document how local rules can be built with Scala 3

Author: bjaglin

src/sbt-test/sbt-scalafix/local-rules/build.sbt

@@ -17,19 +17,36 @@ inThisBuild(
 val rules = project
   .disablePlugins(ScalafixPlugin)
   .settings(
-    libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion,
+    libraryDependencies +=
+      ("ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion)
+        .cross(CrossVersion.for3Use2_13),
     libraryDependencies += "joda-time" % "joda-time" % "2.10.6"
   )
 
 val service = project
   .dependsOn(rules % ScalafixConfig)
   .settings(
     // SyntacticRule
-    libraryDependencies += "ch.epfl.scala" %% "example-scalafix-rule" % "3.0.0" % ScalafixConfig
+    libraryDependencies +=
+      ("ch.epfl.scala" %% "example-scalafix-rule" % "3.0.0")
+        .cross(CrossVersion.for3Use2_13) % ScalafixConfig
   )
 
 // SameProjectSyntacticRule
 val sameproject = project
   .settings(
-    libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion % ScalafixConfig
+    libraryDependencies +=
+      ("ch.epfl.scala" %% "scalafix-core" % Versions.scalafixVersion)
+        .cross(CrossVersion.for3Use2_13) % ScalafixConfig,
+    // Since sbt 1.10.x (https://github.com/sbt/sbt/pull/7480), scala3-library is not automatically added
+    // to non-standard configurations, but is needed by the Scala 3 compiler, so it must be added explicitly
+    // if no dependency brings it implicitly, which is the case here because the only dependency is for3Use2_13.
+    libraryDependencies ++= {
+      if (scalaBinaryVersion.value == "3")
+        Seq(
+          "org.scala-lang" %% "scala3-library" % scalaVersion.value % ScalafixConfig
+        )
+      else
+        Nil
+    }
   )

src/sbt-test/sbt-scalafix/local-rules/rules/src/test/scala/local/Boom.scala renamed to src/sbt-test/sbt-scalafix/local-rules/rules/src/main/scala/local/Syntactic.scala

@@ -1,15 +1,27 @@
+# try to run a failing local rule without prior explicit rule compilation
+-> service / scalafix LocalSyntacticRule
+
+# ensure updates to the rule definition is reflected in the next run
+$ copy-file rules/src/test/scala/local/NoOp.scala rules/src/main/scala/local/Syntactic.scala
 > service / scalafix LocalSyntacticRule
 
-# ensure updates to the rule definition (that we make sure compiles first) is reflected in the next run
-> rules / Test / compile
-$ copy-file rules/src/test/scala/local/Boom.scala rules/src/main/scala/local/Boom.scala
-$ delete rules/src/main/scala/local/NoOp.scala
--> service / scalafix LocalSyntacticRule
+# run a rule defined in ScalafixConfig
+> sameproject / scalafix SameProjectSyntacticRule
 
 # make sure scalafixDependencies is also honored by running a rule from a remote JAR
 > service / scalafix CollectHead
 
 # run a rule included from a remote JAR referenced via the Scalafix ivy config
 > service / scalafix SyntacticRule
 
+
+# switch all projects to Scala 3 to ...
+> set ThisBuild / scalaVersion := "3.6.4"
+
+# ... test that local rules can be written in Scala 3 ...
+> service / scalafix LocalSyntacticRule
 > sameproject / scalafix SameProjectSyntacticRule
+
+# ... while still supporting community rules written against Scala 2.13 (until https://github.com/scalacenter/scalafix/issues/2041)
+> service / scalafix CollectHead
+> service / scalafix SyntacticRule