Wrap all names containing $ reserved character in backticks for all script wrappers

[Gedochao]

Fixes #4374
This is an alternative to #4375, as per #4375 (comment)

What's this about

We no longer can just use $ in field names in the *.sc script wrapper, as otherwise on Scala 3.9+ (currently rc/nightly) we get:

[warn] .../snippet.scala:12:15
[warn] The identifier `args$opt0` should not contain `$`, which is reserved for internal compiler use.
[warn]   private var args$opt0 = Option.empty[Array[String]]
[warn]               ^^^^^^^^^
[warn] .../snippet.scala:3:13
[warn] The identifier `snippet$_` should not contain `$`, which is reserved for internal compiler use.
[warn] final class snippet$_ {
[warn]             ^^^^^^^^^
[warn] .../snippet.scala:13:7
[warn] The identifier `args$set` should not contain `$`, which is reserved for internal compiler use.
[warn]   def args$set(args: Array[String]): Unit = {
[warn]       ^^^^^^^^
[warn] .../snippet.scala:16:7
[warn] The identifier `args$opt` should not contain `$`, which is reserved for internal compiler use.
[warn]   def args$opt: Option[Array[String]] = args$opt0
[warn]       ^^^^^^^^
[warn] .../snippet.scala:17:7
[warn] The identifier `args` should not contain `$`, which is reserved for internal compiler use.
[warn]   def args$: Array[String] = args$opt.getOrElse {
[warn]       ^^^^^

What changes

Rather than migrating the wrapper off-$, this approach wraps everything in backticks.

For a given script:

// hello.sc
println("Hello")
println(args.head)

The under-the-hood generated code wrapper looked roughly like this:

final class hello$_ {
  def args = hello_sc.args$
  def scriptPath = """hello.sc"""
  // ... your script code here ...
}
object hello_sc {
  private var args$opt0 = Option.empty[Array[String]]
  def args$set(args: Array[String]): Unit = { args$opt0 = Some(args) }
  def args$opt: Option[Array[String]] = args$opt0
  def args$: Array[String] = args$opt.getOrElse {
    sys.error("No arguments passed to this script")
  }
  lazy val script = new hello$_
  def main(args: Array[String]): Unit = {
    args$set(args)
    val _ = script.hashCode()
  }
}
export hello_sc.script as `hello`

After this change, this migrates to:

final class `hello$_` {
  def args = `hello_sc.args$`
  def scriptPath = """hello.sc"""
  // ... your script code here ...
}
object hello_sc {
  private var `args$opt0` = Option.empty[Array[String]]
  def `args$set`(args: Array[String]): Unit = { `args$opt0` = Some(args) }
  def `args$opt`: Option[Array[String]] = `args$opt0`
  def `args$`: Array[String] = `args$opt`.getOrElse {
    sys.error("No arguments passed to this script")
  }
  lazy val script = new `hello$_`
  def main(args: Array[String]): Unit = {
    `args$set`(args)
    val _ = script.hashCode()
  }
}
export hello_sc.script as `hello`

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)

How much have your relied on LLM-based tools in this contribution?

extensively, Cursor + Claude

How was the solution tested?

Included new automated test ensuring the warning is not there