add FinalCaseClasses rule to enforce final modifier on case classes and include tests

Author: halotukozak

analyzer/src/main/scala/com/avsystem/commons/analyzer/AnalyzerPlugin.scala

@@ -61,6 +61,7 @@ final class AnalyzerPlugin(val global: Global) extends Plugin { plugin =>
     new BasePackage(global),
     new ImplicitValueClasses(global),
     new FinalValueClasses(global),
+    new FinalCaseClasses(global),
     new ImplicitParamDefaults(global),
     new CatchThrowable(global),
     new ImplicitFunctionParams(global),

analyzer/src/main/scala/com/avsystem/commons/analyzer/FinalCaseClasses.scala

@@ -0,0 +1,15 @@
+package com.avsystem.commons
+package analyzer
+
+import scala.tools.nsc.Global
+
+class FinalCaseClasses(g: Global) extends AnalyzerRule(g, "finalCaseClasses", Level.Warn) {
+
+  import global.*
+
+  def analyze(unit: CompilationUnit): Unit = unit.body.foreach {
+    case cd: ClassDef if !cd.mods.hasFlag(Flag.FINAL) && cd.mods.hasFlag(Flag.CASE) =>
+      report(cd.pos, "Case classes should be marked as final")
+    case _ =>
+  }
+}

analyzer/src/test/scala/com/avsystem/commons/analyzer/FinalCaseClassesTest.scala

@@ -0,0 +1,39 @@
+package com.avsystem.commons
+package analyzer
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class FinalCaseClassesTest extends AnyFunSuite with AnalyzerTest {
+  test("case classes should be marked as final") {
+    assertErrors(2,
+      //language=scala
+      """
+        |object whatever {
+        |  // This should pass - final case class
+        |  final case class GoodCaseClass(x: Int, y: String) {
+        |    def double: Int = x * 2
+        |  }
+        |
+        |  // This should fail - case class not marked as final
+        |  case class BadCaseClass1(x: Int, y: String) {
+        |    def double: Int = x * 2
+        |  }
+        |
+        |  // This should fail - another case class not marked as final
+        |  case class BadCaseClass2[T](x: T, y: String) {
+        |    def double: String = y * 2
+        |  }
+        |
+        |  // Regular class - should not be affected
+        |  class RegularClass(val x: Int, val y: String) {
+        |    def double: Int = x * 2
+        |  }
+        |
+        |  // Regular class with case-like constructor - should not be affected
+        |  class RegularClass2(x: Int, y: String) {
+        |    def double: Int = x * 2
+        |  }
+        |}
+      """.stripMargin)
+  }
+}