Cross building plugins 

Like we are able to cross build against multiple Scala versions, we can cross build sbt 0.13 plugins while staying on sbt 1.x.

crossSbtVersions := Vector("1.2.8", "0.13.18")

If you need to make changes specific to a sbt version, you can now include them into src/main/scala-sbt-0.13 and src/main/scala-sbt-1.0. To switch between the sbt versions use

^^ 0.13.18

[info] Setting `sbtVersion in pluginCrossBuild` to 0.13.18
[info] Set current project to sbt-something (in build file:/xxx/sbt-something/)

or ^compile to cross compile.

Mixing libraries and sbt plugins in a build 

When you want to mix both libraries and sbt plugins into a multi-project build, it’s more convenient to drive the sbt version based on the Scala version.

You can do that as follows:

ThisBuild / crossScalaVersions := Seq("2.10.7", "2.12.10")

lazy val core = (project in file("core"))

lazy val plugin = (project in file("sbt-something"))
  .enablePlugins(SbtPlugin)
  .dependsOn(core)
  .settings(
    // change the sbt version based on Scala version
    pluginCrossBuild / sbtVersion := {
      scalaBinaryVersion.value match {
        case "2.10" => "0.13.18"
        case "2.12" => "1.2.8"
      }
    }
  )

This is a technique discovered by @jroper in sbt-pgp#115. It works because sbt 0.13 and 1.x series use different Scala binary versions.

Using the setting, you can now use Scala cross building commands such as +compile and +publish.