on version mismatch, protect PC dynamic loading behind a flag

project/ScalafixBuild.scala

@@ -58,7 +58,10 @@ object ScalafixBuild extends AutoPlugin with GhpagesKeys {
           val xsource3 = TargetAxis(sv, xsource3 = true)
 
           (prevVersions :+ xsource3).map((sv, _))
-        } :+ (scala3Next, TargetAxis(scala213))
+        } ++ Seq(
+          (scala3Next, TargetAxis(scala213)),
+          (scala3Next, TargetAxis(scala3LTS))
+        )
 
     lazy val publishLocalTransitive =
       taskKey[Unit]("Run publishLocal on this project and its dependencies")

scalafix-rules/src/main/scala-3/scalafix/internal/rule/ExplicitResultTypes.scala

@@ -40,25 +40,36 @@ final class ExplicitResultTypes(
           "To fix this problem, either remove ExplicitResultTypes from .scalafix.conf or upgrade the compiler in your build."
       )
     } else {
-      config.conf // Support deprecated explicitReturnTypes config
-        .getOrElse("explicitReturnTypes", "ExplicitResultTypes")(
-          ExplicitResultTypesConfig.default
-        )
-        .map(c =>
-          new ExplicitResultTypes(
-            c,
-            Option {
-              if (
-                stripPatchVersion(config.scalaVersion) ==
-                  stripPatchVersion(compilerScalaVersion)
-              )
-                PresentationCompilerTypeInferrer
-                  .static(config, new ScalaPresentationCompiler())
+      config.conf.getOrElse("ExplicitResultTypes")(this.config).andThen {
+        conf =>
+          val majorMinorCompilerScalaVersion =
+            stripPatchVersion(compilerScalaVersion)
+
+          val matchingMinors =
+            majorMinorScalaVersion == majorMinorCompilerScalaVersion
+
+          if (
+            !matchingMinors && !conf.fetchScala3CompilerArtifactsOnVersionMismatch
+          ) {
+            Configured.error(
+              s"The ExplicitResultTypes rule was compiled with a different Scala 3 minor ($majorMinorCompilerScalaVersion) " +
+                s"than the target sources ($majorMinorScalaVersion). To fix this problem, make sure you are running the latest version of Scalafix. " +
+                "If that is the case, either change your build to stick to the Scala 3 LTS or Next versions supported by Scalafix, or " +
+                "enable ExplicitResultTypes.fetchScala3CompilerArtifactsOnVersionMismatch in .scalafix.conf in order to try to load what is needed dynamically."
+            )
+          } else {
+            val pcTypeInferrer =
+              if (matchingMinors)
+                PresentationCompilerTypeInferrer.static(
+                  config,
+                  new ScalaPresentationCompiler()
+                )
               else
                 PresentationCompilerTypeInferrer.dynamic(config)
-            }
-          )
-        )
+
+            Configured.Ok(new ExplicitResultTypes(conf, Some(pcTypeInferrer)))
+          }
+      }
     }
   }
 

scalafix-rules/src/main/scala/scalafix/internal/rule/ExplicitResultTypesConfig.scala

@@ -39,6 +39,12 @@ case class ExplicitResultTypesConfig(
     rewriteStructuralTypesToNamedSubclass: Boolean = true,
     @Description("If true, adds result types only to implicit definitions.")
     onlyImplicits: Boolean = false,
+    @Description(
+      "If true and the Scala 3 version Scalafix was compiled with differs from the target files' one," +
+        "attempts to resolve and download compiler artifacts dynamically via Coursier. " +
+        "Disabled by default as this introduces a performance overhead and might not work."
+    )
+    fetchScala3CompilerArtifactsOnVersionMismatch: Boolean = false,
     @Hidden()
     symbolReplacements: Map[String, String] = Map.empty
 )

scalafix-tests/expect/src/test/scala/scalafix/tests/rule/RuleSuite.scala

@@ -39,7 +39,9 @@ class RuleSuite extends AbstractSemanticRuleSuite with AnyFunSuiteLike {
     val versionMismatch =
       stripPatch(RulesBuildInfo.scalaVersion) != stripPatch(props.scalaVersion)
     val explicitResultTypesTest =
-      diffTest.path.input.toNIO.toString.contains("explicitResultTypes")
+      diffTest.path.input.toNIO.toString.contains(
+        "explicitResultTypes" + java.io.File.separator // don't skip tests with a suffix
+      )
 
     // ExplicitResultTypes can only run against sources compiled with the sam
     // binary version as the one used to compile the rule

scalafix-tests/input/src/main/scala-3/test/explicitResultTypesRunAcrossMinors/FetchScala3CompilerArtifactsOnVersionMismatch.scala

@@ -0,0 +1,15 @@
+/*
+rules = ExplicitResultTypes
+ExplicitResultTypes.skipSimpleDefinitions = true
+ExplicitResultTypes.fetchScala3CompilerArtifactsOnVersionMismatch = true
+
+// RuleSuite.scala does run this test for expect3_xTarget3_y with x != y
+// because it is in a different folder than "explicitResultTypes" - the suffix
+// matters. Take that into account if you move this file.
+
+*/
+package test.explicitResultTypesRunAcrossMinors
+
+object FetchScala3CompilerArtifactsOnVersionMismatch {
+  def foo = 1
+}

scalafix-tests/integration/src/test/scala-3/scalafix/tests/rules/ExplicitResultTypesSuite.scala

@@ -2,7 +2,9 @@ package scalafix.tests.rules
 
 import scala.meta.io.AbsolutePath
 
+import metaconfig.Conf
 import metaconfig.Configured
+import metaconfig.typesafeconfig.*
 import org.scalatest.funsuite.AnyFunSuite
 import scalafix.internal.rule.ExplicitResultTypes
 import scalafix.v1.Configuration
@@ -34,4 +36,36 @@ class ExplicitResultTypesSuite extends AnyFunSuite {
     assert(rule.withConfiguration(earlyScala3) == Configured.error(expected))
   }
 
+  test("Other Scala 3 minor versions are not supported by default") {
+    val neitherLTSNorNext =
+      config.withScalaVersion("3.4.0")
+
+    val v =
+      buildinfo.RulesBuildInfo.scalaVersion.split('.').take(2).mkString(".")
+
+    val expected =
+      s"The ExplicitResultTypes rule was compiled with a different Scala 3 minor ($v) than the target sources (3.4). " +
+        "To fix this problem, make sure you are running the latest version of Scalafix. " +
+        "If that is the case, either change your build to stick to the Scala 3 LTS or Next versions supported by Scalafix, or " +
+        "enable ExplicitResultTypes.fetchScala3CompilerArtifactsOnVersionMismatch in .scalafix.conf in order to try to load what is needed dynamically."
+
+    assert(
+      rule.withConfiguration(neitherLTSNorNext) == Configured.error(expected)
+    )
+  }
+
+  test(
+    "fetchScala3CompilerArtifactsOnVersionMismatch unlocks dynamic loading"
+  ) {
+    val neitherLTSNorNext =
+      config.withScalaVersion("3.5.0")
+
+    val conf = Conf
+      .parseString(
+        "ExplicitResultTypes.fetchScala3CompilerArtifactsOnVersionMismatch = true"
+      )
+      .get
+    assert(rule.withConfiguration(neitherLTSNorNext.withConf(conf)).isOk)
+  }
+
 }

scalafix-tests/output/src/main/scala-3/test/explicitResultTypesRunAcrossMinors/FetchScala3CompilerArtifactsOnVersionMismatch.scala

@@ -0,0 +1,5 @@
+package test.explicitResultTypesRunAcrossMinors
+
+object FetchScala3CompilerArtifactsOnVersionMismatch {
+  def foo: Int = 1
+}