`RootModule` and `Cross` cannot be applied to the same module (500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

See https://github.com/orgs/com-lihaoyi/discussions/6 for other bounties and the terms and conditions that bounties operate under

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Both of them are classs, so neither one can be mixed in.

This couldn't be fixed in Scala 2, as they both need to take parameters (implicit for RootModule, explicit for Cross), and so cannot be traits. But now that main is on Scala 3, this should be doable. Apart from making them both traits, we may need to update the macros in CrossMacros.scala and elsewhere to remove any assumption that these types are classes, and update the parser to allow the Segments string to start with a Segment.Cross e.g. [3.6.3].foo.bar.baz

The end result should be an test build in integration/feature demonstrating both a build.mill and a subfolder/package.mill with object package extends RootModule with Cross(...), and the tests exercising both subfolder[foo].bar as well as [foo].bar` query strings