Migrating from sbt 1.x

Changing build.sbt DSL to Scala 3.x

As a reminder, users can build either Scala 2.x or Scala 3.x programs using either sbt 1.x or sbt 2.x. However, the Scala that underlies the build.sbt DSL is determined by the sbt version. In sbt 2.0, we are migrating to Scala 3.x.

This means that if you implement custom tasks or sbt plugins for sbt 2.x, it must be done using Scala 3.x. See Scala 3.x incompatibility table and Scala 2 with -Xsource:3.

// This works on Scala 2.12.20 under -Xsource:3
import sbt.{ given, * }

Bare settings changes

version := "0.1.0"
scalaVersion := "3.3.3"

Bare settings, like the example above, are settings written directly in build.sbt without settings(...).

name := "root" // every subprojects will be named root!
publish / skip := true

Migrating ThisBuild

In sbt 2.x, bare settings settings should no longer be scoped to ThisBuild. One benefit of the new common settings over ThisBuild is that it would act in a more predictable delegation. These settings are inserted between plugins settings and those defined in settings(...), meaning they can be used to define settings like Compile / scalacOptions, which was not possible with ThisBuild.

Migrating to slash syntax

sbt 1.x supported both the sbt 0.13 style syntax and the slash syntax. sbt 2.x removes the support for the sbt 0.13 syntax, so use the slash syntax for both sbt shell and in build.sbt:

<project-id> / Config / intask / key

For example, test:compile will no longer work on the shell. Use Test/compile instead. See syntactic Scalafix rule for unified slash syntax for semi-automated migration of build.sbt files.

Cross building sbt plugins

In sbt 2.x, if you cross build an sbt plugin with Scala 3.x and 2.12.x, it will automatically cross build against sbt 1.x and sbt 2.x:

// using sbt 2.x
lazy val plugin = (projectMatrix in file("plugin"))
  .enablePlugins(SbtPlugin)
  .settings(
    name := "sbt-vimquit",
  )
  .jvmPlatform(scalaVersions = Seq("3.6.2", "2.12.20"))

If you use projectMatrix, make sure to move the plugin to a subdirectory like plugin/. Otherwise, the synthetic root project will also pick up the src/.

Cross building sbt plugin with sbt 1.x

Use sbt 1.10.2 or later, if you want to cross build using sbt 1.x.

// using sbt 1.x
lazy val scala212 = "2.12.20"
lazy val scala3 = "3.6.2"
ThisBuild / crossScalaVersions := Seq(scala212, scala3)

lazy val plugin = (project in file("plugin"))
  .enablePlugins(SbtPlugin)
  .settings(
    name := "sbt-vimquit",
    (pluginCrossBuild / sbtVersion) := {
      scalaBinaryVersion.value match {
        case "2.12" => "1.5.8"
        case _      => "2.0.0-M3"
      }
    },
  )

Changes to %%

In sbt 2.x, ModuleID's %% operator has become platform-aware. For JVM subprojects, %% works as before, encoding Scala suffix (for example _3) on Maven repositories.

Migrating %%% operator

When Scala.JS or Scala Native becomes available on sbt 2.x, %% will encode both the Scala version (such as _3) and the platform suffix (_sjs1 etc). As a result, %%% can be replaced with %%:

libraryDependencies += "org.scala-js" %% "scalajs-dom" % "2.8.0"

Use .platform(Platform.jvm) in case where JVM libraries are needed.

The PluginCompat technique

To use the same *.scala source but target both sbt 1.x and 2.x, we can create a shim, for example an object named PluginCompat in both src/main/scala-2.12/ and src/main/scala-3/.

Migrating Classpath type

sbt 2.x changed the Classpath type to be an alias of the Seq[Attributed[xsbti.HashedVirtualFileRef]] type. The following is a shim created to work with classpaths from both sbt 1.x and 2.x.

// src/main/scala-3/PluginCompat.scala

package sbtfoo

import java.nio.file.{ Path => NioPath }
import sbt.*
import xsbti.{ FileConverter, HashedVirtualFileRef, VirtualFile }

private[sbtfoo] object PluginCompat:
  type FileRef = HashedVirtualFileRef
  type Out = VirtualFile

  def toNioPath(a: Attributed[HashedVirtualFileRef])(using conv: FileConverter): NioPath =
    conv.toPath(a.data)
  inline def toFile(a: Attributed[HashedVirtualFileRef])(using conv: FileConverter): File =
    toNioPath(a).toFile()
  def toNioPaths(cp: Seq[Attributed[HashedVirtualFileRef]])(using conv: FileConverter): Vector[NioPath] =
    cp.map(toNioPath).toVector
  inline def toFiles(cp: Seq[Attributed[HashedVirtualFileRef]])(using conv: FileConverter): Vector[File] =
    toNioPaths(cp).map(_.toFile())
end PluginCompat

and here's for sbt 1.x:

// src/main/scala-2.12/PluginCompat.scala

package sbtfoo

private[sbtfoo] object PluginCompat {
  type FileRef = java.io.File
  type Out = java.io.File

  def toNioPath(a: Attributed[File])(implicit conv: FileConverter): NioPath =
    a.data.toPath()
  def toFile(a: Attributed[File])(implicit conv: FileConverter): File =
    a.data
  def toNioPaths(cp: Seq[Attributed[File]])(implicit conv: FileConverter): Vector[NioPath] =
    cp.map(_.data.toPath()).toVector
  def toFiles(cp: Seq[Attributed[File]])(implicit conv: FileConverter): Vector[File] =
    cp.map(_.data).toVector
}

Now we can import PluginCompat.* and use toNioPaths(...) etc to absorb the differences between sbt 1.x and 2.x. The above demonstrates how we can absorb the classpath type change, and convert it into a vector of NIO Paths.