How to get default values of class fields in a compiler plugin?

Question
1

I have an annotation defined in the source, which a compiler plugin shall transform:

class dslRoot(val aConstField: String = "hi") extends StaticAnnotation

I can match this part in the plugin:

override def transformValDef(tree: tpd.ValDef)(using ctx: Context): tpd.Tree =
    if tree.name.toString == "aConstField" then
      report.error(
        s"""${ctx.owner.fullName}
           |${tree.rhs.tpe}
           |""".stripMargin
      )
    tree

And I get NoType instead of a ConstantType

How can I get the default value during the transformation?

2

“Default getters” for constructor parameters are members of the companion class.

See Applications.findDefaultGetter for example code.

class C(x: String = "hello")

Use -Vprint:typer to see

  final module class C() extends AnyRef() { this: C.type =>
    def $lessinit$greater$default$1: String @uncheckedVariance = "hello"
  }
1 Like
3

Locating the getter now works; I wonder how to get the right-hand side (in this case, the default value) without introducing another compiler plugin phase.

4

If you mean how to cope with companions, you can transformStats since companions are always defined together.

An example is my recent PR which examines companions at flatten.

I haven’t taken a second look at that code, so it might be suboptimal or wrong. There may be existing utility methods that simplify everything. Please let me know if you find them.