Cannot compile SBT plugin under Scala 2.13.0

[mr-git]

steps

SBT version: 1.2.8, 1.3.0

build.sbt:

lazy val `plugin-with-bug` = project.in(file("."))
sbtPlugin := true
scalaVersion := "2.13.0"
crossScalaVersions := Seq("2.12.9", "2.13.0")

src/main/scala/Bug.scala:

import sbt._

object Bug extends AutoPlugin {
  val key = settingKey[String]("key")
  override def requires: Plugins = plugins.JvmPlugin
  override def trigger: PluginTrigger = allRequirements
  override def globalSettings: Seq[Setting[_]] = Seq()
}

Run command: +compile or ++2.13.0; compile

problem

IJ]sbt:plugin-with-bug> ++2.13.0; compile
[info] Setting Scala version to 2.13.0 on 1 projects.
[info] Reapplying settings...
[info] Set current project to plugin-with-bug (in build file:/.../git/sbt-issue/)
[info] Compiling 1 Scala source to /.../sbt-issue/target/scala-2.13/sbt-1.0/classes ...
[error] /.../git/sbt-issue/src/main/scala/Bug.scala:4:31: exception during macro expansion: 
[error] java.lang.ClassNotFoundException: scala.Serializable
[error] 	at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:436)
[error] 	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
[error] 	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
[error] 	at sbt.std.KeyMacro$.settingKeyImpl(KeyMacro.scala:20)
[error]   val key = settingKey[String]("key")
[error]                               ^
[error] one error found

expectation

There are no compilation errors.
Compilation for Scala 2.13.0 must succeed.

notes

[eed3si9n]

sbt plugin should be created like this:

lazy val root = (project in file("."))
  .enablePlugins(SbtPlugin)
  .settings(
    name := "sbt-something"
  )

and Scala version should not be specified unless you're cross building between sbt 1.x and 0.13.
There's no sbt available under 2.13.0, so expectation doesn't make sense.

See also https://www.scala-sbt.org/1.x/docs/Plugins.html

[mr-git]

thank you!

conclusion:

• SBT 0.x runs only on Scala 2.10.x
• SBT 1.x runs only on Scala 2.12.x
• SBT 2.x will run only on Scala 2.13.x or 3.0.x
• SBT 3.x will run only on Scala 3.0.x or Scala 3.1.x

It would be lovely to see such table/mapping in SBT docs for plugin development

[logc]

I ran into this exact issue by starting my plugin from sbt new scala/scala-seed.g8 and then following the SBT reference manual. Thanks for the solution, and the reference of SBT/Scala versions! Both should probably go into the manual

The plugin doc is confusing. It shows "bare" configurations in the complete example, and it starts by introducing Commands, then advises against using them ...

So, I created a minimal working example in this repository: https://github.com/logc/sbt-hello . Please comment there if you find that it does not follow current best practices. Otherwise, I hope it helps someone else!

[eed3si9n]

@logc I've split this into a new issue - #6131