Fix #14488: Scala.js: Add compiler support for scala.Enumeration.

[sjrd]

This is the same logic that is used in the Scala.js compiler plugin for Scala 2.

We catch ValDefs of the forms

  val SomeField = Value
  val SomeOtherField = Value(5)

and rewrite them as

  val SomeField = Value("SomeField")
  val SomeOtherField = Value(5, "SomeOtherField")

For calls to Value and new Val without name that we cannot rewrite, we emit warnings.

[sjrd]

Play JSON actually actively tests for the presence of the bug:
https://github.com/dotty-staging/play-json/blob/f65a409f4dc129f7adc72f048abbfbfc937a740f/play-json/js/src/test/scala-3/play/api/libs/json/EnumSpec.scala#L14-22
What am I supposed to do with that!?

[SethTisue]

@sjrd perhaps you could

set `play-json`.js / Test / unmanagedSources / excludeFilter := HiddenFileFilter || "EnumSpec.scala"`

a little trick I've employed about a million times in the Scala 2 community build

[dwijnand]

Change the test, I'd say, as it's tracking progression rather than spec or regression.

[sjrd]

set `play-json`.js / Test / unmanagedSources / excludeFilter := HiddenFileFilter || "EnumSpec.scala"`

a little trick I've employed about a million times in the Scala 2 community build

Thanks, I'm trying that now.

Change the test, I'd say, as it's tracking progression rather than spec or regression.

I guess that's also a possibility. But that means it's a commit in our fork that is neither a cherry-pick from upstream, nor something I can PR back upstream anytime soon. Is that OK?

[dwijnand]

Yep, or at least to me it is.

[sjrd]

This test uses extends Val without explicit name, which is not supported (and was never supported in Scala 2 either).

[sjrd]

Yep, or at least to me it is.

Alright, that worked as well.

@dwijnand Would you mind reviewing this PR, perhaps? It's mostly a port of the Enum-related aspect of
https://github.com/scala-js/scala-js/blob/v1.10.1/compiler/src/main/scala/org/scalajs/nscplugin/PrepJSInterop.scala

[sjrd]

Ping @dwijnand ?

[sjrd]

Thanks 🙏