Can/should I use both -Xsource:3 and -source:3.0-migration in a cross-compiled codebase?

[armanbilge]

I'd like to refresh the compiler flags configured by sbt-typelevel which is used to cross-compile several libraries for both Scala 2 and 3. I have a question about:

• -Xsource:3 in Scala 2, which "enables some Scala 3 syntax and behavior"

• -source:3.0-migration in Scala 3, which "accepts some of the old Scala 2.13 syntax"

Ref: https://docs.scala-lang.org/scala3/guides/migration/tooling-tour.html

Does it make sense to use both of these flags in a cross-compiled codebase? I ask, because the former sounds like "make Scala 2 accept Scala 3-ish sources" and the latter sounds like "make Scala 3 accept Scala 2-ish sources". So I wonder if we should pick one strategy (aim for either Scala 2-ish or Scala 3-ish sources) and use the one flag that makes sense for that.

Also what about -source:3.3-migration? 😅 Can that be mixed with -Xsource:3?

Case Study

Consider the following program:

case class Foo private (bar: String)

object Main {
  def main(args: Array[String]): Unit =
    println(Foo("bar"))
}

Actually, this doesn't compile out-of-the-box with Scala 3.3.0.

[error] object Foo does not take parameters
[error]     println(Foo("bar"))
[error]             ^^^

Now here is what happens with various Scala version and compiler flag configurations:

Options / Scala	2.13.10	3.3.0
no options	✅	❌
-Xsource:3	❌	n/a
-source:3.0-migration	n/a	✅
-source:3.3-migration	n/a	❌

Analysis:

• -Xsource:3 and -source:3.0-migration have the opposite effect on their respective compilers. So using them together seems pretty dumb.
• -Xsource:3 and -source:3.3-migration in this specific situation have the same result. So maybe they can be used together?

[som-snytt]

There is an old ticket somewhere to document -Xsource for Scala 2.

The purpose of -Xsource:3 was to get source in a place where it compiles straight under scala 3.

[smarter]

I don't think a plugin should unilaterally touch -source, since it can lead to a lot of confusion down the road without the user realizing what the culprit is. For example -source:3.0-migration can break Scala 3 code because it changes implicit resolution rules (e.g., nested implicits will lead to ambiguity errors, but Scala 3 APIs using context function types routinely rely on that).

[armanbilge]

and ideally it should only be used temporarily during a migration

Hmm, you mean like migrating from Scala 2 to Scala 3? What about for libraries that are permanently cross-compiled?

[smarter]

Such libraries need to be written in the common subset of Scala 2 and 3 which has the same meaning in both languages (and use version-specific sources when that's not possible).

[armanbilge]

Ok, thanks! So if I'm understanding correctly, cross-compiling libraries should use neither -Xsource:3 nor -source:3.x-migration ?

[smarter]

If you're cross-compiling, then I think -Xsource:3 isn't a problem because the things it reject will also be rejected by Scala 3 anyway, and it will mean you can use new syntax, but it's not a big deal either way.

[som-snytt]

I disagree that -Xsource:3 doesn't matter. Modulo bugs, you can use leading infix. You can't use early defs. It was a bigger commitment when it included the implicits resolution change.