Finding lambda captured values (or their classes) in Scala 3

I'm looking for a way to find values (or their classes) that are captured by lambda (for serialization - something like Spark) in Scala 3 (I don't need Scala 2 support):

val a = "abc"
val f = () => a + "xyz"
serialize(f) // Should detect a / String as captured value

Doing this in runtime is kinda easy (iterating over f.getClass.getDeclaredFields), but I would like to do it in compile time.

I was trying to inspect time of lambda in Macro, but it's detected as plain scala.Function0 without any interesting info.

I wonder if I can do some tree inspection, but I would really like to avoid that - I feel like I would have to copy compiler internals to catch all edge cases.

Try the following macro

import scala.quoted.*

inline def serialize(x: Any): Unit = ${serializeImpl('x)}

def serializeImpl(x: Expr[Any])(using Quotes): Expr[Unit] = {
  import quotes.reflect.*

  def owners(s: Symbol): List[Symbol] = s :: List.unfold(s)(s1 => Option.when(s1.maybeOwner != Symbol.noSymbol)((s1.maybeOwner, s1.maybeOwner)))

  val symbol = x.asTerm.underlying.symbol
  val rhs = symbol.tree match {
    case ValDef(_, _, Some(rhs)) => rhs
  }

  val traverser = new TreeTraverser {
    override def traverseTree(tree: Tree)(owner: Symbol): Unit = {
      tree match {
        case Ident(name) =>
          val symbol1 = tree.symbol
          val pos1 = symbol1.pos.get
          println(s"identifier: $name, defined inside lambda: ${owners(symbol1).contains(symbol)}, defined in current file: ${pos1.sourceFile == SourceFile.current}")

        case _ =>
      }

      super.traverseTree(tree)(owner)
    }
  }

  traverser.traverseTree(rhs)(rhs.symbol)

  '{()}
}

Usage:

object App1 {
  val b = "bbb"
}

import App1.b

object App {
  val a = "abc"
  val f = () => { val x = "uvw"; a + b + x + "xyz"}
  serialize(f)
}

//scalac: identifier: a, defined inside lambda: false, defined in current file: true
//scalac: identifier: b, defined inside lambda: false, defined in current file: false
//scalac: identifier: x, defined inside lambda: true, defined in current file: true
//scalac: identifier: $anonfun, defined inside lambda: true, defined in current file: true