[toString] replacement, originally discussed in https://kotlinlang.slack.com/archives/C7L3JB43G/p1572884683059400

toStringReplacement.meta.kt

package arrow.meta.plugins.helloWorld

import arrow.meta.Meta
import arrow.meta.Plugin
import arrow.meta.invoke
import arrow.meta.phases.analysis.ElementScope
import arrow.meta.quotes.ClassScope
import arrow.meta.quotes.Scope
import arrow.meta.quotes.Transform
import arrow.meta.quotes.classOrObject
import arrow.meta.quotes.func
import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
import org.jetbrains.kotlin.psi.KtClass
import org.jetbrains.kotlin.psi.KtNamedFunction

val Meta.toStringReplacement: Plugin
  get() =
    "toStringReplacement" {
      meta(
        /**
         * Replaces [toString] with a custom impl when [toString already exists as an override]
         */
        func(KtNamedFunction::toStringPresent) { toString ->
          Transform.replace(
            replacing = toString,
            newDeclaration = toStringReplacement()
          )
        },
        /**
         * Adds [toString] with a custom impl to all data classes that have no [toString] method
         */
        classOrObject(KtClass::dataClassInheritsToString) { dataClass ->
          Transform.replace(
            replacing = dataClass,
            newDeclaration = dataClassWithAddedToString(ctx)
          )
        }
      )
    }

/**
 * Used when a data class does not have [toString]
 */
private fun ClassScope.dataClassWithAddedToString(elementScope: ElementScope): ClassScope =
  elementScope.run {
    """|$`@annotationEntries` $kind $name $`(typeParameters)` $`(valueParameters)` : $supertypes {
       |  $body
       |  ${toStringReplacement()}
       |}
       |""".`class`
  }

/**
 * The [toString] replacement function
 */
private fun ElementScope.toStringReplacement(): Scope<KtNamedFunction> =
  """|override fun toString(): String = 
     |  "replaced!"
     |""".function.synthetic

/**
 * Matches all data classes that do not include [toString] as an override
 */
private fun KtClass.dataClassInheritsToString(): Boolean =
  isData() && declarations.filterIsInstance<KtNamedFunction>().all { !it.toStringPresent() }

/**
 * A [toString] function is considered when it's in a data class and it's an override 
 */
private fun KtNamedFunction.toStringPresent(): Boolean =
  name == "toString" &&
    (psiOrParent as? KtClass)?.isData() == true &&
    modifierList?.hasModifier(KtModifierKeywordToken.keywordModifier("override")) == true