From dcee4fbc139763df06930591cc9a5dcc2025075f Mon Sep 17 00:00:00 2001
From: Tagir Valeev <tagir.valeev@jetbrains.com>
Date: Mon, 21 Jul 2025 12:31:17 +0200
Subject: [PATCH] [java-inspections] IDEA-376166 return null inspection should
 consider contract as well as `@Nullable`

GitOrigin-RevId: 2fb103a97887bd068454702b520f86e83aa38a36
---
 .../src/com/siyeh/ig/bugs/ReturnNullInspection.java  | 11 +++++++++--
 .../siyeh/igtest/bugs/return_null/ReturnNull.java    | 12 ++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/java/java-impl/src/com/siyeh/ig/bugs/ReturnNullInspection.java b/java/java-impl/src/com/siyeh/ig/bugs/ReturnNullInspection.java
index f47025c3ab5a..5bad38f68721 100644
--- a/java/java-impl/src/com/siyeh/ig/bugs/ReturnNullInspection.java
+++ b/java/java-impl/src/com/siyeh/ig/bugs/ReturnNullInspection.java
@@ -10,6 +10,8 @@ import com.intellij.codeInsight.options.JavaInspectionControls;
 import com.intellij.codeInspection.CommonQuickFixBundle;
 import com.intellij.codeInspection.LocalQuickFix;
 import com.intellij.codeInspection.dataFlow.DfaPsiUtil;
+import com.intellij.codeInspection.dataFlow.JavaMethodContractUtil;
+import com.intellij.codeInspection.dataFlow.StandardMethodContract;
 import com.intellij.codeInspection.options.OptPane;
 import com.intellij.codeInspection.util.OptionalUtil;
 import com.intellij.java.syntax.parser.JavaKeywords;
@@ -19,6 +21,7 @@ import com.intellij.openapi.project.Project;
 import com.intellij.psi.*;
 import com.intellij.psi.controlFlow.DefUseUtil;
 import com.intellij.psi.util.PsiTreeUtil;
+import com.intellij.util.containers.ContainerUtil;
 import com.siyeh.InspectionGadgetsBundle;
 import com.siyeh.ig.BaseInspection;
 import com.siyeh.ig.BaseInspectionVisitor;
@@ -33,7 +36,7 @@ import org.jetbrains.annotations.NonNls;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
-import java.util.Arrays;
+import java.util.List;
 
 import static com.intellij.codeInspection.options.OptPane.checkbox;
 import static com.intellij.codeInspection.options.OptPane.pane;
@@ -187,6 +190,10 @@ public final class ReturnNullInspection extends BaseInspection {
       if (DfaPsiUtil.getTypeNullability(returnType) == Nullability.NULLABLE) {
         return;
       }
+      if (!lambda && JavaMethodContractUtil.hasExplicitContractAnnotation(method)) {
+        List<StandardMethodContract> contracts = JavaMethodContractUtil.getMethodContracts(method);
+        if (ContainerUtil.exists(contracts, c -> c.getReturnValue().isNull())) return;
+      }
 
       if (CollectionUtils.isCollectionClassOrInterface(returnType)) {
         if (m_reportCollectionMethods) {
@@ -213,7 +220,7 @@ public final class ReturnNullInspection extends BaseInspection {
           return false;
         }
         final PsiElement[] refs = DefUseUtil.getRefs(codeBlock, variable, element);
-        return Arrays.stream(refs).anyMatch(this::isInNullableContext);
+        return ContainerUtil.exists(refs, this::isInNullableContext);
       }
       else if (parent instanceof PsiExpressionList) {
         final PsiElement grandParent = parent.getParent();
diff --git a/java/java-tests/testData/ig/com/siyeh/igtest/bugs/return_null/ReturnNull.java b/java/java-tests/testData/ig/com/siyeh/igtest/bugs/return_null/ReturnNull.java
index bf8334343290..dce705d9503b 100644
--- a/java/java-tests/testData/ig/com/siyeh/igtest/bugs/return_null/ReturnNull.java
+++ b/java/java-tests/testData/ig/com/siyeh/igtest/bugs/return_null/ReturnNull.java
@@ -55,4 +55,16 @@ class Test {
     void foo() {
         Function<String, @Nullable String> f = s  -> null;
     }
+}
+class WithContract {
+  @org.jetbrains.annotations.Contract("!null -> !null; null -> null")
+  public static String maybeTrim(String s) {
+    if (s == null) return null;
+    return s.trim();
+  }
+
+  public static String maybeTrimNoContract(String s) {
+    if (s == null) return <warning descr="Return of 'null'">null</warning>;
+    return s.trim();
+  }
 }
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