disable filters with empty method checks for interfaces

Third party plugins now support mutating annotations, so it is now
possible for mutations to be applied to interfaces. This results
in warnings in the logs from some filters that expect non
empty methods. An explicit checks for interfaces will prevent
the majority of these messages (though they may still occur
for annotations on abstract methods).

Author: hcoles

pitest-entry/src/main/java/org/pitest/mutationtest/build/intercept/equivalent/EqualsPerformanceShortcutFilter.java

@@ -61,6 +61,13 @@ public void begin(ClassTree clazz) {
   @Override
   public Collection<MutationDetails> intercept(
       Collection<MutationDetails> mutations, Mutater m) {
+
+    // skip for mutations to annotations on interfaces
+    // to avoid generating empty method warnings.
+    if (this.currentClass.isInterface()) {
+      return mutations;
+    }
+
    final List<MutationDetails> doNotTouch = mutations.stream()
            .filter(inEqualsMethod().negate())
            .collect(Collectors.toList());

pitest-entry/src/main/java/org/pitest/mutationtest/build/intercept/javafeatures/ForEachLoopFilter.java

@@ -184,6 +184,13 @@ public void begin(ClassTree clazz) {
   @Override
   public Collection<MutationDetails> intercept(
       Collection<MutationDetails> mutations, Mutater m) {
+
+    // skip for mutations to annotations on interfaces
+    // to avoid generating empty method warnings.
+    if (this.currentClass.isInterface()) {
+      return mutations;
+    }
+
     return mutations.stream().filter(mutatesIteratorLoopPlumbing().negate())
             .collect(Collectors.toList());
   }

pitest-entry/src/main/java/org/pitest/mutationtest/build/intercept/timeout/AvoidForLoopCounterFilter.java

@@ -49,7 +49,7 @@
 import org.pitest.util.Log;
 
 /**
- * Removes mutants that affect for loop counters as these have
+ * Removes mutants that effect for loop counters as these have
  * a high chance of timing out.
  */
 public class AvoidForLoopCounterFilter implements MutationInterceptor {
@@ -152,6 +152,13 @@ public void begin(ClassTree clazz) {
   @Override
   public Collection<MutationDetails> intercept(
       Collection<MutationDetails> mutations, Mutater m) {
+
+    // skip for mutations to annotations on interfaces
+    // to avoid generating empty method warnings.
+    if (this.currentClass.isInterface()) {
+      return mutations;
+    }
+
     return mutations.stream()
             .filter(mutatesAForLoopCounter().negate())
             .collect(Collectors.toList());

pitest-entry/src/main/java/org/pitest/mutationtest/build/intercept/timeout/InfiniteLoopFilter.java

@@ -43,6 +43,13 @@ public void begin(ClassTree clazz) {
   @Override
   public Collection<MutationDetails> intercept(
       Collection<MutationDetails> mutations, Mutater m) {
+
+    // skip for mutations to annotations on interfaces
+    // to avoid generating empty method warnings.
+    if (this.currentClass.isInterface()) {
+      return mutations;
+    }
+
     final Map<Location,Collection<MutationDetails>> buckets = FCollection.bucket(mutations, mutationToLocation());
 
     final List<MutationDetails> willTimeout = new ArrayList<>();