don't infer @Nullable from source when there's a contract about it (IDEA-137323)

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/DfaUtil.java

@@ -166,7 +166,7 @@ public class DfaUtil {
 
     if (rc == RunnerResult.OK) {
       if (hasNulls.get()) {
-        return Nullness.NULLABLE;
+        return InferenceFromSourceUtil.suppressNullable(method) ? Nullness.UNKNOWN : Nullness.NULLABLE;
       }
       if (hasNotNulls.get() && !hasUnknowns.get()) {
         return Nullness.NOT_NULL;

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/InferenceFromSourceUtil.java

@@ -72,4 +72,15 @@ public class InferenceFromSourceUtil {
 
     return false;
   }
+
+  static boolean suppressNullable(PsiMethod method) {
+    if (method.getParameterList().getParametersCount() == 0) return false;
+
+    for (MethodContract contract : ControlFlowAnalyzer.getMethodContracts(method)) {
+      if (contract.returnValue == MethodContract.ValueConstraint.NULL_VALUE) {
+        return true;
+      }
+    }
+    return false;
+  }
 }

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/NullityInference.java

@@ -117,7 +117,7 @@ public class NullityInference {
       });
       
       if (hasNulls.get()) {
-        return Nullness.NULLABLE;
+        return InferenceFromSourceUtil.suppressNullable(method) ? Nullness.UNKNOWN : Nullness.NULLABLE;
       }
       
       if (hasErrors.get() || hasUnknowns.get() || delegates.size() > 1) {

java/java-tests/testSrc/com/intellij/codeInspection/NullityInferenceFromSourceTestCase.groovy

@@ -87,10 +87,29 @@ String bar() { return "z"; }
     assert inferNullity(parse('Object foo() { return () -> { return null; }; }')) == NOT_NULL
   }
 
+  void "test in presence of explicit null contract"() {
+    assert inferNullity(parse('''
+@Contract("null->null")
+Object foo(Object o) { if (o == null) return null; return 2; }
+''')) == UNKNOWN
+  }
+  void "test in presence of inferred null contract"() {
+    assert inferNullity(parse('''
+Object foo(Object o) { if (o == null) return null; return 2; }
+''')) == UNKNOWN
+  }
+
+  void "test in presence of fail contract"() {
+    assert inferNullity(parse('''
+@Contract("null->fail")
+Object foo(Object o) { if (o == null) return o.hashCode(); return 2; }
+''')) == NOT_NULL
+  }
+
   protected abstract Nullness inferNullity(PsiMethod method)
 
   protected PsiMethod parse(String method) {
-    return myFixture.addClass("final class Foo { $method }").methods[0]
+    return myFixture.addClass("import org.jetbrains.annotations.*; final class Foo { $method }").methods[0]
   }
 
   static class LightInferenceTest extends NullityInferenceFromSourceTestCase {