Scala 3 build.sc

[lolgab]

Mill is gaining adoption, but being a Scala 3 only build tool, would help it position itself as a next-generation build tool for Scala.

Sbt will be soon running on Scala 3, so it would be nice if Mill did the same.

If we want to avoid breaking changes, we cross-compile Mill to Scala 3 and let users decide which version they want to use, either via the meta-build, a config file or a directive (like //> using scala 3 from scala-cli).

[lihaoyi]

I think a breaking change for Scala 3 is fine IMO. Trying to keep things cross-building adds considerable internal complexity, and giving the user a choice adds considerable user-facing complexity. We should just make the jump; maybe not immediately, but perhaps mid/late this year?

All of Mill's upstream Scala libraries are Scala 3 compatible, if I'm not mistaken. The normal code should be trivial to make cross-compatible (e.g. adding missing return type ascriptions for implicits), and the generated code should be similar (since it's just code). Most of the "heavy lifting" is already moved to T.workers that live in separate classloaders

So the main work seems to be (a) porting the Applicative task macros to Scala 3 and (b) porting the line number fixer plugin to Scala 3. Neither of them seems terribly difficult, though we would need someone familiar with Scala 3 metaprogramming for (a) and Scala 3 internals for (b).

[kubukoz]

All of Mill's upstream Scala libraries are Scala 3 compatible

Coursier will require a _2.13 prefix afaik, but it should be fine... I think.

[lihaoyi]

Superseded by #3152