UnusedMethodParameter honors @unused annotation now (#440)

* Scala 2.13-only for @unused

* Fixes #340. Excusing arguments annotated with @unused

src/main/scala/com/sksamuel/scapegoat/inspections/unneccesary/UnusedMethodParameter.scala

@@ -43,6 +43,9 @@ class UnusedMethodParameter
             }
           }
 
+          private def isParameterExcused(param: ValDef): Boolean =
+            param.symbol.annotations.exists(_.atp.toString == "scala.annotation.unused")
+
           /**
            * For constructor params, some params become vals / fields of the class:
            *   1. all params in the first argument list for case classes
@@ -109,8 +112,9 @@ class UnusedMethodParameter
                 for {
                   vparams <- vparamss
                   vparam  <- vparams
-                } if (!usesParameter(vparam.name.toString, rhs))
+                } if (!isParameterExcused(vparam) && !usesParameter(vparam.name.toString, rhs)) {
                   context.warn(tree.pos, self, s"Unused method parameter ($vparam).")
+                }
               case _ => continue(tree)
             }
           }

src/test/scala-2.13/com/sksamuel/scapegoat/inspections/unnecessary/UnusedMethodParameterTest213.scala

@@ -0,0 +1,27 @@
+package com.sksamuel.scapegoat.inspections.unnecessary
+
+import com.sksamuel.scapegoat.InspectionTest
+import com.sksamuel.scapegoat.inspections.unneccesary.UnusedMethodParameter
+
+class UnusedMethodParameterTest213 extends InspectionTest {
+
+  override val inspections = Seq(new UnusedMethodParameter)
+
+  "UnusedMethodParameter in Scala 2.13" - {
+    "should not report warning" - {
+      "for unused parameters if they are annotated with @unused (#340)" in {
+        val code =
+          """| import scala.annotation.unused
+             |  class Test {
+             |   def foo(@unused a:String, b: Int): Unit = {
+             |     println(b)
+             |   }
+             | } """.stripMargin
+
+        compileCodeSnippet(code)
+        compiler.scapegoat.feedback.warnings.size shouldBe 0
+        compiler.scapegoat.feedback.warns.size shouldBe 0
+      }
+    }
+  }
+}