Do not generate DefaultImpls if empty

compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java

@@ -334,7 +334,7 @@ public class ClassFileFactory implements OutputFileCollection {
         public abstract String asText(ClassBuilderFactory factory);
     }
 
-    public void removeInlinedClasses(Set<String> classNamesToRemove) {
+    public void removeClasses(Set<String> classNamesToRemove) {
         for (String classInternalName : classNamesToRemove) {
             generators.remove(classInternalName + ".class");
         }

compiler/backend/src/org/jetbrains/kotlin/codegen/InterfaceImplBodyCodegen.kt

@@ -32,8 +32,12 @@ import org.jetbrains.kotlin.psi.KtClassOrObject
 import org.jetbrains.kotlin.resolve.DescriptorToSourceUtils
 import org.jetbrains.kotlin.resolve.DescriptorUtils
 import org.jetbrains.kotlin.resolve.jvm.diagnostics.DelegationToTraitImpl
+import org.jetbrains.kotlin.resolve.jvm.diagnostics.JvmDeclarationOrigin
 import org.jetbrains.kotlin.resolve.jvm.jvmSignature.JvmMethodSignature
 import org.jetbrains.kotlin.resolve.scopes.KtScope
+import org.jetbrains.org.objectweb.asm.AnnotationVisitor
+import org.jetbrains.org.objectweb.asm.FieldVisitor
+import org.jetbrains.org.objectweb.asm.MethodVisitor
 import org.jetbrains.org.objectweb.asm.Opcodes.ACC_FINAL
 import org.jetbrains.org.objectweb.asm.Opcodes.ACC_PUBLIC
 import org.jetbrains.org.objectweb.asm.Opcodes.V1_6
@@ -45,7 +49,9 @@ public class InterfaceImplBodyCodegen(
         v: ClassBuilder,
         state: GenerationState,
         parentCodegen: MemberCodegen<*>?
-) : ClassBodyCodegen(aClass, context, v, state, parentCodegen) {
+) : ClassBodyCodegen(aClass, context, InterfaceImplBodyCodegen.InterfaceImplClassBuilder(v), state, parentCodegen) {
+    private var isAnythingGenerated: Boolean = false
+        get() = (v as InterfaceImplClassBuilder).isAnythingGenerated
 
     override fun generateDeclaration() {
         v.defineClass(
@@ -57,6 +63,7 @@ public class InterfaceImplBodyCodegen(
     }
 
     override fun classForInnerClassRecord(): ClassDescriptor? {
+        if (!isAnythingGenerated) return null
         if (DescriptorUtils.isLocal(descriptor)) return null
         val classDescriptorImpl = ClassDescriptorImpl(
                 descriptor, Name.identifier(JvmAbi.DEFAULT_IMPLS_CLASS_NAME),
@@ -144,9 +151,59 @@ public class InterfaceImplBodyCodegen(
     }
 
     override fun generateKotlinAnnotation() {
+        (v as InterfaceImplClassBuilder).stopCounting()
+
         val av = v.newAnnotation(AsmUtil.asmDescByFqNameWithoutInnerClasses(KOTLIN_INTERFACE_DEFAULT_IMPLS), true)
         av.visit(JvmAnnotationNames.VERSION_FIELD_NAME, JvmAbi.VERSION.toArray())
         av.visitEnd()
         AsmUtil.writeKotlinSyntheticClassAnnotation(v, state)
     }
+
+    override fun done() {
+        super.done()
+        if (!isAnythingGenerated) {
+            state.factory.removeClasses(setOf(typeMapper.mapDefaultImpls(descriptor).internalName))
+        }
+    }
+
+    private class InterfaceImplClassBuilder(private val v: ClassBuilder) : DelegatingClassBuilder() {
+        private var shouldCount: Boolean = true
+        var isAnythingGenerated: Boolean = false
+            private set
+
+        fun stopCounting() {
+            shouldCount = false
+        }
+
+        override fun getDelegate() = v
+
+        override fun newField(
+                origin: JvmDeclarationOrigin,
+                access: Int,
+                name: String,
+                desc: String,
+                signature: String?,
+                value: Any?
+        ): FieldVisitor {
+            if (shouldCount) isAnythingGenerated = true
+            return super.newField(origin, access, name, desc, signature, value)
+        }
+
+        override fun newMethod(
+                origin: JvmDeclarationOrigin,
+                access: Int,
+                name: String,
+                desc: String,
+                signature: String?,
+                exceptions: Array<out String>?
+        ): MethodVisitor {
+            if (shouldCount) isAnythingGenerated = true
+            return super.newMethod(origin, access, name, desc, signature, exceptions)
+        }
+
+        override fun newAnnotation(desc: String, visible: Boolean): AnnotationVisitor {
+            if (shouldCount) isAnythingGenerated = true
+            return super.newAnnotation(desc, visible)
+        }
+    }
 }

compiler/backend/src/org/jetbrains/kotlin/codegen/inline/InlineCodegen.java

@@ -170,7 +170,7 @@ public class InlineCodegen extends CallGenerator {
 
         codegen.propagateChildReifiedTypeParametersUsages(result.getReifiedTypeParametersUsages());
 
-        state.getFactory().removeInlinedClasses(result.getClassesToRemove());
+        state.getFactory().removeClasses(result.getClassesToRemove());
 
         codegen.markLineNumberAfterInlineIfNeeded();
     }