Source Compatibility

Scala 3 does NOT guarantee source compatibility between different minor language versions (e.g. some syntax valid in 3.x might get deprecated and then phased out in 3.y for y > x). There are also some syntax structures that were valid in Scala 2 but are not anymore in Scala 3. However the compiler provides a possibility to specify the desired version of syntax used in a particular file or globally for a run of the compiler to make migration between versions easier.

The default Scala language syntax version currently supported by the Dotty compiler is 3.0. There are also other language versions that can be specified instead:

  • 3.0-migration: Same as 3.0 but with a Scala 2 compatibility mode that helps moving Scala 2.13 sources over to Scala 3. In particular, it

    • flags some Scala 2 constructs that are disallowed in Scala 3 as migration warnings instead of hard errors,
    • changes some rules to be more lenient and backwards compatible with Scala 2.13
    • gives some additional warnings where the semantics has changed between Scala 2.13 and 3.0
    • in conjunction with -rewrite, offer code rewrites from Scala 2.13 to 3.0.
  • future: A preview of changes introduced in the next versions after 3.0. In the doc pages here we refer to the language version with these changes as 3.1, but it might be that some of these changes will be rolled out in later 3.x versions.

Some Scala 2 specific idioms will be dropped in this version. The feature set supported by this version will be refined over time as we approach its release.

  • future-migration: Same as future but with additional helpers to migrate from 3.0. Similarly to the helpers available under 3.0-migration, these include migration warnings and optional rewrites.

There are two ways to specify a language version :

  • with a -source command line setting, e.g. -source 3.0-migration.
  • with a scala.language import at the top of a source file, e.g:
package p
import scala.language.`future-migration`

class C { ... }

Language imports supersede command-line settings in the source files where they are specified. Only one language import specifying a source version is allowed in a source file, and it must come before any definitions in that file.

Note: The Scala 3 Migration Guide gives further information to help the Scala programmer moving from Scala 2.13 to Scala 3.