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Scala 3 Migration Guide

Prerequisites

Language

The migration to Scala 3 is made easier thanks to the interoperability between Scala 2.13 and Scala 3, as described in the Compatibility Reference page.

However, there are a few prerequisites that a Scala 2.13 project must meet before being ported to Scala 3:

  • It must not depend on a macro library that has not yet been ported to Scala 3.
  • It must not use a compiler plugin that has no equivalent in Scala 3.
  • It must not depend on scala-reflect.

The following paragraphs explain how to check those prerequisites and, in case they are unmet, what you can do about it.

If you are ready to proceed with the migration you can jump straight to the sbt Migration Tutorial.

Macro dependencies

A macro library is a Scala library that exposes a macro method.

Those libraries tend to be more expressive and as such they are widely used in Scala 2. We can mention as examples:

But the Scala 3 compiler cannot expand Scala 2.13 macros. So, before jumping to Scala 3, you should make sure that your project does not depend on a macro library that has not yet been ported.

You can find the migration status of many macro libraries in the Scala Macro Libraries page. Hopefully many will already be ported by the time you read these lines.

For each of these macro dependencies in your project, you need to upgrade it to a cross-built version—a version available on both Scala 2.13 and Scala 3.

Let’s take a quick example.

The dependency to "scalatest" %% "scalatest" % "3.0.9" must be upgraded because:

  • The scalatest API is based on some macro definitions.
  • The 3.0.9 version is not published for Scala 3.

We can upgrade it to version 3.2.7, which is cross-published in Scala 2.13 and Scala 3.

libraryDependencies += "org.scalatest" %% "scalatest" % "3.2.7"

Compiler plugins

The Scala 2 compiler plugins are not compatible with Scala 3.

Compiler plugins are generally configured in the build.sbt file by one of these settings:

// build.sbt
libraryDependencies +=
  compilerPlugin("org.typelevel" %% "kind-projector" % "0.11.0" cross CrossVersion.full)

addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.11.0" cross CrossVersion.full)

Some compiler plugins may also be automatically added by an sbt plugin.

You can find all configured compiler plugins by looking at the compiler options of your project.

sbt:example> show example / Compile / scalacOptions
[info] * -Xplugin:target/compiler_plugins/wartremover_2.13.6-2.4.15.jar
[info] * -Xplugin:target/compiler_plugins/semanticdb-scalac_2.13.6-4.4.18.jar
[info] * -Yrangepos
[info] * -P:semanticdb:targetroot:/example/target/scala-2.13/meta

In the above example we can see that two compiler plugins are used: wartremover and semanticdb. For each of these plugins, we need to check that there is an alternative solution, or we need to disable it.

Alternative solutions to the most used compiler plugins are given below.

SemanticDB

The support of SemanticDB is now shipped into the Scala 3 compiler:

  • The -Ysemanticdb option activates the generation of semanticDB files.
  • The -semanticdb-target option can be used to specify the output directory of semanticDB files.

sbt is able to configure SemanticDB automatically with this single setting: semanticdbEnabled := true.

Scala.js

The Scala.js compilation on Scala 3 does not rely on a compiler plugin anymore.

To compile your Scala.js project you can use the sbt-scalajs plugin version 1.5.0 or higher.

// project/plugins.sbt
addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.5.0")

Scala Native

Scala 3 does not yet support Scala Native.

If you project is cross-built to Scala Native, you can port it to Scala 3. But you will not be able to compile for the Native platform.

Kind Projector

A subset of the Kind Projector syntax is supported by Scala 3 under the -Ykind-projector option.

AdditionalLy, we now have the following features that make kind-projector not needed in many cases:

You can learn more about the Kind Projector migration in its dedicated page.

Runtime reflection

scala-reflect will not be ported to Scala 3 because it exposes Scala 2 compiler internals that do not exist in Scala 3.

If your project depends on scala-reflect, or consumes instances of the Manifest class, it cannot be compiled by the Scala 3 compiler. To remedy this situation, you can try to re-implement the corresponding parts of the code, using Java reflection or the Scala 3 metaprogramming features.

If scala-reflect is transitively added in your classpath, you probably need to upgrade the dependency that brings it.

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