The new 2.13 collections have now been merged onto the 2.13.x scala/scala branch, in https://github.com/scala/scala/pull/6498. This is the single most important pull request in the entire 2.13 cycle.
The compiler bootstraps with the new collections and compiler performance is on par with what it was before.
We’ll release 2.13.0-M4 once the lowest tiers of the 2.13 community build are green (we’re working directly with library authors on this) and once we have enough documentation and Scalafix rewrites ready that we can reasonably ask library authors to publish for M4.
There are already a number of green or very-nearly-green projects in the 2.13 community build, so I’d say that’s going fine.
We had originally hoped to go straight from M4 to RC1, but we concluded that probably isn’t realistic, so we expect there will be an M5.
Seth Tisue / Scala team / Lightbend, Inc.