Scala 2 with -Xsource:3

The Scala 2.13 compiler issues helpful migration warnings with the -Xsource:3 flag.

Before moving to the Scala 3 compiler, it’s recommended to enable this flag in Scala 2 and address the new warnings.

Usage information is shown with scalac -Xsource:help.

Migration vs cross-building

With Scala 2.13.14 and newer, the -Xsource:3 flag supports two scenarios:

• Xsource:3 enables warnings relevant for migrating a codebase to Scala 3. In addition to new warnings, the flag enables certain benign Scala 3 syntaxes such as import p.*.
• Adding the -Xsource-features:<features> flag is useful to reduce the maintenance burden of projects that cross-build between Scala 2 and 3. Certain language constructs have been backported from Scala 3 in order to improve compatibility. Instead of warning about a behavior change in Scala 3, it adopts the new behavior.

Warnings as errors, and quick fixes

By default, Scala 3 migration warnings emitted by Scala 2.13 are reported as errors, using the default configuration, -Wconf:cat=scala3-migration:e. This ensures that migration messaging is more visible. Diagnostics can be emitted as warnings by specifying -Wconf:cat=scala3-migration:w. Typically, emitting warnings instead of errors will cause more diagnostics to be reported.

The @nowarn annotation can be used in program sources to suppress individual warnings. Diagnostics are suppressed before they are promoted to errors, so that @nowarn takes precedence over -Wconf and -Werror.

The Scala 2.13 compiler implements quick fixes for many Scala 3 migration warnings. Quick fixes are displayed in Metals-based IDEs (not yet in IntelliJ), or they can be applied directly to the source code using the -quickfix flag, for example -quickfix:cat=scala3-migration. See also scala -quickfix:help.

Enabled Scala 3 syntax

The -Xsource:3 flag enables the following Scala 3 syntaxes in Scala 2:

• import p.*
• import p.m as n
• import p.{given, *}
• case C(xs*) as an alias for case C(xs @ _*)
• A & B type intersection as an alias for A with B
• Selecting a method x.f performs an eta-expansion (x.f _), even without an expected type

Scala 3 migration warnings in detail

Many Scala 3 migration warnings are easy to understand, e.g., for implicit definitions without an explicit type:

scala> object O { implicit val s = "" }
                               ^
       error: Implicit definition must have explicit type (inferred String) [quickfixable]

Enabling Scala 3 features with -Xsource-features

Certain Scala 3 language changes have been backported and can be enabled using -Xsource-features; usage and available features are shown with -Xsource-features:help.

When enabling a feature, the corresponding migration warning is no longer issued.

scala> raw"\u0061"
           ^
       warning: Unicode escapes in raw interpolations are deprecated; use literal characters instead
val res0: String = a

scala> :setting -Xsource:3

scala> raw"\u0061"
           ^
       error: Unicode escapes in raw interpolations are ignored in Scala 3 (or with -Xsource-features:unicode-escapes-raw); use literal characters instead
       Scala 3 migration messages are errors under -Xsource:3. Use -Wconf / @nowarn to filter them or add -Xmigration to demote them to warnings.
       Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=scala3-migration, site=res1

scala> :setting -Xsource-features:unicode-escapes-raw

scala> raw"\u0061"
val res1: String = \u0061

For every such language feature, a migration warning is issued under plain -Xsource:3. Enabling the feature silences the warning and adopts the changed behavior. To avoid silent language changes when upgrading to a new Scala 2.13 version, it is recommended to enable features explicitly or use a group (e.g., -Xsource-features:v2.13.14).

-Xsource:3-cross is a shorthand for -Xsource:3 -Xsource-features:_.

Changes in language semantics

The following table shows backported Scala 3 language semantics available in -Xsource-features / -Xsource:3-cross.

Example 1:

  def f =
    1
    + 2

Example 2:

class C(x: Int) {
  def x(): Int = x // allowed in Scala 2, double definition error in Scala 3
}

Changes affecting binary encoding

As of Scala 2.13.15, there are 3 changes in -Xsource-features that affect binary encoding of classfiles:

1. case-apply-copy-access: the constructor modifiers of case classes (case class C private[p] (x: Int)) are copied to the synthetic apply and copy methods.
2. case-companion-function: the synthetic companion objects of case classes no longer extend FunctionN.
3. infer-override: overriding methods without an explicit return type inherit the return type from the parent (instead of using the inferred type of the method body).

For projects that are already cross-building between Scala 2 and 3 with existing releases for both, enabling these changes breaks binary compatibility (make sure to use MiMa to detect such changes). For example, if a library defines

trait A { def f: Object }
class B extends A { def f = "hi" }

• enabling -Xsource-features:infer-override breaks binary compatibility on Scala 2.13: existing releases have A.f: String, the new version will have A.f: Object
• adding an explicit result type A.f: String breaks binary compatibility on Scala 3: existing releases have A.f: Object

It is possible to work around this using version-dependent source files, see scala/scala-xml#675 as an example.

Instead of implementing such workarounds, it might be easier not to enable changes affecting binary encoding (-Xsource-features:v2.13.14,-case-apply-copy-access,-case-companion-function,-infer-override).