rolled with it.

Here's where I'm at (the interesting bits):

...
 override def prepareForUnit(tree: Tree)(using ctx: Context): Context =
    //find the printLn method
    val predef = requiredModule("scala.Predef")
    printlnSym = predef.requiredMethod("println", List(defn.AnyType))
    ctx

  override def transformDefDef(tree: DefDef)(using ctx: Context): Tree =
    val sym = tree.symbol

    // ignore abstract and synthetic methods
    if tree.rhs.isEmpty|| sym.isOneOf(Synthetic | Deferred | Private | Accessor)
    then return tree
    try {
      println("\n\n\n\n")
      println("========================== tree ==========================")
      println(tree.show)

      // val body = {tree.rhs}
      val body = ValDef(
        newSymbol(
        tree.symbol, termName("body"), tree.symbol.flags, tree.rhs.tpe),
        Block(Nil, tree.rhs)
      )
      
      // println(body)
      val bodyRef  = ref(body.symbol)
      val printRes = ref(printlnSym).appliedTo(bodyRef)

      // shove it all together in a block
      val rhs1 = tpd.Block(body :: printRes :: Nil, bodyRef)

      //replace RHS with new
      val newDefDef = cpy.DefDef(tree)(rhs = rhs1)
      println("====================== transformed ======================")
      println(newDefDef.show)
      newDefDef
    } catch {
      case e =>
        println("====================== error ===========================")
        println(e)
        println(e.printStackTrace)
        tree
    }
...

test program for compiler plugin

object Test extends App:
    def foo: String = "forty two"
    def bar(x: String): Int = x.length
    def baz(x: String, y: Int): String = x + y

    baz(foo, bar(foo))

output during compile using plugin (exactly what I wanted! I got very excited at this point)

========================== tree ==========================
def foo: String = "forty two"
====================== transformed ======================
def foo: String = 
  {
    val body: ("forty two" : String) = 
      {
        "forty two"
      }
    println(body)
    body
  }


========================== tree ==========================
def bar(x: String): Int = x.length()
====================== transformed ======================
def bar(x: String): Int = 
  {
    val body: Int = 
      {
        x.length()
      }
    println(body)
    body
  }


========================== tree ==========================
def baz(x: String, y: Int): String = x.+(y)
====================== transformed ======================
def baz(x: String, y: Int): String = 
  {
    val body: String = 
      {
        x.+(y)
      }
    println(body)
    body
  }

output during runtime :'( (this changes depending on the code it's running on, but always the same theme)

Exception in thread "main" java.lang.VerifyError: Bad local variable type
Exception Details:
  Location:
    testing/Test$.body$2()I @0: aload_1
  Reason:
    Type top (current frame, locals[1]) is not assignable to reference type
  Current Frame:
    bci: @0
    flags: { }
    locals: { 'testing/Test$' }
    stack: { }
  Bytecode:
    0000000: 2bb6 007d ac

    at testing.Test.main(Example.scala)

Edit: I'm using scala 3.1.2

Answer 1

I was using the existing flags when creating my new symbol. Instead, I needed to use the Local flag, which I suppose makes sense.