Can a Scala compiler plugin transform the autogenerated accessor methods of scala case classes?

Question

After the parser phase of the Scalac process, the following case class

case class ExampleCaseClass(var s:String, var i:Int) extends ContextuallyMutable

takes the intermediate form:

Clazz(case class ExampleCaseClass extends ContextuallyMutable with scala.Product with scala.Serializable {
  <caseaccessor> <paramaccessor> var s: String = _;
  <caseaccessor> <paramaccessor> var i: Int = _;
  def <init>(s: String, i: Int) = {
    super.<init>();
    ()
  }
})

However, a run time reflection call:

ExampleCaseClass("Can a Scala compiler plugin transform the autogenerated accessor methods of scala case classes?", 42).getClass.getMethods.foreach(println(_))

reveals many more public methods:

public boolean ExampleCaseClass.equals(java.lang.Object)
public java.lang.String ExampleCaseClass.toString()
public int ExampleCaseClass.hashCode()
public static ExampleCaseClass ExampleCaseClass.apply(java.lang.String,int)
public int ExampleCaseClass.i()
public java.lang.String ExampleCaseClass.s()
public ExampleCaseClass ExampleCaseClass.copy(java.lang.String,int)
public void ExampleCaseClass.i_$eq(int)
public scala.collection.Iterator ExampleCaseClass.productElementNames()
public java.lang.String ExampleCaseClass.productElementName(int)
public void ExampleCaseClass.s_$eq(java.lang.String)
public int ExampleCaseClass.copy$default$2()
public boolean ExampleCaseClass.canEqual(java.lang.Object)
public java.lang.String ExampleCaseClass.productPrefix()
public int ExampleCaseClass.productArity()
public java.lang.Object ExampleCaseClass.productElement(int)
public scala.collection.Iterator ExampleCaseClass.productIterator()
public java.lang.String ExampleCaseClass.copy$default$1()
public static scala.Function1 ExampleCaseClass.tupled()
public static scala.Option ExampleCaseClass.unapply(ExampleCaseClass)
public static scala.Function1 ExampleCaseClass.curried()
public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
public final void java.lang.Object.wait() throws java.lang.InterruptedException
public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
public final native java.lang.Class java.lang.Object.getClass()
public final native void java.lang.Object.notify()
public final native void java.lang.Object.notifyAll()

Clearly some subsequent compiler phase creates the property accessor methods:

public int ExampleCaseClass.i()
public java.lang.String ExampleCaseClass.s()
public void ExampleCaseClass.i_$eq(int)
public void ExampleCaseClass.s_$eq(java.lang.String)

Which compilation phase generates these accessor methods and what manner of compiler plugin (or other means) might prevent or transform them?

The enquirer has already run numerous experiments removing or reshaping the:

  <caseaccessor> <paramaccessor> var s: String = _;
  <caseaccessor> <paramaccessor> var i: Int = _;

portions of the case class, and also with injecting the desired accessor methods in advance but no combination has met the desired outcome. They either fail to compile because of naming conflicts that arise in subsequent compilation phases, or they alter the parameter names in constructor, apply, and accessor methods.

Can a scala compiler plugin transform synthetic accessors at all? Does the Java Compiler introduce these methods? If so, should the enquirer look to Javac plugins and what analogues might serve the Scala.js and Scala native compilation targets?

Thank you for any consideration.

Answer 1

case class expansion happens in more than one place, see another question.

Instead of writing a new plugin just to disallow using var it would be much better to add a new rule to Wartremover or ScalaFix. As a matter of the fact, these rules already exist:

• disallow var with Wartremover - combine it with fatal warnings to fail compilation on var
• disallow var with ScalaFix

If you want to add more elaborate rule... it would still be easier just to write your own Wartremover/ScalaFix rule (the latter might be preferred as it is already supported in Scala 3).

And if you really need a custom compiler plugin to mess with code generated by compiler... take a look at better-toString plugin. It adds its own phase after "parser" phase. But I wouldn't hope for removing the autogenerated implementations. At best you can override them manually where specs allows you to.

Answer 2

The enquirer found a Scala 3 solution with persistence and help from examples:

1. better-tostring a plugin that demonstrates conditional method insertion by Polyvariant.
2. Compiler Plugin Development in Scala 3 Example and Tutorial by Scala Center dev: Fengyun Liu. The video provided insights into compiler phases and the example shed light on method body generation syntax. In particular, available documentation doesn't readily clarify how to call println from a method body generated as a Tree by a compiler plugin, but Liu's example plugin demonstrated requireModule and requiredMethod.
3. Scala 3 Compiler Plugin Documentation offers a very nice template for how to start writing a plugin. The final solution looked very similar.

As of 26 Nov 2021, no solution exists for Scala 2.13, but maybe that just means it is time to upgrade.

final class BlockMutatorPlugin extends StandardPlugin {
  override val name: String = "BlockMutatorPlugin"
  override val description: String = "Scala Compiler Plugin for blocking ContextuallyMutable setter methods."
  override def init(options: List[String]): List[PluginPhase] = List(new BlockContextuallyMutableSetters)
}

class BlockContextuallyMutableSetters extends PluginPhase {

  val phaseName = "blockGetter"

  /* Running this plugin after phases before ElimErasedValueType
resulted in the replacement of the generated setter methods by the synthetic
default versions.  By the time that this ElimErasedValueType phase ends, the 
defaults already existed, so this plugin could augment them safely.  */

  override val runsAfter = Set(ElimErasedValueType.name)

  private var printBlocked: Tree = _

  override def prepareForTemplate(tree: tpd.Template)(using ctx: Context): Context = {
    val cnsl = requiredModule("scala.Predef")
    val prntln: PreName = "println".toTermName
    val say = cnsl.requiredMethod(prntln, List[Types.Type](ctx.definitions.ObjectType))
    printBlocked = ref(say).appliedTo(Literal(Constant("Blocked!")))
    ctx
  }

  override def transformTemplate(tree: Template)(using ctx: Context): Tree = {
    if (tree.parents.filter(_.symbol.name.toString.equals("ContextuallyMutable")).nonEmpty) {
      cpy.Template(tree)(
        body = tree.body.collect {
          case dd: DefDef if dd.name.isSetterName => DefDef(
            dd.symbol.asInstanceOf[Symbols.TermSymbol],
            printBlocked
          )
          case x => x
        }
      ).asInstanceOf[Tree]
    } else tree
  }
}

The most important part of this effort involved discovering which compiler phase this plugin should follow. Similar efforts in Scala 2.13.6 have failed so far; the only remaining impediment to the Scala 2 solution sought by this original Stack Overflow question. As such, the enquirer will not mark his own answer as the accepted solution unless future edits avail Scala 2. Until that time, your response may claim that designation.

For any inclined to try compile this example, the code above requires the following import statements:

import dotty.tools.dotc.ast.tpd
import tpd.*
import dotty.tools.dotc.core.*
import Names.PreName
import Symbols.{ClassSymbol, requiredMethod, requiredModule}
import Decorators.*
import NameOps.*
import Contexts.Context
import Constants.Constant
import dotty.tools.dotc.plugins.*
import dotty.tools.dotc.transform.ElimErasedValueType