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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,85 @@ | ||
| --- | ||
| title: Scala 2 with -Xsource:3 | ||
| type: chapter | ||
| description: This section describes the Scala 2 compiler's -Xsource:3 flag | ||
| num: 7 | ||
| previous-page: tooling-tour | ||
| next-page: tooling-migration-mode | ||
| --- | ||
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| 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. | ||
| In addition to new warnigns, the flag enables certain benign Scala 3 syntax such as `import p.*`. | ||
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| This page explains the details behind the flag, an overview is shown using `scalac -Xsource:help`. | ||
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| ## Migration vs cross-building | ||
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| With Scala 2.13.13 and newer, the `-Xsource:3` flag comes in two variants: | ||
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| - `Xsource:3` enables warnings relevant for migrating a codebase to Scala 3. | ||
| - `Xsource:3-cross` is useful for projects that cross-build between Scala 2 and 3 for a longer period of time. | ||
| For certain language constructs that trigger a warning with `-Xsource:3`, the behavior changes to match Scala 3. | ||
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| Details about individual warnings are listed below on this page. | ||
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| ## Fatal warnings and quick fixes | ||
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| By default, Scala 3 migration warnings emitted by Scala 2.13 are fatal, i.e., they are reported as errors. | ||
| This can be changed using `-Wconf`, for example `-Wconf:cat=scala3-migration:w` changes them to be reported as warnings. | ||
| Alternatively, `-Xmigration` has the same effect. | ||
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| The [`@nowarn` annotation](https://www.scala-lang.org/api/current/scala/annotation/nowarn.html) can be used to suppress individual warnings, which also works with fatal warnings enabled. | ||
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| 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), and they can be applied directly to the source code using the `-quickfix` flag, for example `-quickfix:cat=scala3-migration`. | ||
| See also `scalac -quickfix:help`. | ||
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| ## Enabled Scala 3 syntax | ||
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| The `-Xsource:3` flag enables the following Scala 3 syntax in Scala 2: | ||
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| - `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 | ||
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| ## Scala 3 migration warnings in detail | ||
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| Many Scala 3 migration warnings are easy to understand and identical under `-Xsource:3` and `-Xsource:3-cross`, e.g., for implicit definitions without an explicit type: | ||
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| {% highlight text %} | ||
| scala> object O { implicit val s = "" } | ||
| ^ | ||
| error: Implicit definition must have explicit type (inferred String) [quickfixable] | ||
| {% endhighlight %} | ||
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| The following table explains warnings where where the behavior changes between `-Xsource:3` and `-Xsource:3-cross`. | ||
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| | Feature | `-Xsource:3` | `-Xsource:3-cross` | | ||
| |--- |--- |--- | | ||
| | `(x: Any) + ""` is deprecated | deprecation warning | does not compile, implicit `any2stringadd` is not inferred | | ||
| | Unicode escapes in triple-quoted strings and raw interpolations (`"""\u0061"""`) | fatal warning, escape is processed | escape is not processed | | ||
| | Leading infix operators continue the previous line <sup>1</sup> | fatal warning, second line is a separate expression | operation continues the previous line | | ||
| | Desugaring of string interpolators using `StringContext` | fatal warning if the interpolation references a `StringContext` in scope different from `scala.StringContext` | desugaring always uses `scala.StringContext` | | ||
| | Modifiers of `apply` and `copy` methods of case classes with non-public constructors: `case class C private[p] (x: Int)` | fatal warning about the generated `apply` and `copy` being public | `apply` and `copy` inherit the access modifiers from the constructor | | ||
| | Inferred type of an overriding member <sup>2</sup> | fatal warning, the type of `B.f` is `String` | the type of `B.f` is `Object` | | ||
| | An implicit for type `p.A` is found in the package prefix `p` | fatal warning | the package prefix `p` is no longer part of the implicit search scope | | ||
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| Example 1: | ||
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| {% highlight text %} | ||
| def f = | ||
| 1 | ||
| + 2 | ||
| {% endhighlight %} | ||
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| Example 2: | ||
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| {% highlight text %} | ||
| trait A { def f: Object } | ||
| class B extends A { def f = "hi" } | ||
| {% endhighlight %} |
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