Diagnostics for impossible/useless casts

idea/src/org/jetbrains/jet/lang/annotations/LabelsAnnotator.java

@@ -17,7 +17,7 @@ import org.jetbrains.jet.lexer.JetTokens;
 
 public class LabelsAnnotator implements Annotator {
     public void annotate(@NotNull PsiElement element, @NotNull final AnnotationHolder holder) {
-        if (ApplicationManager.getApplication().isUnitTestMode()) return;
+//        if (ApplicationManager.getApplication().isUnitTestMode()) return;
         element.accept(new JetVisitor() {
             @Override
             public void visitPrefixExpression(JetPrefixExpression expression) {

idea/src/org/jetbrains/jet/lang/types/JetTypeInferrer.java

@@ -590,12 +590,11 @@ public class JetTypeInferrer {
                     result = targetType;
                 }
                 else if (operationType == JetTokens.AS_KEYWORD) {
-                    // TODO : Check for cast impossibility
+                    checkForCastImpossibility(expression, actualType, targetType);
                     result = targetType;
                 }
                 else if (operationType == JetTokens.AS_SAFE) {
-                    // TODO : Check for cast impossibility
+                    checkForCastImpossibility(expression, actualType, targetType);
-
                     result = TypeUtils.makeNullable(targetType);
                 }
                 else {
@@ -604,6 +603,25 @@ public class JetTypeInferrer {
             }
         }
 
+        private void checkForCastImpossibility(JetBinaryExpressionWithTypeRHS expression, JetType actualType, JetType targetType) {
+            if (actualType == null) return;
+
+            JetTypeChecker typeChecker = semanticServices.getTypeChecker();
+            if (!typeChecker.isSubtypeOf(targetType, actualType)) {
+                if (typeChecker.isSubtypeOf(actualType, targetType)) {
+                    semanticServices.getErrorHandler().genericWarning(expression.getOperationSign().getNode(), "No cast needed, use ':' instead");
+                }
+                else {
+                    semanticServices.getErrorHandler().genericError(expression.getOperationSign().getNode(), "This cast can never succeed");
+                }
+            }
+            else {
+                if (typeChecker.isSubtypeOf(actualType, targetType)) {
+                    semanticServices.getErrorHandler().genericWarning(expression.getOperationSign().getNode(), "No cast needed");
+                }
+            }
+        }
+
         @Override
         public void visitTupleExpression(JetTupleExpression expression) {
             List<JetExpression> entries = expression.getEntries();

idea/testData/checker/Casts.jet

@@ -5,12 +5,12 @@ fun test() : Unit {
   x : Int?
   y : Int
   x as Int : Int
-  y as Int : Int
+  y <warning>as</warning> Int : Int
-  x as Int? : Int?
+  x <warning>as</warning> Int? : Int?
-  y as Int? : Int?
+  y <warning>as</warning> Int? : Int?
   x as? Int : Int?
-  y as? Int : Int?
+  y <warning>as?</warning> Int : Int?
-  x as? Int? : Int?
+  x <warning>as?</warning> Int? : Int?
-  y as? Int? : Int?
+  y <warning>as?</warning> Int? : Int?
   ()
 }

idea/tests/org/jetbrains/jet/JetTestCaseBase.java

@@ -46,7 +46,7 @@ public abstract class JetTestCaseBase extends LightDaemonAnalyzerTestCase {
 
     @Override
     protected void runTest() throws Throwable {
-        doTest(getTestFilePath(), false, false);
+        doTest(getTestFilePath(), true, false);
     }
 
     @NotNull