Ban using reference equality for MemorySegments.

See openjdk/jdk#30501

PiperOrigin-RevId: 891791638

Author: kluever

core/src/main/java/com/google/errorprone/bugpatterns/MemorySegmentReferenceEquality.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2026 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugPattern.SeverityLevel.ERROR;
+import static com.google.errorprone.util.ASTHelpers.getType;
+import static com.google.errorprone.util.ASTHelpers.isSubtype;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.suppliers.Supplier;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.tools.javac.code.Type;
+
+/** A {@link BugChecker}; see the associated {@link BugPattern} annotation for details. */
+@BugPattern(summary = "Do not compare MemorySegments using reference equality.", severity = ERROR)
+public final class MemorySegmentReferenceEquality extends AbstractReferenceEquality {
+
+  private static final Supplier<Type> MEMORY_SEGMENT_TYPE =
+      VisitorState.memoize(state -> state.getTypeFromString("java.lang.foreign.MemorySegment"));
+
+  @Override
+  protected boolean matchArgument(ExpressionTree tree, VisitorState state) {
+    Type type = getType(tree);
+    Type memorySegmentType = MEMORY_SEGMENT_TYPE.get(state);
+    return (type == null || memorySegmentType == null)
+        ? false
+        : isSubtype(type, memorySegmentType, state);
+  }
+}

core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java

@@ -243,6 +243,7 @@
 import com.google.errorprone.bugpatterns.MathAbsoluteNegative;
 import com.google.errorprone.bugpatterns.MathRoundIntLong;
 import com.google.errorprone.bugpatterns.MemoizeConstantVisitorStateLookups;
+import com.google.errorprone.bugpatterns.MemorySegmentReferenceEquality;
 import com.google.errorprone.bugpatterns.MethodCanBeStatic;
 import com.google.errorprone.bugpatterns.MisformattedTestData;
 import com.google.errorprone.bugpatterns.MisleadingEmptyVarargs;
@@ -808,6 +809,7 @@ public static ScannerSupplier warningChecks() {
           LoopConditionChecker.class,
           LossyPrimitiveCompare.class,
           MathRoundIntLong.class,
+          MemorySegmentReferenceEquality.class,
           MislabeledAndroidString.class,
           MisleadingEmptyVarargs.class,
           MisleadingEscapedSpace.class,

core/src/test/java/com/google/errorprone/bugpatterns/MemorySegmentReferenceEqualityTest.java

@@ -0,0 +1,218 @@
+/*
+ * Copyright 2026 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.bugpatterns;
+
+import static com.google.common.truth.TruthJUnit.assume;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/**
+ * @author kak@google.com (Kurt Alfred Kluever)
+ */
+@RunWith(JUnit4.class)
+public class MemorySegmentReferenceEqualityTest {
+
+  private final BugCheckerRefactoringTestHelper refactoringHelper =
+      BugCheckerRefactoringTestHelper.newInstance(MemorySegmentReferenceEquality.class, getClass());
+
+  @Before
+  public void setUp() {
+    assume().that(Runtime.version().feature()).isAtLeast(22);
+  }
+
+  @Test
+  public void positiveCase_equal() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+
+            class Test {
+              boolean f(MemorySegment a, MemorySegment b) {
+                return a == b;
+              }
+            }
+            """)
+        .addOutputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+            import java.util.Objects;
+
+            class Test {
+              boolean f(MemorySegment a, MemorySegment b) {
+                return Objects.equals(a, b);
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void positiveCase_notEqual() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+
+            class Test {
+              boolean f(MemorySegment a, MemorySegment b) {
+                return a != b;
+              }
+            }
+            """)
+        .addOutputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+            import java.util.Objects;
+
+            class Test {
+              boolean f(MemorySegment a, MemorySegment b) {
+                return !Objects.equals(a, b);
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void negativeCase_null() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+
+            class Test {
+              boolean f(MemorySegment b) {
+                return b == null;
+              }
+
+              boolean g(MemorySegment b) {
+                return b != null;
+              }
+
+              boolean h(MemorySegment b) {
+                return null == b;
+              }
+
+              boolean i(MemorySegment b) {
+                return null != b;
+              }
+            }
+            """)
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void compareToNullConstant() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+
+            class Test {
+              boolean f(MemorySegment a) {
+                return a == MemorySegment.NULL;
+              }
+
+              boolean g(MemorySegment a) {
+                return a != MemorySegment.NULL;
+              }
+
+              boolean h(MemorySegment a) {
+                return MemorySegment.NULL == a;
+              }
+
+              boolean i(MemorySegment a) {
+                return MemorySegment.NULL != a;
+              }
+            }
+            """)
+        .addOutputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+            import java.util.Objects;
+
+            class Test {
+              boolean f(MemorySegment a) {
+                return Objects.equals(a, MemorySegment.NULL);
+              }
+
+              boolean g(MemorySegment a) {
+                return !Objects.equals(a, MemorySegment.NULL);
+              }
+
+              boolean h(MemorySegment a) {
+                return Objects.equals(MemorySegment.NULL, a);
+              }
+
+              boolean i(MemorySegment a) {
+                return !Objects.equals(MemorySegment.NULL, a);
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void compareToNullConstant_notEqual() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+
+            class Test {
+              boolean f(MemorySegment a) {
+                return a != MemorySegment.NULL;
+              }
+
+              boolean g(MemorySegment a) {
+                return MemorySegment.NULL != a;
+              }
+            }
+            """)
+        .addOutputLines(
+            "Test.java",
+            """
+            import java.lang.foreign.MemorySegment;
+            import java.util.Objects;
+
+            class Test {
+              boolean f(MemorySegment a) {
+                return !Objects.equals(a, MemorySegment.NULL);
+              }
+
+              boolean g(MemorySegment a) {
+                return !Objects.equals(MemorySegment.NULL, a);
+              }
+            }
+            """)
+        .doTest();
+  }
+}

docs/bugpattern/MemorySegmentReferenceEquality.md

@@ -0,0 +1,27 @@
+`MemorySegment` is a value-based class. Comparing `MemorySegment` instances
+using `==` or `!=` is bug-prone because `MemorySegment` implementations may not
+be unique for the same underlying memory. Use `Objects.equals()` (or `.equals()`
+if the receiver is known to be non-null) instead.
+
+For example:
+
+```java
+MemorySegment seg = ...;
+if (seg == MemorySegment.NULL) { // reference equality
+  ...
+}
+```
+
+should be:
+
+```java
+MemorySegment seg = ...;
+if (Objects.equals(seg, MemorySegment.NULL)) { // value equality
+  ...
+}
+```
+
+Reference equality between any two `MemorySegment` instances (e.g., `a == b`) is
+flagged by this check, and `Objects.equals(a, b)` is the preferred alternative.
+
+See also https://bugs.openjdk.org/browse/JDK-8381012