google · copybara-service · Jul 1, 2026 · Jul 1, 2026
diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/AbstractReturnValueIgnored.java b/core/src/main/java/com/google/errorprone/bugpatterns/AbstractReturnValueIgnored.java
@@ -69,6 +69,7 @@
 import com.google.errorprone.matchers.Description;
 import com.google.errorprone.matchers.Matcher;
 import com.google.errorprone.matchers.UnusedReturnValueMatcher;
+import com.google.errorprone.util.SourceVersion;
 import com.sun.source.tree.ExpressionStatementTree;
 import com.sun.source.tree.ExpressionTree;
 import com.sun.source.tree.LambdaExpressionTree;
@@ -337,18 +338,23 @@ && matchingMethods(
     if (parent instanceof ExpressionStatementTree
         && constantExpressions.constantExpression(invocationTree, state).isEmpty()
         && considerBlanketFixes) {
-      ImmutableSet<String> identifiersInScope =
-          findAllIdents(state).stream().map(v -> v.name.toString()).collect(toImmutableSet());
-      concat(Stream.of("unused"), range(2, 10).mapToObj(i -> "unused" + i))
-          // TODO(b/72928608): Handle even local variables declared *later* within this scope.
-          // TODO(b/250568455): Also check whether we have suggested this name before in this scope.
-          .filter(n -> !identifiersInScope.contains(n))
-          .findFirst()
-          .ifPresent(
-              n ->
-                  fixes.put(
-                      "Suppress error by assigning to a variable",
-                      prefixWith(parent, format("var %s = ", n))));
+      if (SourceVersion.supportsUnnamedVariablesAndPatterns(state.context)) {
+        fixes.put("Suppress error by assigning to a variable", prefixWith(parent, "var _ = "));
+      } else {
+        ImmutableSet<String> identifiersInScope =
+            findAllIdents(state).stream().map(v -> v.name.toString()).collect(toImmutableSet());
+        concat(Stream.of("unused"), range(2, 10).mapToObj(i -> "unused" + i))
+            // TODO(b/72928608): Handle even local variables declared *later* within this scope.
+            // TODO(b/250568455): Also check whether we have suggested this name before in this
+            // scope.
+            .filter(n -> !identifiersInScope.contains(n))
+            .findFirst()
+            .ifPresent(
+                n ->
+                    fixes.put(
+                        "Suppress error by assigning to a variable",
+                        prefixWith(parent, format("var %s = ", n))));
+      }
     }
     if (parent instanceof ExpressionStatementTree && considerBlanketFixes) {
       if (constantExpressions.constantExpression(invocationTree, state).isPresent()) {

diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/CheckReturnValue.java b/core/src/main/java/com/google/errorprone/bugpatterns/CheckReturnValue.java
@@ -70,6 +70,7 @@
 import com.google.errorprone.matchers.Description;
 import com.google.errorprone.matchers.Matcher;
 import com.google.errorprone.util.ASTHelpers;
+import com.google.errorprone.util.SourceVersion;
 import com.sun.source.tree.ClassTree;
 import com.sun.source.tree.ExpressionStatementTree;
 import com.sun.source.tree.ExpressionTree;
@@ -329,7 +330,10 @@ public Description matchClass(ClassTree tree, VisitorState state) {
 
   private Description describeInvocationResultIgnored(
       ExpressionTree invocationTree, String shortCall, MethodSymbol symbol, VisitorState state) {
-    String assignmentToUnused = "var unused = ...";
+    String assignmentToUnused =
+        SourceVersion.supportsUnnamedVariablesAndPatterns(state.context)
+            ? "var _ = ..."
+            : "var unused = ...";
     String message =
         invocationResultIgnored(shortCall, assignmentToUnused, apiTrailer(symbol, state));
     return buildDescription(invocationTree)
@@ -373,7 +377,11 @@ protected Description describeReturnValueIgnored(MemberReferenceTree tree, Visit
             + "."
             + state.getTypes().findDescriptorSymbol(getType(tree).asElement()).getSimpleName();
     String methodReference = state.getSourceForNode(tree);
-    String assignmentLambda = parensAndMaybeEllipsis + " -> { var unused = ...; }";
+    String assignmentLambda =
+        parensAndMaybeEllipsis
+            + (SourceVersion.supportsUnnamedVariablesAndPatterns(state.context)
+                ? " -> { var _ = ...; }"
+                : " -> { var unused = ...; }");
     String message =
         methodReferenceIgnoresResult(
             shortCall,

diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/Varifier.java b/core/src/main/java/com/google/errorprone/bugpatterns/Varifier.java
@@ -82,8 +82,8 @@ public Description matchVariable(VariableTree tree, VisitorState state) {
         || hasImplicitType(tree, state)) {
       return NO_MATCH;
     }
-    // Foo unused = ...;
-    if (symbol.getSimpleName().contentEquals("unused")) {
+    // Foo unused = ...; or Foo _ = ...;
+    if (symbol.getSimpleName().contentEquals("unused") || symbol.getSimpleName().isEmpty()) {
       return fix(tree);
     }
     // MyException exception = assertThrows(MyException.class, () -> ...);

diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/CheckReturnValueTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/CheckReturnValueTest.java
@@ -16,6 +16,7 @@
 
 package com.google.errorprone.bugpatterns;
 
+import static com.google.common.truth.TruthJUnit.assume;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.common.base.Joiner;
@@ -47,6 +48,11 @@ public class CheckReturnValueTest {
 
   @Test
   public void positiveCases() {
+    /*
+     * TODO(cpovirk): Split or otherwise modify this test to work when run under Java 21, where we
+     * suggest `var unused =` instead of `var _ =`.
+     */
+    assume().that(Runtime.version().feature()).isAtLeast(22);
     compilationHelper
         .addSourceLines(
             "CheckReturnValuePositiveCases.java",
@@ -72,7 +78,7 @@ public void foo() {
     int i = 1;
     // BUG: Diagnostic contains: The result of `increment(...)` must be used
     //
-    // If you really don't want to use the result, then assign it to a variable: `var unused = ...`.
+    // If you really don't want to use the result, then assign it to a variable: `var _ = ...`.
     //
     // If callers of `increment(...)` shouldn't be required to use its result, then annotate it with
     // `@CanIgnoreReturnValue`.
@@ -109,7 +115,7 @@ public void testResolvedToVoidMethodReference(boolean predicate) {
     // To use the result, you may need to restructure your code.
     //
     // If you really don't want to use the result, then switch to a lambda that assigns it to a
-    // variable: `() -> { var unused = ...; }`.
+    // variable: `() -> { var _ = ...; }`.
     //
     // If callers of `increment()` shouldn't be required to use its result, then annotate it with
     // `@CanIgnoreReturnValue`.
@@ -127,7 +133,7 @@ public void testConstructorResolvedToVoidMethodReference() {
     // To use the result, you may need to restructure your code.
     //
     // If you really don't want to use the result, then switch to a lambda that assigns it to a
-    // variable: `() -> { var unused = ...; }`.
+    // variable: `() -> { var _ = ...; }`.
     //
     // If callers of `MyObject()` shouldn't be required to use its result, then annotate it with
     // `@CanIgnoreReturnValue`.
@@ -167,7 +173,7 @@ class MySubObject3 extends MyObject {
 
     // BUG: Diagnostic contains: The result of `new MyObject()` must be used
     //
-    // If you really don't want to use the result, then assign it to a variable: `var unused = ...`.
+    // If you really don't want to use the result, then assign it to a variable: `var _ = ...`.
     //
     // If callers of `MyObject()` shouldn't be required to use its result, then annotate it with
     // `@CanIgnoreReturnValue`.
@@ -1382,6 +1388,11 @@ String makeBarOrThrow() {
 
   @Test
   public void doesNotSuggestCanIgnoreReturnValueForOtherFile() {
+    /*
+     * This is the one test of introducing an unused variable that we run targeting Java 22+ to test
+     * that we suggest `var unused =` instead of `var _ =` there.
+     */
+    assume().that(Runtime.version().feature()).isAtLeast(22);
     refactoringHelper
         .addInputLines(
             "Lib.java",
@@ -1417,7 +1428,7 @@ void foo(Lib l) {
             @CheckReturnValue
             class Test {
               void foo(Lib l) {
-                var unused = l.makeBarOrThrow();
+                var _ = l.makeBarOrThrow();
               }
             }
             """)
@@ -1427,6 +1438,7 @@ void foo(Lib l) {
   @Test
   public void suggestsVarUnusedForConstructor() {
     refactoringHelper
+        .setArgs("-source", "21")
         .addInputLines(
             "Test.java",
             """
@@ -1453,6 +1465,7 @@ void go() {
   @Test
   public void suggestsVarUnused2() {
     refactoringHelper
+        .setArgs("-source", "21")
         .addInputLines(
             "Lib.java",
             """
@@ -1492,6 +1505,7 @@ void foo(Lib lib) {
   @Test
   public void inheritsCanIgnoreReturnValue() {
     refactoringHelper
+        .setArgs("-source", "21")
         .addInputLines(
             "Super.java",
             """

diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/ReturnValueIgnoredTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/ReturnValueIgnoredTest.java
@@ -763,6 +763,7 @@ public void proto_buildPartial() {
   @Test
   public void refactoringDeletesConstantExpressionCall() {
     refactoringHelper
+        .setArgs("-source", "21")
         .addInputLines(
             "Test.java",
             """
@@ -825,6 +826,7 @@ void f(Optional<Integer> o) {
   @Test
   public void refactoringDoesNotAssignToOriginalForTypeArgumentMismatch() {
     refactoringHelper
+        .setArgs("-source", "21")
         .addInputLines(
             "Test.java",
             """

diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/VarifierTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/VarifierTest.java
@@ -16,6 +16,8 @@
 
 package com.google.errorprone.bugpatterns;
 
+import static com.google.common.truth.TruthJUnit.assume;
+
 import com.google.errorprone.BugCheckerRefactoringTestHelper;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -420,4 +422,29 @@ void test() {
         .expectUnchanged()
         .doTest();
   }
+
+  @Test
+  public void varUnnamed() {
+    assume().that(Runtime.version().feature()).isAtLeast(22);
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            """
+            class Test {
+              public void trim(String string) {
+                String _ = string.trim();
+              }
+            }
+            """)
+        .addOutputLines(
+            "Test.java",
+            """
+            class Test {
+              public void trim(String string) {
+                var _ = string.trim();
+              }
+            }
+            """)
+        .doTest();
+  }
 }