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Scala 3 Migration Guide

Scala 2 with -Xsource:3

    Language

      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 withscalac -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 asimport 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 alsoscala -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 forcase C(xs @ _*)
      • A & B type intersection as an alias forA with B

      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>objectO{implicitvals=""}^error:Implicitdefinitionmusthaveexplicittype(inferredString)[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:Unicodeescapesinrawinterpolationsaredeprecated;useliteralcharactersinsteadvalres0:String=ascala>:setting-Xsource:3scala>raw"\u0061"^error:UnicodeescapesinrawinterpolationsareignoredinScala3(orwith-Xsource-features:unicode-escapes-raw);useliteralcharactersinsteadScala3migrationmessagesareerrorsunder-Xsource:3.Use-Wconf/@nowarntofilterthemoradd-Xmigrationtodemotethemtowarnings.Applicable-Wconf/@nowarnfiltersforthisfatalwarning:msg=<partofthemessage>,cat=scala3-migration,site=res1scala>:setting-Xsource-features:unicode-escapes-rawscala>raw"\u0061"valres1: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.

      Feature flag-Xsource:3 behavior-Xsource-features /-Xsource:3-cross behavior
      any2stringadd:(x: Any) + "" is deprecateddeprecation warningdoes not compile, implicitany2stringadd is not inferred
      unicode-escapes-raw: unicode escapes in triple-quoted strings and raw interpolations ("""\u0061""")fatal warning, escape is processedescape is not processed
      leading-infix: leading infix operators continue the previous line1fatal warning, second line is a separate expressionoperation continues the previous line
      string-context-scope: desugaring of string interpolators usingStringContextfatal warning if the interpolation references aStringContext in scope different fromscala.StringContextdesugaring always usesscala.StringContext
      package-prefix-implicits: an implicit for typep.A is found in the package prefixpfatal warningthe package prefixp is no longer part of the implicit search scope
      implicit-resolution: specificity during implicit searchfatal warninguse Scala-3-styledownwards comparisons for implicit search and overloading resolution
      case-apply-copy-access: modifiers of synthetic methodsfatal warningconstructor modifiers are used for apply / copy methods of case classes
      case-companion-function: companions are Functionsfatal warning at use sitesynthetic case companion objects no longer extend FunctionN, but are adapted at use site with warning
      infer-override: override type inferencefatal warninginferred type of member uses type of overridden member
      double-definitions: definitions differing in empty parens2fatal warningdouble definition error
      eta-expand-always:x.f eta-expands (x.f _) even without an expected typecompilation error (“missing argument list”)x.f expands to a function value

      Example 1:

      deff=1+2

      Example 2:

      classC(x:Int){defx():Int=x// allowed in Scala 2, double definition error in Scala 3}

      Changes affecting binary encoding

      As of Scala 2.13.18, 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 syntheticapply andcopy methods.
      2. case-companion-function: the synthetic companion objects of case classes no longer extendFunctionN.
      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 useMiMa to detect such changes). For example, if a library defines

      traitA{deff:Object}classBextendsA{deff="hi"}
      • enabling-Xsource-features:infer-override breaks binary compatibility on Scala 2.13: existing releases haveA.f: String, the new version will haveA.f: Object
      • adding an explicit result typeA.f: String breaks binary compatibility on Scala 3: existing releases haveA.f: Object

      It is possible to work around this using version-dependent source files, seescala/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).

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