report 'trait' keyword as deprecated, provide quickfix for replacing with 'interface'

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -142,6 +142,7 @@ public interface Errors {
 
     DiagnosticFactory0<JetDelegatorToSuperClass> SUPERTYPE_NOT_INITIALIZED = DiagnosticFactory0.create(ERROR);
 
+    DiagnosticFactory0<PsiElement> DEPRECATED_TRAIT_KEYWORD = DiagnosticFactory0.create(WARNING);
     DiagnosticFactory0<JetTypeReference> DELEGATION_NOT_TO_TRAIT = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<JetTypeReference> SUPERTYPE_NOT_A_CLASS_OR_TRAIT = DiagnosticFactory0.create(ERROR);
 

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -262,6 +262,7 @@ public class DefaultErrorMessages {
         MAP.put(ENUM_ENTRY_AFTER_ENUM_MEMBER, "Enum entry ''{0}'' is not allowed after a member", NAME);
         MAP.put(DELEGATION_IN_TRAIT, "Interfaces cannot use delegation");
         MAP.put(DELEGATION_NOT_TO_TRAIT, "Only interfaces can be delegated to");
+        MAP.put(DEPRECATED_TRAIT_KEYWORD, "'trait' keyword is deprecated, use 'interface' instead");
         MAP.put(UNMET_TRAIT_REQUIREMENT, "Super interface ''{0}'' requires subclasses to extend ''{1}''", NAME, NAME);
         MAP.put(NO_CONSTRUCTOR, "This class does not have a constructor");
         MAP.put(NOT_A_CLASS, "Not a class");

compiler/frontend/src/org/jetbrains/kotlin/resolve/DeclarationsChecker.java

@@ -247,6 +247,11 @@ public class DeclarationsChecker {
         checkTypeParameters(aClass);
 
         if (aClass.isInterface()) {
+            ASTNode traitKeyword = aClass.getNode().findChildByType(JetTokens.TRAIT_KEYWORD);
+            if (traitKeyword != null) {
+                trace.report(Errors.DEPRECATED_TRAIT_KEYWORD.on(traitKeyword.getPsi()));
+            }
+
             checkTraitModifiers(aClass);
             checkConstructorInTrait(aClass);
         }

compiler/testData/diagnostics/tests/DeprecatedTraitKeyword.kt

@@ -0,0 +1,2 @@
+<!DEPRECATED_TRAIT_KEYWORD!>trait<!> A {
+}

compiler/testData/diagnostics/tests/DeprecatedTraitKeyword.txt

@@ -0,0 +1,7 @@
+package
+
+internal interface A {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java

@@ -181,6 +181,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
             doTest(fileName);
         }
 
+        @TestMetadata("DeprecatedTraitKeyword.kt")
+        public void testDeprecatedTraitKeyword() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/DeprecatedTraitKeyword.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("DiamondFunction.kt")
         public void testDiamondFunction() throws Exception {
             String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/DiamondFunction.kt");