Flag setProtoField(Enum.UNRECOGNIZED), as this will fail at runtime.

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

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2024 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.matchers.Description.NO_MATCH;
+import static com.google.errorprone.matchers.Matchers.instanceMethod;
+import static com.google.errorprone.util.ASTHelpers.getSymbol;
+import static com.google.errorprone.util.ASTHelpers.isSubtype;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.MethodInvocationTreeMatcher;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.suppliers.Supplier;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.tools.javac.code.Type;
+import java.util.regex.Pattern;
+
+/** A BugPattern; see the summary. */
+@BugPattern(
+    summary =
+        "Setting a proto field to an UNRECOGNIZED value will result in an exception at runtime when"
+            + " building.",
+    severity = ERROR)
+public final class SetUnrecognized extends BugChecker implements MethodInvocationTreeMatcher {
+  @Override
+  public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+    if (!IS_PROTO_SETTER.matches(tree, state)) {
+      return NO_MATCH;
+    }
+    if (tree.getArguments().size() != 1) {
+      return NO_MATCH;
+    }
+    ExpressionTree arg = tree.getArguments().get(0);
+    var argSymbol = getSymbol(arg);
+    if (!argSymbol.getSimpleName().contentEquals("UNRECOGNIZED")) {
+      return NO_MATCH;
+    }
+    if (!isSubtype(argSymbol.owner.type, ENUM_LITE.get(state), state)) {
+      return NO_MATCH;
+    }
+    return describeMatch(tree);
+  }
+
+  private static final Matcher<ExpressionTree> IS_PROTO_SETTER =
+      instanceMethod()
+          .onDescendantOfAny("com.google.protobuf.MessageLite.Builder")
+          .withNameMatching(Pattern.compile("(add|set).*"));
+
+  private static final Supplier<Type> ENUM_LITE =
+      VisitorState.memoize(
+          state -> state.getTypeFromString("com.google.protobuf.Internal.EnumLite"));
+}

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

@@ -338,6 +338,7 @@
 import com.google.errorprone.bugpatterns.SelfAssignment;
 import com.google.errorprone.bugpatterns.SelfComparison;
 import com.google.errorprone.bugpatterns.SelfEquals;
+import com.google.errorprone.bugpatterns.SetUnrecognized;
 import com.google.errorprone.bugpatterns.ShortCircuitBoolean;
 import com.google.errorprone.bugpatterns.ShouldHaveEvenArgs;
 import com.google.errorprone.bugpatterns.SizeGreaterThanOrEqualsZero;
@@ -808,6 +809,7 @@ public static ScannerSupplier warningChecks() {
           SelfAssignment.class,
           SelfComparison.class,
           SelfEquals.class,
+          SetUnrecognized.class,
           ShouldHaveEvenArgs.class,
           SizeGreaterThanOrEqualsZero.class,
           StreamToString.class,

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

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2024 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 com.google.errorprone.CompilationTestHelper;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class SetUnrecognizedTest {
+  private final CompilationTestHelper compilationHelper =
+      CompilationTestHelper.newInstance(SetUnrecognized.class, getClass());
+
+  @Test
+  public void positive() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.errorprone.bugpatterns.proto.Proto3Test.TestProto3Enum;",
+            "import com.google.errorprone.bugpatterns.proto.Proto3Test.TestProto3Message;",
+            "class Test {",
+            "  void test() {",
+            "    // BUG: Diagnostic contains:",
+            "    TestProto3Message.newBuilder().setMyEnum(TestProto3Enum.UNRECOGNIZED);",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void negative() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import com.google.errorprone.bugpatterns.proto.Proto3Test.TestProto3Enum;",
+            "import com.google.errorprone.bugpatterns.proto.Proto3Test.TestProto3Message;",
+            "class Test {",
+            "  void test() {",
+            "    TestProto3Message.newBuilder().setMyEnum(TestProto3Enum.VALUE_1);",
+            "  }",
+            "}")
+        .doTest();
+  }
+}

core/src/test/proto/proto3_test.proto

@@ -21,6 +21,13 @@ package com.google.errorprone.bugpatterns;
 option java_package = "com.google.errorprone.bugpatterns.proto";
 
 message TestProto3Message {
-  string myString = 1;
-  bytes myBytes = 2;
+  string my_string = 1;
+  bytes my_bytes = 2;
+  TestProto3Enum my_enum = 3;
+}
+
+enum TestProto3Enum {
+  UNKNOWN = 0;
+  VALUE_1 = 1;
+  VALUE_2 = 2;
 }