Emit mixin forwarders for JUnit-annotated trait methods by default

JUnit 4 does not support default methods. For better user experience,
this commit makes the compiler generate mixin forwarders for inherited
trait methods that carry a JUnit annotation.

The -Yjunit-trait-methods-no-forwarders flag disables this behavior.

This supersedes the scala-js/scala-2.12-junit-mixin-plugin compiler
plugin.

2 files changed, 8 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index 7ef606b6ef..047ea3c751 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/src/compiler/scala/tools/nsc/settings/Warnings.scala
@@ -25,7 +25,7 @@ trait Warnings {
   // currently considered too noisy for general use
   val warnUnusedImport     = BooleanSetting("-Ywarn-unused-import", "Warn when imports are unused.")
 
-  val nowarnDefaultJunitMethods = BooleanSetting("-Ynowarn-default-junit-methods", "Don't warn when a JUnit @Test method is generated as a default method (not supported in JUnit 4).")
+  val junitTraitMethodsNoForwarders = BooleanSetting("-Yjunit-trait-methods-no-forwarders", "Don't generate forwarders for JUnit annotated trait methods (JUnit 4 does not support default methods).")
 
   // Experimental lint warnings that are turned off, but which could be turned on programmatically.
   // They are not activated by -Xlint and can't be enabled on the command line because they are not
diff --git a/src/compiler/scala/tools/nsc/transform/Mixin.scala b/src/compiler/scala/tools/nsc/transform/Mixin.scala
index f8e6b630d1..58f32b9aca 100644
--- a/src/compiler/scala/tools/nsc/transform/Mixin.scala
+++ b/src/compiler/scala/tools/nsc/transform/Mixin.scala
@@ -286,12 +286,16 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
                 case _ => false
               }
 
-              if (existsCompetingMethod(clazz.baseClasses))
+              def generateJUnitForwarder: Boolean = {
+                !settings.junitTraitMethodsNoForwarders &&
+                  member.annotations.nonEmpty &&
+                  JUnitAnnotations.exists(annot => annot.exists && member.hasAnnotation(annot))
+              }
+
+              if (existsCompetingMethod(clazz.baseClasses) || generateJUnitForwarder)
                 genForwarder(required = true)
               else if (settings.XgenMixinForwarders)
                 genForwarder(required = false)
-              else if (!settings.nowarnDefaultJunitMethods && JUnitTestClass.exists && member.hasAnnotation(JUnitTestClass))
-                warning(member.pos, "JUnit tests in traits that are compiled as default methods are not executed by JUnit 4. JUnit 5 will fix this issue.")
             }
 
           case _        =>