https://stackoverflow.com/questions/74497076/auto-generate-companion-object-for-case-class-in-scala

build.sbt

ThisBuild / version := "0.1.0-SNAPSHOT"

//ThisBuild / scalaVersion := "2.13.10"
ThisBuild / scalaVersion := "2.12.17"

lazy val macroAnnotationSettings = Seq(
  scalacOptions ++= (CrossVersion.partialVersion(scalaVersion.value) match {
    case Some((2, v)) if v >= 13 => Seq("-Ymacro-annotations")
    case _ => Nil
  }),
  libraryDependencies ++= (CrossVersion.partialVersion(scalaVersion.value) match {
    case Some((2, v)) if v <= 12 =>
      Seq(compilerPlugin("org.scalamacros" % "paradise" % "2.1.1" cross CrossVersion.full))
    case _ => Nil
  })
)

lazy val core = project
  .settings(
    macroAnnotationSettings,
    scalacOptions ++= Seq(
      "-Ymacro-debug-lite",
    ),
  )
  .dependsOn(macros)

lazy val macros = project
  .settings(
    macroAnnotationSettings,
    libraryDependencies ++= Seq(
      scalaOrganization.value % "scala-reflect" % scalaVersion.value,
    ),
  )

Macros.scala

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

object Macros {
  @compileTimeOnly("enable macro paradise to expand macro annotations")
  class GenerateCompanion extends StaticAnnotation {
    def macroTransform(annottees: Any*): Any = macro GenerateCompanion.impl
  }

  object GenerateCompanion {
    def impl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
      import c.universe._

      def vals(paramss: Seq[Seq[ValDef]]): Seq[ValDef] =
        paramss.flatten.map(p => {
          val name = p.name.toString
          q"val ${TermName(underscoreToCamel(name))}: String = $name"
        })

      annottees match {
        case (c@q"$_ class $tpname[..$_] $_(...$paramss) extends { ..$_ } with ..$_ { $_ => ..$_ }") :: Nil =>
          q"""
          $c
          object ${tpname.toTermName} {
            ..${vals(paramss)}
          }
          """

        case (c@q"$_ class $tpname[..$_] $_(...$paramss) extends { ..$_ } with ..$_ { $_ => ..$_ }") ::
          q"$mods object $tname extends { ..$earlydefns } with ..$parents { $self => ..$body }" :: Nil =>
          q"""
           $c
           $mods object $tname extends { ..$earlydefns } with ..$parents { $self =>
            ..$body
            ..${vals(paramss)}
           }
          """
      }
    }

    def underscoreToCamel(name: String): String = "_([a-z\\d])".r.replaceAllIn(name, _.group(1).toUpperCase)
  }
}

Main.scala

import Macros.GenerateCompanion

object Main extends App {
  @GenerateCompanion
  class Pizza(val crust_type: String, val foo_foo: Int)

  object Pizza {
    def bar: String = "bar"
  }

  println(Pizza.crustType) //crust_type
  println(Pizza.fooFoo) //foo_foo
  println(Pizza.bar) //bar

}

log

/usr/local/graalvm-ee-java8-19.3.0/bin/java -server -Xmx1536M -Dsbt.supershell=false -Didea.managed=true -Dfile.encoding=UTF-8 -Didea.installation.dir=/media/data/idea-IU-211.7628.21 -jar /home/dmitin/.local/share/JetBrains/IntelliJIdea2022.2/Scala/launcher/sbt-launch.jar early(addPluginSbtFile=\"\"\"/tmp/idea2.sbt\"\"\") "; set ideaPort in Global := 39859 ; idea-shell"
[info] welcome to sbt 1.8.0 (Oracle Corporation Java 1.8.0_231)
[info] loading global plugins from /home/dmitin/.sbt/1.0/plugins
[info] loading settings for project scalademo18-build from idea2.sbt ...
[info] loading project definition from /media/data/Projects1/scalademo18/project
[info] loading settings for project scalademo18 from build.sbt ...
[info] set current project to scalademo18 (in build file:/media/data/Projects1/scalademo18/)
[info] Defining Global / ideaPort
[info] The new value will be used by Compile / compile, Test / compile and 3 others.
[info] 	Run `last` for details.
[info] Reapplying settings...
[info] set current project to scalademo18 (in build file:/media/data/Projects1/scalademo18/)
[IJ]clean
[success] Total time: 0 s, completed 19.11.2022 6:36:58
[IJ]compile
[info] compiling 1 Scala source to /media/data/Projects1/scalademo18/macros/target/scala-2.12/classes ...
[info] done compiling
[info] compiling 1 Scala source to /media/data/Projects1/scalademo18/core/target/scala-2.12/classes ...
performing macro expansion new GenerateCompanion().macroTransform(class Pizza extends scala.AnyRef {
  <paramaccessor> val crust_type: String = _;
  <paramaccessor> val foo_foo: Int = _;
  def <init>(crust_type: String, foo_foo: Int) = {
    super.<init>();
    ()
  }
}, object Pizza extends scala.AnyRef {
  def <init>() = {
    super.<init>();
    ()
  };
  def bar: String = "bar"
}) at source-/media/data/Projects1/scalademo18/core/src/main/scala/Main.scala,line-4,offset=62
{
  class Pizza extends scala.AnyRef {
    <paramaccessor> val crust_type: String = _;
    <paramaccessor> val foo_foo: Int = _;
    def <init>(crust_type: String, foo_foo: Int) = {
      super.<init>();
      ()
    }
  };
  object Pizza extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    def bar: String = "bar";
    val crustType: String = "crust_type";
    val fooFoo: String = "foo_foo"
  };
  ()
}
Block(List(ClassDef(Modifiers(), TypeName("Pizza"), List(), Template(List(Select(Ident(scala), TypeName("AnyRef"))), noSelfType, List(ValDef(Modifiers(PARAMACCESSOR), TermName("crust_type"), Ident(TypeName("String")), EmptyTree), ValDef(Modifiers(PARAMACCESSOR), TermName("foo_foo"), Ident(TypeName("Int")), EmptyTree), DefDef(Modifiers(), termNames.CONSTRUCTOR, List(), List(List(ValDef(Modifiers(PARAM | PARAMACCESSOR), TermName("crust_type"), Ident(TypeName("String")), EmptyTree), ValDef(Modifiers(PARAM | PARAMACCESSOR), TermName("foo_foo"), Ident(TypeName("Int")), EmptyTree))), TypeTree(), Block(List(pendingSuperCall), Literal(Constant(()))))))), ModuleDef(Modifiers(), TermName("Pizza"), Template(List(Select(Ident(scala), TypeName("AnyRef"))), noSelfType, List(DefDef(Modifiers(), termNames.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(pendingSuperCall), Literal(Constant(())))), DefDef(Modifiers(), TermName("bar"), List(), List(), Ident(TypeName("String")), Literal(Constant("bar"))), ValDef(Modifiers(), TermName("crustType"), Ident(TypeName("String")), Literal(Constant("crust_type"))), ValDef(Modifiers(), TermName("fooFoo"), Ident(TypeName("String")), Literal(Constant("foo_foo"))))))), Literal(Constant(())))
[info] done compiling
[success] Total time: 7 s, completed 19.11.2022 6:37:07